JavaScript for impatient programmers (ES2022 edition)

Dr. Axel Rauschmayer

2022

JavaScript for impatient programmers (ES2022 edition)

1. JavaScript for impatient programmers (ES2022 edition)

2. Before you buy the book

1. About the content

2. Previewing and buying this book

3. About the author

4. Acknowledgements

3. FAQ: book and supplementary material

1. How to read this book

2. I own a digital version

3. I own the print version

4. Notations and conventions

4. History and evolution of JavaScript

1. How JavaScript was created

2. Standardizing JavaScript

3. Timeline of ECMAScript versions

4. Ecma Technical Committee 39 (TC39)

5. The TC39 process

6. FAQ: TC39 process

7. Evolving JavaScript: Don’t break the web

5. New JavaScript features

1. New in ECMAScript 2022

2. New in ECMAScript 2021

3. New in ECMAScript 2020

4. New in ECMAScript 2019

5. New in ECMAScript 2018

6. New in ECMAScript 2017

7. New in ECMAScript 2016

8. Source of this chapter

6. FAQ: JavaScript

1. What are good references for JavaScript?

2. How do I find out what JavaScript features are supported where?

3. Where can I look up what features are planned for JavaScript?

4. Why does JavaScript fail silently so often?

5. Why can’t we clean up JavaScript, by removing quirks and outdated features?

6. How can I quickly try out a piece of JavaScript code?

7. Using JavaScript: the big picture

1. What are you learning in this book?

2. The structure of browsers and Node.js

3. JavaScript references

4. Further reading

8. Syntax

1. An overview of JavaScript’s syntax

2. (Advanced)

3. Identifiers

4. Statement vs. expression

5. Ambiguous syntax

6. Semicolons

7. Automatic semicolon insertion (ASI)

8. Semicolons: best practices

9. Strict mode vs. sloppy mode

9. Consoles: interactive JavaScript command lines

1. Trying out JavaScript code

2. The console.* API: printing data and more

10. Assertion API

1. Assertions in software development

2. How assertions are used in this book

3. Normal comparison vs. deep comparison

4. Quick reference: module assert

11. Getting started with quizzes and exercises

1. Quizzes

2. Exercises

3. Unit tests in JavaScript

12. Variables and assignment

1. let

2. const

3. Deciding between const and let

4. The scope of a variable

5. (Advanced)

6. Terminology: static vs. dynamic

7. Global variables and the global object

8. Declarations: scope and activation

9. Closures

13. Values

1. What’s a type?

2. JavaScript’s type hierarchy

3. The types of the language specification

4. Primitive values vs. objects

5. The operators typeof and instanceof: what’s the type of a value?

6. Classes and constructor functions

7. Converting between types

14. Operators

1. Making sense of operators

2. The plus operator (+)

3. Assignment operators

4. Equality: == vs. ===

5. Ordering operators

6. Various other operators

15. The non-values undefined and null

1. undefined vs. null

2. Occurrences of undefined and null

3. Checking for undefined or null

4. The nullish coalescing operator (??) for default values [ES2020]

5. undefined and null don’t have properties

6. The history of undefined and null

16. Booleans

1. Converting to boolean

2. Falsy and truthy values

3. Truthiness-based existence checks

4. Conditional operator (? :)

5. Binary logical operators: And (x && y), Or (x || y)

6. Logical Not (!)

17. Numbers

1. Numbers are used for both floating point numbers and integers

2. Number literals

3. Arithmetic operators

4. Converting to number

5. Error values

6. The precision of numbers: careful with decimal fractions

7. (Advanced)

8. Background: floating point precision

9. Integer numbers in JavaScript

10. Bitwise operators

11. Quick reference: numbers

18. Math

1. Data properties

2. Exponents, roots, logarithms

3. Rounding

4. Trigonometric Functions

5. Various other functions

6. Sources

19. Bigints – arbitrary-precision integers [ES2020] (advanced)

1. Why bigints?

2. Bigints

3. Bigint literals

4. Reusing number operators for bigints (overloading)

5. The wrapper constructor BigInt

6. Coercing bigints to other primitive types

7. TypedArrays and DataView operations for 64-bit values

8. Bigints and JSON

9. FAQ: Bigints

20. Unicode – a brief introduction (advanced)

1. Code points vs. code units

2. Encodings used in web development: UTF-16 and UTF-8

3. Grapheme clusters – the real characters

21. Strings

1. Cheat sheet: strings

2. Plain string literals

3. Accessing JavaScript characters

4. String concatenation via +

5. Converting to string

6. Comparing strings

7. Atoms of text: code points, JavaScript characters, grapheme clusters

8. Quick reference: Strings

22. Using template literals and tagged templates

1. Disambiguation: “template”

2. Template literals

3. Tagged templates

4. Examples of tagged templates (as provided via libraries)

5. Raw string literals

6. (Advanced)

7. Multiline template literals and indentation

8. Simple templating via template literals

23. Symbols

1. Symbols are primitives that are also like objects

2. The descriptions of symbols

3. Use cases for symbols

4. Publicly known symbols

5. Converting symbols

24. Control flow statements

1. Controlling loops: break and continue

2. Conditions of control flow statements

3. if statements [ES1]

4. switch statements [ES3]

5. while loops [ES1]

6. do-while loops [ES3]

7. for loops [ES1]

8. for-of loops [ES6]

9. for-await-of loops [ES2018]

10. for-in loops (avoid) [ES1]

11. Recomendations for looping

25. Exception handling

1. Motivation: throwing and catching exceptions

2. throw

3. The try statement

4. Error and its subclasses

5. Chaining errors

26. Callable values

1. Kinds of functions

2. Ordinary functions

3. Specialized functions

4. Summary: kinds of callable values

5. Returning values from functions and methods

6. Parameter handling

7. Methods of functions: .call(), .apply(), .bind()

27. Evaluating code dynamically: eval(), new Function() (advanced)

1. eval()

2. new Function()

3. Recommendations

28. Modules

1. Cheat sheet: modules

2. JavaScript source code formats

3. Before we had modules, we had scripts

4. Module systems created prior to ES6

5. ECMAScript modules

6. Named exports and imports

7. Default exports and imports

8. More details on exporting and importing

9. npm packages

10. Naming modules

11. Module specifiers

12. import.meta – metadata for the current module [ES2020]

13. Loading modules dynamically via import() [ES2020] (advanced)

14. Top-level await in modules [ES2022] (advanced)

15. Polyfills: emulating native web platform features (advanced)

29. Objects

1. Cheat sheet: objects

2. What is an object?

3. Fixed-layout objects

4. Spreading into object literals (...) [ES2018]

5. Methods and the special variable this

6. Optional chaining for property getting and method calls [ES2020] (advanced)

7. Dictionary objects (advanced)

8. Property attributes and freezing objects (advanced)

9. Prototype chains

10. FAQ: objects

30. Classes [ES6]

1. Cheat sheet: classes

2. The essentials of classes

3. The internals of classes

4. Prototype members of classes

5. Instance members of classes [ES2022]

6. Static members of classes

7. Subclassing

8. The methods and accessors of Object.prototype (advanced)

9. FAQ: classes

31. Where are the remaining chapters?

JavaScript for impatient programmers (ES2022 edition)

Before you buy the book

- 1.1 About the content

- 1.1.1 What’s in this book?

- 1.1.2 What is not covered by this book?

- 1.1.3 Isn’t this book too long for impatient people?

- 1.2 Previewing and buying this book

- 1.2.1 How can I preview the book, the exercises, and the quizzes?

- 1.2.2 How can I buy a digital version of this book?

- 1.2.3 How can I buy the print version of this book?

- 1.3 About the author

- 1.4 Acknowledgements

About the content

What’s in this book?

This book makes JavaScript less challenging to learn for newcomers by offering a modern view that is as consistent as possible.

Highlights:

- Get started quickly by initially focusing on modern features.

- Test-driven exercises and quizzes available for most chapters.

- Covers all essential features of JavaScript, up to and including ES2022.

- Optional advanced sections let you dig deeper.

No prior knowledge of JavaScript is required, but you should know how to program.

What is not covered by this book?

- Some advanced language features are not explained, but references to appropriate material are provided – for example, to my other JavaScript books at ExploringJS.com, which are free to read online.

- This book deliberately focuses on the language. Browser-only features, etc. are not described.

Isn’t this book too long for impatient people?

There are several ways in which you can read this book. One of them involves skipping much of the content in order to get started quickly. For details, see §2.1.1 “In which order should I read the content in this book?”.

Previewing and buying this book

How can I preview the book, the exercises, and the quizzes?

Go to the homepage of this book:

- All essential chapters of this book are free to read online.

- The first half of the test-driven exercises can be downloaded.

- The first half of the quizzes can be tried online.

How can I buy a digital version of this book?

There are two digital versions of JavaScript for impatient programmers:

- Ebooks: PDF, EPUB, MOBI, HTML (all without DRM)

- Ebooks plus exercises and quizzes

The home page of this book describes how you can buy them.

How can I buy the print version of this book?

The print version of JavaScript for impatient programmers is available on Amazon.

About the author

Dr. Axel Rauschmayer specializes in JavaScript and web development. He has been developing web applications since 1995. In 1999, he was technical manager at a German internet startup that later expanded internationally. In 2006, he held his first talk on Ajax. In 2010, he received a PhD in Informatics from the University of Munich.

Since 2011, he has been blogging about web development at 2ality.com and has written several books on JavaScript. He has held trainings and talks for companies such as eBay, Bank of America, and O’Reilly Media.

He lives in Munich, Germany.

Acknowledgements

- Cover by Fran Caye

- Parts of this book were edited by Adaobi Obi Tulton.

- Thanks for answering questions, discussing language topics, etc.:

- Allen Wirfs-Brock (@awbjs)

- Benedikt Meurer (@bmeurer)

- Brian Terlson (@bterlson)

- Daniel Ehrenberg (@littledan)

- Jordan Harband (@ljharb)

- Maggie Johnson-Pint (@maggiepint)

- Mathias Bynens (@mathias)

- Myles Borins (@MylesBorins)

- Rob Palmer (@robpalmer2)

- Šime Vidas (@simevidas)

- And many others

- Thanks for reviewing:

- Johannes Weber (@jowe)

[Generated: 2022-01-03 14:16]

FAQ: book and supplementary material

- 2.1 How to read this book

- 2.1.1 In which order should I read the content in this book?

- 2.1.2 Why are some chapters and sections marked with “(advanced)”?

- 2.1.3 Why are some chapters marked with “(bonus)”?

- 2.2 I own a digital version

- 2.2.1 How do I submit feedback and corrections?

- 2.2.2 How do I get updates for the downloads I bought at Payhip?

- 2.2.3 Can I upgrade from package “Ebooks” to package “Ebooks + exercises + quizzes”?

- 2.3 I own the print version

- 2.3.1 Can I get a discount for a digital version?

- 2.3.2 Can I submit an error or see submitted errors?

- 2.3.3 Is there an online list with the URLs in this book?

- 2.4 Notations and conventions

- 2.4.1 What is a type signature? Why am I seeing static types in this book?

- 2.4.2 What do the notes with icons mean?

This chapter answers questions you may have and gives tips for reading this book.

How to read this book

In which order should I read the content in this book?

This book is three books in one:

- You can use it to get started with JavaScript as quickly as possible. This “mode” is for impatient people:

- Start reading with §6 “Using JavaScript: the big picture”.

- Skip all chapters and sections marked as “advanced”, and all quick references.

- It gives you a comprehensive look at current JavaScript. In this “mode”, you read everything and don’t skip advanced content and quick references.

- It serves as a reference. If there is a topic that you are interested in, you can find information on it via the table of contents or via the index. Due to basic and advanced content being mixed, everything you need is usually in a single location.

The quizzes and exercises play an important part in helping you practice and retain what you have learned.

Why are some chapters and sections marked with “(advanced)”?

Several chapters and sections are marked with “(advanced)”. The idea is that you can initially skip them. That is, you can get a quick working knowledge of JavaScript by only reading the basic (non-advanced) content.

As your knowledge evolves, you can later come back to some or all of the advanced content.

Why are some chapters marked with “(bonus)”?

The bonus chapters are only available in the paid versions of this book (print and ebook). They are listed in the full table of contents.

I own a digital version

How do I submit feedback and corrections?

The HTML version of this book (online, or ad-free archive in the paid version) has a link at the end of each chapter that enables you to give feedback.

How do I get updates for the downloads I bought at Payhip?

- The receipt email for the purchase includes a link. You’ll always be able to download the latest version of the files at that location.

- If you opted into emails while buying, you’ll get an email whenever there is new content. To opt in later, you must contact Payhip (see bottom of payhip.com).

Can I upgrade from package “Ebooks” to package “Ebooks + exercises + quizzes”?

Yes. The instructions for doing so are on the homepage of this book.

I own the print version

Can I get a discount for a digital version?

If you bought the print version, you can get a discount for a digital version. The homepage of the print version explains how.

Alas, the reverse is not possible: you cannot get a discount for the print version if you bought a digital version.

Can I submit an error or see submitted errors?

On the homepage of the print version, you can submit errors and see submitted errors.

Is there an online list with the URLs in this book?

The homepage of the print version has a list with all the URLs that you see in the footnotes of the print version.

Notations and conventions

What is a type signature? Why am I seeing static types in this book?

For example, you may see:

Number.isFinite(num: number): boolean

That is called the type signature of Number.isFinite(). This notation, especially the static types number of num and boolean of the result, are not real JavaScript. The notation is borrowed from the compile-to-JavaScript language TypeScript (which is mostly just JavaScript plus static typing).

Why is this notation being used? It helps give you a quick idea of how a function works. The notation is explained in detail in “Tackling TypeScript”, but is usually relatively intuitive.

What do the notes with icons mean?

Reading instructions

Explains how to best read the content.

External content

Points to additional, external, content.

Tip

Gives a tip related to the current content.

Question

Asks and answers a question pertinent to the current content (think FAQ).

Warning

Warns about pitfalls, etc.

Details

Provides additional details, complementing the current content. It is similar to a footnote.

Exercise

Mentions the path of a test-driven exercise that you can do at that point.

Quiz

Indicates that there is a quiz for the current (part of a) chapter.

History and evolution of JavaScript

- 3.1 How JavaScript was created

- 3.2 Standardizing JavaScript

- 3.3 Timeline of ECMAScript versions

- 3.4 Ecma Technical Committee 39 (TC39)

- 3.5 The TC39 process

- 3.5.1 Tip: Think in individual features and stages, not ECMAScript versions

- 3.6 FAQ: TC39 process

- 3.6.1 How is [my favorite proposed feature] doing?

- 3.6.2 Is there an official list of ECMAScript features?

- 3.7 Evolving JavaScript: Don’t break the web

How JavaScript was created

JavaScript was created in May 1995 in 10 days, by Brendan Eich. Eich worked at Netscape and implemented JavaScript for their web browser, Netscape Navigator.

The idea was that major interactive parts of the client-side web were to be implemented in Java. JavaScript was supposed to be a glue language for those parts and to also make HTML slightly more interactive. Given its role of assisting Java, JavaScript had to look like Java. That ruled out existing solutions such as Perl, Python, TCL, and others.

Initially, JavaScript’s name changed several times:

- Its code name was Mocha.

- In the Netscape Navigator 2.0 betas (September 1995), it was called LiveScript.

- In Netscape Navigator 2.0 beta 3 (December 1995), it got its final name, JavaScript.

Standardizing JavaScript

There are two standards for JavaScript:

- ECMA-262 is hosted by Ecma International. It is the primary standard.

- ISO/IEC 16262 is hosted by the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC). This is a secondary standard.

The language described by these standards is called ECMAScript, not JavaScript. A different name was chosen because Sun (now Oracle) had a trademark for the latter name. The “ECMA” in “ECMAScript” comes from the organization that hosts the primary standard.

The original name of that organization was ECMA, an acronym for European Computer Manufacturers Association. It was later changed to Ecma International (with “Ecma” being a proper name, not an acronym) because the organization’s activities had expanded beyond Europe. The initial all-caps acronym explains the spelling of ECMAScript.

In principle, JavaScript and ECMAScript mean the same thing. Sometimes the following distinction is made:

- The term JavaScript refers to the language and its implementations.

- The term ECMAScript refers to the language standard and language versions.

Therefore, ECMAScript 6 is a version of the language (its 6th edition).

Timeline of ECMAScript versions

This is a brief timeline of ECMAScript versions:

- ECMAScript 1 (June 1997): First version of the standard.

- ECMAScript 2 (June 1998): Small update to keep ECMA-262 in sync with the ISO standard.

- ECMAScript 3 (December 1999): Adds many core features – “[…] regular expressions, better string handling, new control statements [do-while, switch], try/catch exception handling, […]”

- ECMAScript 4 (abandoned in July 2008): Would have been a massive upgrade (with static typing, modules, namespaces, and more), but ended up being too ambitious and dividing the language’s stewards.

- ECMAScript 5 (December 2009): Brought minor improvements – a few standard library features and strict mode.

- ECMAScript 5.1 (June 2011): Another small update to keep Ecma and ISO standards in sync.

- ECMAScript 6 (June 2015): A large update that fulfilled many of the promises of ECMAScript 4. This version is the first one whose official name – ECMAScript 2015 – is based on the year of publication.

- ECMAScript 2016 (June 2016): First yearly release. The shorter release life cycle resulted in fewer new features compared to the large ES6.

- ECMAScript 2017 (June 2017). Second yearly release.

- Subsequent ECMAScript versions (ES2018, etc.) are always ratified in June.

Ecma Technical Committee 39 (TC39)

TC39 is the committee that evolves JavaScript. Its members are, strictly speaking, companies: Adobe, Apple, Facebook, Google, Microsoft, Mozilla, Opera, Twitter, and others. That is, companies that are usually fierce competitors are working together for the good of the language.

Every two months, TC39 has meetings that member-appointed delegates and invited experts attend. The minutes of those meetings are public in a GitHub repository.

The TC39 process

With ECMAScript 6, two issues with the release process used at that time became obvious:

- If too much time passes between releases then features that are ready early, have to wait a long time until they can be released. And features that are ready late, risk being rushed to make the deadline.

- Features were often designed long before they were implemented and used. Design deficiencies related to implementation and use were therefore discovered too late.

In response to these issues, TC39 instituted the new TC39 process:

- ECMAScript features are designed independently and go through stages, starting at 0 (“strawman”), ending at 4 (“finished”).

- Especially the later stages require prototype implementations and real-world testing, leading to feedback loops between designs and implementations.

- ECMAScript versions are released once per year and include all features that have reached stage 4 prior to a release deadline.

The result: smaller, incremental releases, whose features have already been field-tested. Fig. 1 illustrates the TC39 process.

Figure 1: Each ECMAScript feature proposal goes through stages that are numbered from 0 to 4. Champions are TC39 members that support the authors of a feature. Test 262 is a suite of tests that checks JavaScript engines for compliance with the language specification.

ES2016 was the first ECMAScript version that was designed according to the TC39 process.

Tip: Think in individual features and stages, not ECMAScript versions

Up to and including ES6, it was most common to think about JavaScript in terms of ECMAScript versions – for example, “Does this browser support ES6 yet?”

Starting with ES2016, it’s better to think in individual features: once a feature reaches stage 4, you can safely use it (if it’s supported by the JavaScript engines you are targeting). You don’t have to wait until the next ECMAScript release.

FAQ: TC39 process

How is [my favorite proposed feature] doing?

If you are wondering what stages various proposed features are in, consult the GitHub repository proposals.

Is there an official list of ECMAScript features?

Yes, the TC39 repo lists finished proposals and mentions in which ECMAScript versions they were introduced.

Evolving JavaScript: Don’t break the web

One idea that occasionally comes up is to clean up JavaScript by removing old features and quirks. While the appeal of that idea is obvious, it has significant downsides.

Let’s assume we create a new version of JavaScript that is not backward compatible and fix all of its flaws. As a result, we’d encounter the following problems:

- JavaScript engines become bloated: they need to support both the old and the new version. The same is true for tools such as IDEs and build tools.

- Programmers need to know, and be continually conscious of, the differences between the versions.

- You can either migrate all of an existing code base to the new version (which can be a lot of work). Or you can mix versions and refactoring becomes harder because you can’t move code between versions without changing it.

- You somehow have to specify per piece of code – be it a file or code embedded in a web page – what version it is written in. Every conceivable solution has pros and cons. For example, strict mode is a slightly cleaner version of ES5. One of the reasons why it wasn’t as popular as it should have been: it was a hassle to opt in via a directive at the beginning of a file or a function.

So what is the solution? Can we have our cake and eat it? The approach that was chosen for ES6 is called “One JavaScript”:

- New versions are always completely backward compatible (but there may occasionally be minor, hardly noticeable clean-ups).

- Old features aren’t removed or fixed. Instead, better versions of them are introduced. One example is declaring variables via let – which is an improved version of var.

- If aspects of the language are changed, it is done inside new syntactic constructs. That is, you opt in implicitly. For example, yield is only a keyword inside generators (which were introduced in ES6). And all code inside modules and classes (both introduced in ES6) is implicitly in strict mode.

Quiz

See quiz app.

New JavaScript features

- 4.1 New in ECMAScript 2022

- 4.2 New in ECMAScript 2021

- 4.3 New in ECMAScript 2020

- 4.4 New in ECMAScript 2019

- 4.5 New in ECMAScript 2018

- 4.6 New in ECMAScript 2017

- 4.7 New in ECMAScript 2016

- 4.8 Source of this chapter

This chapter lists what’s new in ES2016–ES2022 in reverse chronological order. It starts after ES2015 (ES6) because that release has too many features to list here.

New in ECMAScript 2022

ES2022 will probably become a standard in June 2022. The following proposals have reached stage 4 and are scheduled to be part of that standard:

- New members of classes:

- Properties (public slots) can now be created via:

- Instance public fields

- Static public fields

- Private slots are new and can be created via:

- Private fields (instance private fields and static private fields)

- Private methods and accessors (non-static and static)

- Static initialization blocks

- Private slot checks (“ergonomic brand checks for private fields”): The following expression checks if obj has a private slot #privateSlot:

#privateSlot in obj

- Top-level await in modules: We can now use await at the top levels of modules and don’t have to enter async functions or methods anymore.

- error.cause: Error and its subclasses now let us specify which error caused the current one:

new Error('Something went wrong', {cause: otherError})

- Method .at() of indexable values lets us read an element at a given index (like the bracket operator []) and supports negative indices (unlike the bracket operator).

> ['a', 'b', 'c'].at(0)

'a'

> ['a', 'b', 'c'].at(-1)

'c'

The following “indexable” types have method .at():

- string

- Array

- All Typed Array classes: Uint8Array etc.

- RegExp match indices: If we add a flag to a regular expression, using it produces match objects that record the start and end index of each group capture.

- Object.hasOwn(obj, propKey) provides a safe way to check if an object obj has an own property with the key propKey. In contrast to Object.prototype.hasOwnProperty, it works with all objects.

More features may still be added to ES2022

If that happens, this book will be updated accordingly.

New in ECMAScript 2021

The following features were added in ECMAScript 2021:

- String.prototype.replaceAll() lets us replace all matches of a regular expression or a string (.replace() only replaces the first occurrence of a string):

> 'abbbaab'.replaceAll('b', 'x')

'axxxaax'

- Promise.any() and AggregateError: Promise.any() returns a Promise that is fulfilled as soon as the first Promise in an iterable of Promises is fulfilled. If there are only rejections, they are put into an AggregateError which becomes the rejection value.

We use Promise.any() when we are only interested in the first fulfilled Promise among several.

- Logical assignment operators:

a ||= b

a &&= b

a ??= b

- Underscores (_) as separators in:

- Number literals: 123_456.789_012

- Bigint literals: 6_000_000_000_000_000_000_000_000n

- WeakRefs: This feature is beyond the scope of this book. For more information on it, see its proposal.

New in ECMAScript 2020

The following features were added in ECMAScript 2020:

- New module features:

- Dynamic imports via import(): The normal import statement is static: We can only use it at the top levels of modules and its module specifier is a fixed string. import() changes that. It can be used anywhere (including conditional statements) and we can compute its argument.

- import.meta contains metadata for the current module. Its first widely supported property is import.meta.url which contains a string with the URL of the current module’s file.

- Namespace re-exporting: The following expression imports all exports of module 'mod' in a namespace object ns and exports that object.

export * as ns from 'mod';

- Optional chaining for property accesses and method calls. One example of optional chaining is:

value.?prop

This expression evaluates to undefined if value is either undefined or null. Otherwise, it evaluates to value.prop. This feature is especially useful in chains of property reads when some of the properties may be missing.

- Nullish coalescing operator (??):

value ?? defaultValue

This expression is defaultValue if value is either undefined or null and value otherwise. This operator lets us use a default value whenever something is missing.

Previously the Logical Or operator (||) was used in this case but it has downsides here because it returns the default value whenever the left-hand side is falsy (which isn’t always correct).

- Bigints – arbitrary-precision integers: Bigints are a new primitive type. It supports integer numbers that can be arbitrarily large (storage for them grows as necessary).

- String.prototype.matchAll(): This method throws if flag /g isn’t set and returns an iterable with all match objects for a given string.

- Promise.allSettled() receives an iterable of Promises. It returns a Promise that is fulfilled once all the input Promises are settled. The fulfillment value is an Array with one object per input Promise – either one of:

- { status: 'fulfilled', value: «fulfillment value» }

- { status: 'rejected', reason: «rejection value» }

- globalThis provides a way to access the global object that works both on browsers and server-side platforms such as Node.js and Deno.

- for-in mechanics: This feature is beyond the scope of this book. For more information on it, see its proposal.

New in ECMAScript 2019

The following features were added in ECMAScript 2019:

- Array method .flatMap() works like .map() but lets the callback return Arrays of zero or more values instead of single values. The returned Arrays are then concatenated and become the result of .flatMap(). Use cases include:

- Filtering and mapping at the same time

- Mapping single input values to multiple output values

- Array method .flat() converts nested Arrays into flat Arrays. Optionally, we can tell it at which depth of nesting it should stop flattening.

- Object.fromEntries() creates an object from an iterable over entries. Each entry is a two-element Array with a property key and a property value.

- String methods: .trimStart() and .trimEnd() work like .trim() but remove whitespace only at the start or only at the end of a string.

- Optional catch binding: We can now omit the parameter of a catch clause if we don’t use it.

- Symbol.prototype.description is a getter for reading the description of a symbol. Previously, the description was included in the result of .toString() but couldn’t be accessed individually.

These new ES2019 features are beyond the scope of this book:

- JSON superset: See 2ality blog post.

- Well-formed JSON.stringify(): See 2ality blog post.

- Function.prototype.toString() revision: See 2ality blog post.

New in ECMAScript 2018

The following features were added in ECMAScript 2018:

- Asynchronous iteration is the asynchronous version of synchronous iteration. It is based on Promises:

- With synchronous iterables, we can immediately access each item. With asynchronous iterables, we have to await before we can access an item.

- With synchronous iterables, we use for-of loops. With asynchronous iterables, we use for-await-of loops.

- Spreading into object literals: By using spreading (...) inside an object literal, we can copy the properties of another object into the current one. One use case is to create a shallow copy of an object obj:

const shallowCopy = {...obj};

- Rest properties (destructuring): When object-destructuring a value, we can now use rest syntax (...) to get all previously unmentioned properties in an object.

const {a, ...remaining} = {a: 1, b: 2, c: 3};

assert.deepEqual(remaining, {b: 2, c: 3});

- Promise.prototype.finally() is related to the finally clause of a try-catch-finally statement – similarly to how the Promise method .then() is related to the try clause and .catch() is related to the catch clause.

On other words: The callback of .finally() is executed regardless of whether a Promise is fulfilled or rejected.

- New Regular expression features:

- RegExp named capture groups: In addition to accessing groups by number, we can now name them and access them by name:

const matchObj = '---756---'.match(/(?<digits>[0-9]+)/)

assert.equal(matchObj.groups.digits, '756');

- RegExp lookbehind assertions complement lookahead assertions:

- Positive lookbehind: (?<=X) matches if the current location is preceded by 'X'.

- Negative lookbehind: (?<!X) matches if the current location is not preceded by '(?<!X)'.

- s (dotAll) flag for regular expressions. If this flag is active, the dot matches line terminators (by default, it doesn’t).

- RegExp Unicode property escapes give us more power when matching sets of Unicode code points – for example:

> /^\p{Lowercase_Letter}+$/u.test('aüπ')

true

> /^\p{White_Space}+$/u.test('\n \t')

true

> /^\p{Script=Greek}+$/u.test('ΩΔΨ')

true

- Template literal revision allows text with backslashes in tagged templates that is illegal in string literals – for example:

windowsPath`C:\uuu\xxx\111`

latex`\unicode`

New in ECMAScript 2017

The following features were added in ECMAScript 2017:

- Async functions (async/await) let us use synchronous-looking syntax to write asynchronous code.

- Object.values() returns an Array with the values of all enumerable string-keyed properties of a given object.

- Object.entries() returns an Array with the key-value pairs of all enumerable string-keyed properties of a given object. Each pair is encoded as a two-element Array.

- String padding: The string methods .padStart() and .padEnd() insert padding text until the receivers are long enough:

> '7'.padStart(3, '0')

'007'

> 'yes'.padEnd(6, '!')

'yes!!!'

- Trailing commas in function parameter lists and calls: Trailing commas have been allowed in Arrays literals since ES3 and in Object literals since ES5. They are now also allowed in function calls and method calls.

- The following two features are beyond the scope of this book:

- Object.getOwnPropertyDescriptors() (see “Deep JavaScript”)

- Shared memory and atomics (see proposal)

New in ECMAScript 2016

The following features were added in ECMAScript 2016:

- Array.prototype.includes() checks if an Array contains a given value.

- Exponentiation operator (**):

> 4 ** 2

16

Source of this chapter

ECMAScript feature lists were taken from the TC39 page on finished proposals.

FAQ: JavaScript

- 5.1 What are good references for JavaScript?

- 5.2 How do I find out what JavaScript features are supported where?

- 5.3 Where can I look up what features are planned for JavaScript?

- 5.4 Why does JavaScript fail silently so often?

- 5.5 Why can’t we clean up JavaScript, by removing quirks and outdated features?

- 5.6 How can I quickly try out a piece of JavaScript code?

What are good references for JavaScript?

Please consult §6.3 “JavaScript references”.

How do I find out what JavaScript features are supported where?

This book usually mentions if a feature is part of ECMAScript 5 (as required by older browsers) or a newer version. For more detailed information (including pre-ES5 versions), there are several good compatibility tables available online:

- ECMAScript compatibility tables for various engines (by kangax, webbedspace, zloirock)

- Node.js compatibility tables (by William Kapke)

- Mozilla’s MDN web docs have tables for each feature that describe relevant ECMAScript versions and browser support.

- “Can I use…” documents what features (including JavaScript language features) are supported by web browsers.

Where can I look up what features are planned for JavaScript?

Please consult the following sources:

- §3.5 “The TC39 process” describes how upcoming features are planned.

- §3.6 “FAQ: TC39 process” answers various questions regarding upcoming features.

Why does JavaScript fail silently so often?

JavaScript often fails silently. Let’s look at two examples.

First example: If the operands of an operator don’t have the appropriate types, they are converted as necessary.

> '3' * '5'

15

Second example: If an arithmetic computation fails, you get an error value, not an exception.

> 1 / 0

Infinity

The reason for the silent failures is historical: JavaScript did not have exceptions until ECMAScript 3. Since then, its designers have tried to avoid silent failures.

Why can’t we clean up JavaScript, by removing quirks and outdated features?

This question is answered in §3.7 “Evolving JavaScript: Don’t break the web”.

How can I quickly try out a piece of JavaScript code?

§8.1 “Trying out JavaScript code” explains how to do that.

Using JavaScript: the big picture

- 6.1 What are you learning in this book?

- 6.2 The structure of browsers and Node.js

- 6.3 JavaScript references

- 6.4 Further reading

In this chapter, I’d like to paint the big picture: what are you learning in this book, and how does it fit into the overall landscape of web development?

What are you learning in this book?

This book teaches the JavaScript language. It focuses on just the language, but offers occasional glimpses at two platforms where JavaScript can be used:

- Web browser

- Node.js

Node.js is important for web development in three ways:

- You can use it to write server-side software in JavaScript.

- You can also use it to write software for the command line (think Unix shell, Windows PowerShell, etc.). Many JavaScript-related tools are based on (and executed via) Node.js.

- Node’s software registry, npm, has become the dominant way of installing tools (such as compilers and build tools) and libraries – even for client-side development.

The structure of browsers and Node.js

Figure 2: The structure of the two JavaScript platforms web browser and Node.js. The APIs “standard library” and “platform API” are hosted on top of a foundational layer with a JavaScript engine and a platform-specific “core”.

The structures of the two JavaScript platforms web browser and Node.js are similar (fig. 2):

- The foundational layer consists of the JavaScript engine and platform-specific “core” functionality.

- Two APIs are hosted on top of this foundation:

- The JavaScript standard library is part of JavaScript proper and runs on top of the engine.

- The platform API are also available from JavaScript – it provides access to platform-specific functionality. For example:

- In browsers, you need to use the platform-specific API if you want to do anything related to the user interface: react to mouse clicks, play sounds, etc.

- In Node.js, the platform-specific API lets you read and write files, download data via HTTP, etc.

JavaScript references

When you have a question about a JavaScript, a web search usually helps. I can recommend the following online sources:

- MDN web docs: cover various web technologies such as CSS, HTML, JavaScript, and more. An excellent reference.

- Node.js Docs: document the Node.js API.

- ExploringJS.com: My other books on JavaScript go into greater detail than this book and are free to read online. You can look up features by ECMAScript version:

- ES1–ES5: Speaking JavaScript

- ES6: Exploring ES6

- ES2016–ES2017: Exploring ES2016 and ES2017

- Etc.

Further reading

- A bonus chapter provides a more comprehensive look at web development.

Syntax

- 7.1 An overview of JavaScript’s syntax

- 7.1.1 Basic constructs

- 7.1.2 Modules

- 7.1.3 Classes

- 7.1.4 Exception handling

- 7.1.5 Legal variable and property names

- 7.1.6 Casing styles

- 7.1.7 Capitalization of names

- 7.1.8 More naming conventions

- 7.1.9 Where to put semicolons?

- 7.2 (Advanced)

- 7.3 Identifiers

- 7.3.1 Valid identifiers (variable names, etc.)

- 7.3.2 Reserved words

- 7.4 Statement vs. expression

- 7.4.1 Statements

- 7.4.2 Expressions

- 7.4.3 What is allowed where?

- 7.5 Ambiguous syntax

- 7.5.1 Same syntax: function declaration and function expression

- 7.5.2 Same syntax: object literal and block

- 7.5.3 Disambiguation

- 7.6 Semicolons

- 7.6.1 Rule of thumb for semicolons

- 7.6.2 Semicolons: control statements

- 7.7 Automatic semicolon insertion (ASI)

- 7.7.1 ASI triggered unexpectedly

- 7.7.2 ASI unexpectedly not triggered

- 7.8 Semicolons: best practices

- 7.9 Strict mode vs. sloppy mode

- 7.9.1 Switching on strict mode

- 7.9.2 Improvements in strict mode

An overview of JavaScript’s syntax

This is a very first look at JavaScript’s syntax. Don’t worry if some things don’t make sense, yet. They will all be explained in more detail later in this book.

This overview is not exhaustive, either. It focuses on the essentials.

Basic constructs

Comments

// single-line comment

/*

Comment with

multiple lines

*/

Primitive (atomic) values

Booleans:

true

false

Numbers:

1.141

-123

The basic number type is used for both floating point numbers (doubles) and integers.

Bigints:

17n

-49n

The basic number type can only properly represent integers within a range of 53 bits plus sign. Bigints can grow arbitrarily large in size.

Strings:

'abc'

"abc"

`String with interpolated values: ${256} and ${true}`

JavaScript has no extra type for characters. It uses strings to represent them.

Assertions

An assertion describes what the result of a computation is expected to look like and throws an exception if those expectations aren’t correct. For example, the following assertion states that the result of the computation 7 plus 1 must be 8:

assert.equal(7 + 1, 8);

assert.equal() is a method call (the object is assert, the method is .equal()) with two arguments: the actual result and the expected result. It is part of a Node.js assertion API that is explained later in this book.

There is also assert.deepEqual() that compares objects deeply.

Logging to the console

Logging to the console of a browser or Node.js:

// Printing a value to standard out (another method call)

console.log('Hello!');

// Printing error information to standard error

console.error('Something went wrong!');

Operators

// Operators for booleans

assert.equal(true && false, false); // And

assert.equal(true || false, true); // Or

// Operators for numbers

assert.equal(3 + 4, 7);

assert.equal(5 - 1, 4);

assert.equal(3 * 4, 12);

assert.equal(10 / 4, 2.5);

// Operators for bigints

assert.equal(3n + 4n, 7n);

assert.equal(5n - 1n, 4n);

assert.equal(3n * 4n, 12n);

assert.equal(10n / 4n, 2n);

// Operators for strings

assert.equal('a' + 'b', 'ab');

assert.equal('I see ' + 3 + ' monkeys', 'I see 3 monkeys');

// Comparison operators

assert.equal(3 < 4, true);

assert.equal(3 <= 4, true);

assert.equal('abc' === 'abc', true);

assert.equal('abc' !== 'def', true);

JavaScript also has a == comparison operator. I recommend to avoid it – why is explained in §13.4.3 “Recommendation: always use strict equality”.

Declaring variables

const creates immutable variable bindings: Each variable must be initialized immediately and we can’t assign a different value later. However, the value itself may be mutable and we may be able to change its contents. In other words: const does not make values immutable.

// Declaring and initializing x (immutable binding):

const x = 8;

// Would cause a TypeError:

// x = 9;

let creates mutable variable bindings:

// Declaring y (mutable binding):

let y;

// We can assign a different value to y:

y = 3 * 5;

// Declaring and initializing z:

let z = 3 * 5;

Ordinary function declarations

// add1() has the parameters a and b

function add1(a, b) {

return a + b;

}

// Calling function add1()

assert.equal(add1(5, 2), 7);

Arrow function expressions

Arrow function expressions are used especially as arguments of function calls and method calls:

const add2 = (a, b) => { return a + b };

// Calling function add2()

assert.equal(add2(5, 2), 7);

// Equivalent to add2:

const add3 = (a, b) => a + b;

The previous code contains the following two arrow functions (the terms expression and statement are explained later in this chapter):

// An arrow function whose body is a code block

(a, b) => { return a + b }

// An arrow function whose body is an expression

(a, b) => a + b

Plain objects

// Creating a plain object via an object literal

const obj = {

first: 'Jane', // property

last: 'Doe', // property

getFullName() { // property (method)

return this.first + ' ' + this.last;

},

};

// Getting a property value

assert.equal(obj.first, 'Jane');

// Setting a property value

obj.first = 'Janey';

// Calling the method

assert.equal(obj.getFullName(), 'Janey Doe');

Arrays

// Creating an Array via an Array literal

const arr = ['a', 'b', 'c'];

assert.equal(arr.length, 3);

// Getting an Array element

assert.equal(arr[1], 'b');

// Setting an Array element

arr[1] = 'β';

// Adding an element to an Array:

arr.push('d');

assert.deepEqual(

arr, ['a', 'β', 'c', 'd']);

Control flow statements

Conditional statement:

if (x < 0) {

x = -x;

}

for-of loop:

const arr = ['a', 'b'];

for (const element of arr) {

console.log(element);

}

// Output:

// 'a'

// 'b'

Modules

Each module is a single file. Consider, for example, the following two files with modules in them:

file-tools.mjs

main.mjs

The module in file-tools.mjs exports its function isTextFilePath():

export function isTextFilePath(filePath) {

return filePath.endsWith('.txt');

}

The module in main.mjs imports the whole module path and the function isTextFilePath():

// Import whole module as namespace object `path`

import * as path from 'path';

// Import a single export of module file-tools.mjs

import {isTextFilePath} from './file-tools.mjs';

Classes

class Person {

constructor(name) {

this.name = name;

}

describe() {

return `Person named ${this.name}`;

}

static logNames(persons) {

for (const person of persons) {

console.log(person.name);

}

}

}

class Employee extends Person {

constructor(name, title) {

super(name);

this.title = title;

}

describe() {

return super.describe() +

` (${this.title})`;

}

}

const jane = new Employee('Jane', 'CTO');

assert.equal(

jane.describe(),

'Person named Jane (CTO)');

Exception handling

function throwsException() {

throw new Error('Problem!');

}

function catchesException() {

try {

throwsException();

} catch (err) {

assert.ok(err instanceof Error);

assert.equal(err.message, 'Problem!');

}

}

Note:

- try-finally and try-catch-finally are also supported.

- We can throw any value, but features such as stack traces are only supported by Error and its subclasses.

Legal variable and property names

The grammatical category of variable names and property names is called identifier.

Identifiers are allowed to have the following characters:

- Unicode letters: A–Z, a–z (etc.)

- $, _

- Unicode digits: 0–9 (etc.)

- Variable names can’t start with a digit

Some words have special meaning in JavaScript and are called reserved. Examples include: if, true, const.

Reserved words can’t be used as variable names:

const if = 123;

// SyntaxError: Unexpected token if

But they are allowed as names of properties:

> const obj = { if: 123 };

> obj.if

123

Casing styles

Common casing styles for concatenating words are:

- Camel case: threeConcatenatedWords

- Underscore case (also called snake case): three_concatenated_words

- Dash case (also called kebab case): three-concatenated-words

Capitalization of names

In general, JavaScript uses camel case, except for constants.

Lowercase:

- Functions, variables: myFunction

- Methods: obj.myMethod

- CSS:

- CSS entity: special-class

- Corresponding JavaScript variable: specialClass

Uppercase:

- Classes: MyClass

- Constants: MY_CONSTANT

- Constants are also often written in camel case: myConstant

More naming conventions

The following naming conventions are popular in JavaScript.

If the name of a parameter starts with an underscore (or is an underscore) it means that this parameter is not used – for example:

arr.map((_x, i) => i)

If the name of a property of an object starts with an underscore then that property is considered private:

class ValueWrapper {

constructor(value) {

this._value = value;

}

}

Where to put semicolons?

At the end of a statement:

const x = 123;

func();

But not if that statement ends with a curly brace:

while (false) {

// ···

} // no semicolon

function func() {

// ···

} // no semicolon

However, adding a semicolon after such a statement is not a syntax error – it is interpreted as an empty statement:

// Function declaration followed by empty statement:

function func() {

// ···

};

Quiz: basic

See quiz app.

(Advanced)

All remaining sections of this chapter are advanced.

Identifiers

Valid identifiers (variable names, etc.)

First character:

- Unicode letter (including accented characters such as é and ü and characters from non-latin alphabets, such as α)

- $

- _

Subsequent characters:

- Legal first characters

- Unicode digits (including Eastern Arabic numerals)

- Some other Unicode marks and punctuations

Examples:

const ε = 0.0001;

const строка = '';

let _tmp = 0;

const $foo2 = true;

Reserved words

Reserved words can’t be variable names, but they can be property names.

All JavaScript keywords are reserved words:

await break case catch class const continue debugger default delete do else export extends finally for function if import in instanceof let new return static super switch this throw try typeof var void while with yield

The following tokens are also keywords, but currently not used in the language:

enum implements package protected interface private public

The following literals are reserved words:

true false null

Technically, these words are not reserved, but you should avoid them, too, because they effectively are keywords:

Infinity NaN undefined async

You shouldn’t use the names of global variables (String, Math, etc.) for your own variables and parameters, either.

Statement vs. expression

In this section, we explore how JavaScript distinguishes two kinds of syntactic constructs: statements and expressions. Afterward, we’ll see that that can cause problems because the same syntax can mean different things, depending on where it is used.

We pretend there are only statements and expressions

For the sake of simplicity, we pretend that there are only statements and expressions in JavaScript.

Statements

A statement is a piece of code that can be executed and performs some kind of action. For example, if is a statement:

let myStr;

if (myBool) {

myStr = 'Yes';

} else {

myStr = 'No';

}

One more example of a statement: a function declaration.

function twice(x) {

return x + x;

}

Expressions

An expression is a piece of code that can be evaluated to produce a value. For example, the code between the parentheses is an expression:

let myStr = (myBool ? 'Yes' : 'No');

The operator _?_:_ used between the parentheses is called the ternary operator. It is the expression version of the if statement.

Let’s look at more examples of expressions. We enter expressions and the REPL evaluates them for us:

> 'ab' + 'cd'

'abcd'

> Number('123')

123

> true || false

true

What is allowed where?

The current location within JavaScript source code determines which kind of syntactic constructs you are allowed to use:

- The body of a function must be a sequence of statements:

function max(x, y) {

if (x > y) {

return x;

} else {

return y;

}

}

- The arguments of a function call or a method call must be expressions:

console.log('ab' + 'cd', Number('123'));

However, expressions can be used as statements. Then they are called expression statements. The opposite is not true: when the context requires an expression, you can’t use a statement.

The following code demonstrates that any expression bar() can be either expression or statement – it depends on the context:

function f() {

console.log(bar()); // bar() is expression

bar(); // bar(); is (expression) statement

}

Ambiguous syntax

JavaScript has several programming constructs that are syntactically ambiguous: the same syntax is interpreted differently, depending on whether it is used in statement context or in expression context. This section explores the phenomenon and the pitfalls it causes.

Same syntax: function declaration and function expression

A function declaration is a statement:

function id(x) {

return x;

}

A function expression is an expression (right-hand side of =):

const id = function me(x) {

return x;

};

Same syntax: object literal and block

In the following code, {} is an object literal: an expression that creates an empty object.

const obj = {};

This is an empty code block (a statement):

{

}

Disambiguation

The ambiguities are only a problem in statement context: If the JavaScript parser encounters ambiguous syntax, it doesn’t know if it’s a plain statement or an expression statement. For example:

- If a statement starts with function: Is it a function declaration or a function expression?

- If a statement starts with {: Is it an object literal or a code block?

To resolve the ambiguity, statements starting with function or { are never interpreted as expressions. If you want an expression statement to start with either one of these tokens, you must wrap it in parentheses:

(function (x) { console.log(x) })('abc');

// Output:

// 'abc'

In this code:

1. We first create a function via a function expression:

function (x) { console.log(x) }

2. Then we invoke that function: ('abc')

The code fragment shown in (1) is only interpreted as an expression because we wrap it in parentheses. If we didn’t, we would get a syntax error because then JavaScript expects a function declaration and complains about the missing function name. Additionally, you can’t put a function call immediately after a function declaration.

Later in this book, we’ll see more examples of pitfalls caused by syntactic ambiguity:

- Assigning via object destructuring

- Returning an object literal from an arrow function

Semicolons

Rule of thumb for semicolons

Each statement is terminated by a semicolon:

const x = 3;

someFunction('abc');

i++;

except statements ending with blocks:

function foo() {

// ···

}

if (y > 0) {

// ···

}

The following case is slightly tricky:

const func = () => {}; // semicolon!

The whole const declaration (a statement) ends with a semicolon, but inside it, there is an arrow function expression. That is, it’s not the statement per se that ends with a curly brace; it’s the embedded arrow function expression. That’s why there is a semicolon at the end.

Semicolons: control statements

The body of a control statement is itself a statement. For example, this is the syntax of the while loop:

while (condition)

statement

The body can be a single statement:

while (a > 0) a--;

But blocks are also statements and therefore legal bodies of control statements:

while (a > 0) {

a--;

}

If you want a loop to have an empty body, your first option is an empty statement (which is just a semicolon):

while (processNextItem() > 0);

Your second option is an empty block:

while (processNextItem() > 0) {}

Automatic semicolon insertion (ASI)

While I recommend to always write semicolons, most of them are optional in JavaScript. The mechanism that makes this possible is called automatic semicolon insertion (ASI). In a way, it corrects syntax errors.

ASI works as follows. Parsing of a statement continues until there is either:

- A semicolon

- A line terminator followed by an illegal token

In other words, ASI can be seen as inserting semicolons at line breaks. The next subsections cover the pitfalls of ASI.

ASI triggered unexpectedly

The good news about ASI is that – if you don’t rely on it and always write semicolons – there is only one pitfall that you need to be aware of. It is that JavaScript forbids line breaks after some tokens. If you do insert a line break, a semicolon will be inserted, too.

The token where this is most practically relevant is return. Consider, for example, the following code:

return

{

first: 'jane'

};

This code is parsed as:

return;

{

first: 'jane';

}

;

That is:

- Return statement without operand: return;

- Start of code block: {

- Expression statement 'jane'; with label first:

- End of code block: }

- Empty statement: ;

Why does JavaScript do this? It protects against accidentally returning a value in a line after a return.

ASI unexpectedly not triggered

In some cases, ASI is not triggered when you think it should be. That makes life more complicated for people who don’t like semicolons because they need to be aware of those cases. The following are three examples. There are more.

Example 1: Unintended function call.

a = b + c

(d + e).print()

Parsed as:

a = b + c(d + e).print();

Example 2: Unintended division.

a = b

/hi/g.exec(c).map(d)

Parsed as:

a = b / hi / g.exec(c).map(d);

Example 3: Unintended property access.

someFunction()

['ul', 'ol'].map(x => x + x)

Executed as:

const propKey = ('ul','ol'); // comma operator

assert.equal(propKey, 'ol');

someFunction()[propKey].map(x => x + x);

Semicolons: best practices

I recommend that you always write semicolons:

- I like the visual structure it gives code – you clearly see where a statement ends.

- There are less rules to keep in mind.

- The majority of JavaScript programmers use semicolons.

However, there are also many people who don’t like the added visual clutter of semicolons. If you are one of them: Code without them is legal. I recommend that you use tools to help you avoid mistakes. The following are two examples:

- The automatic code formatter Prettier can be configured to not use semicolons. It then automatically fixes problems. For example, if it encounters a line that starts with a square bracket, it prefixes that line with a semicolon.

- The static checker ESLint has a rule that you tell your preferred style (always semicolons or as few semicolons as possible) and that warns you about critical issues.

Strict mode vs. sloppy mode

Starting with ECMAScript 5, JavaScript has two modes in which JavaScript can be executed:

- Normal “sloppy” mode is the default in scripts (code fragments that are a precursor to modules and supported by browsers).

- Strict mode is the default in modules and classes, and can be switched on in scripts (how is explained later). In this mode, several pitfalls of normal mode are removed and more exceptions are thrown.

You’ll rarely encounter sloppy mode in modern JavaScript code, which is almost always located in modules. In this book, I assume that strict mode is always switched on.

Switching on strict mode

In script files and CommonJS modules, you switch on strict mode for a complete file, by putting the following code in the first line:

'use strict';

The neat thing about this “directive” is that ECMAScript versions before 5 simply ignore it: it’s an expression statement that does nothing.

You can also switch on strict mode for just a single function:

function functionInStrictMode() {

'use strict';

}

Improvements in strict mode

Let’s look at three things that strict mode does better than sloppy mode. Just in this one section, all code fragments are executed in sloppy mode.

Sloppy mode pitfall: changing an undeclared variable creates a global variable

In non-strict mode, changing an undeclared variable creates a global variable.

function sloppyFunc() {

undeclaredVar1 = 123;

}

sloppyFunc();

// Created global variable `undeclaredVar1`:

assert.equal(undeclaredVar1, 123);

Strict mode does it better and throws a ReferenceError. That makes it easier to detect typos.

function strictFunc() {

'use strict';

undeclaredVar2 = 123;

}

assert.throws(

() => strictFunc(),

{

name: 'ReferenceError',

message: 'undeclaredVar2 is not defined',

});

The assert.throws() states that its first argument, a function, throws a ReferenceError when it is called.

Function declarations are block-scoped in strict mode, function-scoped in sloppy mode

In strict mode, a variable created via a function declaration only exists within the innermost enclosing block:

function strictFunc() {

'use strict';

{

function foo() { return 123 }

}

return foo(); // ReferenceError

}

assert.throws(

() => strictFunc(),

{

name: 'ReferenceError',

message: 'foo is not defined',

});

In sloppy mode, function declarations are function-scoped:

function sloppyFunc() {

{

function foo() { return 123 }

}

return foo(); // works

}

assert.equal(sloppyFunc(), 123);

Sloppy mode doesn’t throw exceptions when changing immutable data

In strict mode, you get an exception if you try to change immutable data:

function strictFunc() {

'use strict';

true.prop = 1; // TypeError

}

assert.throws(

() => strictFunc(),

{

name: 'TypeError',

message: "Cannot create property 'prop' on boolean 'true'",

});

In sloppy mode, the assignment fails silently:

function sloppyFunc() {

true.prop = 1; // fails silently

return true.prop;

}

assert.equal(sloppyFunc(), undefined);

Further reading: sloppy mode

For more information on how sloppy mode differs from strict mode, see MDN.

Quiz: advanced

See quiz app.

Consoles: interactive JavaScript command lines

- 8.1 Trying out JavaScript code

- 8.1.1 Browser consoles

- 8.1.2 The Node.js REPL

- 8.1.3 Other options

- 8.2 The console.* API: printing data and more

- 8.2.1 Printing values: console.log() (stdout)

- 8.2.2 Printing error information: console.error() (stderr)

- 8.2.3 Printing nested objects via JSON.stringify()

Trying out JavaScript code

You have many options for quickly running pieces of JavaScript code. The following subsections describe a few of them.

Browser consoles

Web browsers have so-called consoles: interactive command lines to which you can print text via console.log() and where you can run pieces of code. How to open the console differs from browser to browser. Fig. 3 shows the console of Google Chrome.

To find out how to open the console in your web browser, you can do a web search for “console «name-of-your-browser»”. These are pages for a few commonly used web browsers:

- Apple Safari

- Google Chrome

- Microsoft Edge

- Mozilla Firefox

Figure 3: The console of the web browser “Google Chrome” is open (in the bottom half of window) while visiting a web page.

The Node.js REPL

REPL stands for read-eval-print loop and basically means command line. To use it, you must first start Node.js from an operating system command line, via the command node. Then an interaction with it looks as depicted in fig. 4: The text after > is input from the user; everything else is output from Node.js.

Figure 4: Starting and using the Node.js REPL (interactive command line).

Reading: REPL interactions

I occasionally demonstrate JavaScript via REPL interactions. Then I also use greater-than symbols (>) to mark input – for example:

> 3 + 5

8

Other options

Other options include:

- There are many web apps that let you experiment with JavaScript in web browsers – for example, Babel’s REPL.

- There are also native apps and IDE plugins for running JavaScript.

Consoles often run in non-strict mode

In modern JavaScript, most code (e.g., modules) is executed in strict mode. However, consoles often run in non-strict mode. Therefore, you may occasionally get slightly different results when using a console to execute code from this book.

The console.* API: printing data and more

In browsers, the console is something you can bring up that is normally hidden. For Node.js, the console is the terminal that Node.js is currently running in.

The full console.* API is documented on MDN web docs and on the Node.js website. It is not part of the JavaScript language standard, but much functionality is supported by both browsers and Node.js.

In this chapter, we only look at the following two methods for printing data (“printing” means displaying in the console):

- console.log()

- console.error()

Printing values: console.log() (stdout)

There are two variants of this operation:

console.log(...values: any[]): void

console.log(pattern: string, ...values: any[]): void

Printing multiple values

The first variant prints (text representations of) values on the console:

console.log('abc', 123, true);

// Output:

// abc 123 true

At the end, console.log() always prints a newline. Therefore, if you call it with zero arguments, it just prints a newline.

Printing a string with substitutions

The second variant performs string substitution:

console.log('Test: %s %j', 123, 'abc');

// Output:

// Test: 123 "abc"

These are some of the directives you can use for substitutions:

- %s converts the corresponding value to a string and inserts it.

console.log('%s %s', 'abc', 123);

// Output:

// abc 123

- %o inserts a string representation of an object.

console.log('%o', {foo: 123, bar: 'abc'});

// Output:

// { foo: 123, bar: 'abc' }

- %j converts a value to a JSON string and inserts it.

console.log('%j', {foo: 123, bar: 'abc'});

// Output:

// {"foo":123,"bar":"abc"}

- %% inserts a single %.

console.log('%s%%', 99);

// Output:

// 99%

Printing error information: console.error() (stderr)

console.error() works the same as console.log(), but what it logs is considered error information. For Node.js, that means that the output goes to stderr instead of stdout on Unix.

Printing nested objects via JSON.stringify()

JSON.stringify() is occasionally useful for printing nested objects:

console.log(JSON.stringify({first: 'Jane', last: 'Doe'}, null, 2));

Output:

{

"first": "Jane",

"last": "Doe"

}

Assertion API

- 9.1 Assertions in software development

- 9.2 How assertions are used in this book

- 9.2.1 Documenting results in code examples via assertions

- 9.2.2 Implementing test-driven exercises via assertions

- 9.3 Normal comparison vs. deep comparison

- 9.4 Quick reference: module assert

- 9.4.1 Normal equality

- 9.4.2 Deep equality

- 9.4.3 Expecting exceptions

- 9.4.4 Another tool function

Assertions in software development

In software development, assertions state facts about values or pieces of code that must be true. If they aren’t, an exception is thrown. Node.js supports assertions via its built-in module assert – for example:

import * as assert from 'assert/strict';

assert.equal(3 + 5, 8);

This assertion states that the expected result of 3 plus 5 is 8. The import statement uses the recommended strict version of assert.

How assertions are used in this book

In this book, assertions are used in two ways: to document results in code examples and to implement test-driven exercises.

Documenting results in code examples via assertions

In code examples, assertions express expected results. Take, for example, the following function:

function id(x) {

return x;

}

id() returns its parameter. We can show it in action via an assertion:

assert.equal(id('abc'), 'abc');

In the examples, I usually omit the statement for importing assert.

The motivation behind using assertions is:

- You can specify precisely what is expected.

- Code examples can be tested automatically, which ensures that they really work.

Implementing test-driven exercises via assertions

The exercises for this book are test-driven, via the test framework Mocha. Checks inside the tests are made via methods of assert.

The following is an example of such a test:

// For the exercise, you must implement the function hello().

// The test checks if you have done it properly.

test('First exercise', () => {

assert.equal(hello('world'), 'Hello world!');

assert.equal(hello('Jane'), 'Hello Jane!');

assert.equal(hello('John'), 'Hello John!');

assert.equal(hello(''), 'Hello !');

});

For more information, consult §10 “Getting started with quizzes and exercises”.

Normal comparison vs. deep comparison

The strict equal() uses === to compare values. Therefore, an object is only equal to itself – even if another object has the same content (because === does not compare the contents of objects, only their identities):

assert.notEqual({foo: 1}, {foo: 1});

deepEqual() is a better choice for comparing objects:

assert.deepEqual({foo: 1}, {foo: 1});

This method works for Arrays, too:

assert.notEqual(['a', 'b', 'c'], ['a', 'b', 'c']);

assert.deepEqual(['a', 'b', 'c'], ['a', 'b', 'c']);

Quick reference: module assert

For the full documentation, see the Node.js docs.

Normal equality

- function equal(actual: any, expected: any, message?: string): void

actual === expected must be true. If not, an AssertionError is thrown.

assert.equal(3+3, 6);

- function notEqual(actual: any, expected: any, message?: string): void

actual !== expected must be true. If not, an AssertionError is thrown.

assert.notEqual(3+3, 22);

The optional last parameter message can be used to explain what is asserted. If the assertion fails, the message is used to set up the AssertionError that is thrown.

let e;

try {

const x = 3;

assert.equal(x, 8, 'x must be equal to 8')

} catch (err) {

assert.equal(

String(err),

'AssertionError [ERR_ASSERTION]: x must be equal to 8');

}

Deep equality

- function deepEqual(actual: any, expected: any, message?: string): void

actual must be deeply equal to expected. If not, an AssertionError is thrown.

assert.deepEqual([1,2,3], [1,2,3]);

assert.deepEqual([], []);

// To .equal(), an object is only equal to itself:

assert.notEqual([], []);

- function notDeepEqual(actual: any, expected: any, message?: string): void

actual must not be deeply equal to expected. If it is, an AssertionError is thrown.

assert.notDeepEqual([1,2,3], [1,2]);

Expecting exceptions

If you want to (or expect to) receive an exception, you need throws(): This function calls its first parameter, the function block, and only succeeds if it throws an exception. Additional parameters can be used to specify what that exception must look like.

- function throws(block: Function, message?: string): void

assert.throws(

() => {

null.prop;

}

);

- function throws(block: Function, error: Function, message?: string): void

assert.throws(

() => {

null.prop;

},

TypeError

);

- function throws(block: Function, error: RegExp, message?: string): void

assert.throws(

() => {

null.prop;

},

/^TypeError: Cannot read properties of null \(reading 'prop'\)$/

);

- function throws(block: Function, error: Object, message?: string): void

assert.throws(

() => {

null.prop;

},

{

name: 'TypeError',

message: "Cannot read properties of null (reading 'prop')",

}

);

Another tool function

- function fail(message: string | Error): never

Always throws an AssertionError when it is called. That is occasionally useful for unit testing.

try {

functionThatShouldThrow();

assert.fail();

} catch (_) {

// Success

}

Quiz

See quiz app.

Getting started with quizzes and exercises

- 10.1 Quizzes

- 10.2 Exercises

- 10.2.1 Installing the exercises

- 10.2.2 Running exercises

- 10.3 Unit tests in JavaScript

- 10.3.1 A typical test

- 10.3.2 Asynchronous tests in Mocha

Throughout most chapters, there are quizzes and exercises. These are a paid feature, but a comprehensive preview is available. This chapter explains how to get started with them.

Quizzes

Installation:

- Download and unzip impatient-js-quiz.zip

Running the quiz app:

- Open impatient-js-quiz/index.html in a web browser

- You’ll see a TOC of all the quizzes.

Exercises

Installing the exercises

To install the exercises:

- Download and unzip impatient-js-code.zip

- Follow the instructions in README.txt

Running exercises

- Exercises are referred to by path in this book.

- For example: exercises/quizzes-exercises/first_module_test.mjs

- Within each file:

- The first line contains the command for running the exercise.

- The following lines describe what you have to do.

Unit tests in JavaScript

All exercises in this book are tests that are run via the test framework Mocha. This section gives a brief introduction.

A typical test

Typical test code is split into two parts:

- Part 1: the code to be tested.

- Part 2: the tests for the code.

Take, for example, the following two files:

- id.mjs (code to be tested)

- id_test.mjs (tests)

Part 1: the code

The code itself resides in id.mjs:

export function id(x) {

return x;

}

The key thing here is: everything we want to test must be exported. Otherwise, the test code can’t access it.

Part 2: the tests

Don’t worry about the exact details of tests

You don’t need to worry about the exact details of tests: They are always implemented for you. Therefore, you only need to read them, but not write them.

The tests for the code reside in id_test.mjs:

// npm t demos/quizzes-exercises/id_test.mjs

suite('id_test.mjs');

import * as assert from 'assert/strict'; // (A)

import {id} from './id.mjs'; // (B)

test('My test', () => { // (C)

assert.equal(id('abc'), 'abc'); // (D)

});

The core of this test file is line D – an assertion: assert.equal() specifies that the expected result of id('abc') is 'abc'.

As for the other lines:

- The comment at the very beginning shows the shell command for running the test.

- Line A: We import the Node.js assertion library (in strict assertion mode).

- Line B: We import the function to test.

- Line C: We define a test. This is done by calling the function test():

- First parameter: the name of the test.

- Second parameter: the test code, which is provided via an arrow function. The parameter t gives us access to AVA’s testing API (assertions, etc.).

To run the test, we execute the following in a command line:

npm t demos/quizzes-exercises/id_test.mjs

The t is an abbreviation for test. That is, the long version of this command is:

npm test demos/quizzes-exercises/id_test.mjs

Exercise: Your first exercise

The following exercise gives you a first taste of what exercises are like:

- exercises/quizzes-exercises/first_module_test.mjs

Asynchronous tests in Mocha

Reading

You may want to postpone reading this section until you get to the chapters on asynchronous programming.

Writing tests for asynchronous code requires extra work: The test receives its results later and has to signal to Mocha that it isn’t finished yet when it returns. The following subsections examine three ways of doing so.

Asynchronicity via callbacks

If the callback we pass to test() has a parameter (e.g., done), Mocha switches to callback-based asynchronicity. When we are done with our asynchronous work, we have to call done:

test('divideCallback', (done) => {

divideCallback(8, 4, (error, result) => {

if (error) {

done(error);

} else {

assert.strictEqual(result, 2);

done();

}

});

});

This is what divideCallback() looks like:

function divideCallback(x, y, callback) {

if (y === 0) {

callback(new Error('Division by zero'));

} else {

callback(null, x / y);

}

}

Asynchronicity via Promises

If a test returns a Promise, Mocha switches to Promise-based asynchronicity. A test is considered successful if the Promise is fulfilled and failed if the Promise is rejected or if a settlement takes longer than a timeout.

test('dividePromise 1', () => {

return dividePromise(8, 4)

.then(result => {

assert.strictEqual(result, 2);

});

});

dividePromise() is implemented as follows:

function dividePromise(x, y) {

return new Promise((resolve, reject) => {

if (y === 0) {

reject(new Error('Division by zero'));

} else {

resolve(x / y);

}

});

}

Async functions as test “bodies”

Async functions always return Promises. Therefore, an async function is a convenient way of implementing an asynchronous test. The following code is equivalent to the previous example.

test('dividePromise 2', async () => {

const result = await dividePromise(8, 4);

assert.strictEqual(result, 2);

// No explicit return necessary!

});

We don’t need to explicitly return anything: The implicitly returned undefined is used to fulfill the Promise returned by this async function. And if the test code throws an exception, then the async function takes care of rejecting the returned Promise.

Variables and assignment

- 11.1 let

- 11.2 const

- 11.2.1 const and immutability

- 11.2.2 const and loops

- 11.3 Deciding between const and let

- 11.4 The scope of a variable

- 11.4.1 Shadowing variables

- 11.5 (Advanced)

- 11.6 Terminology: static vs. dynamic

- 11.6.1 Static phenomenon: scopes of variables

- 11.6.2 Dynamic phenomenon: function calls

- 11.7 Global variables and the global object

- 11.7.1 globalThis [ES2020]

- 11.8 Declarations: scope and activation

- 11.8.1 const and let: temporal dead zone

- 11.8.2 Function declarations and early activation

- 11.8.3 Class declarations are not activated early

- 11.8.4 var: hoisting (partial early activation)

- 11.9 Closures

- 11.9.1 Bound variables vs. free variables

- 11.9.2 What is a closure?

- 11.9.3 Example: A factory for incrementors

- 11.9.4 Use cases for closures

These are JavaScript’s main ways of declaring variables:

- let declares mutable variables.

- const declares constants (immutable variables).

Before ES6, there was also var. But it has several quirks, so it’s best to avoid it in modern JavaScript. You can read more about it in Speaking JavaScript.

let

Variables declared via let are mutable:

let i;

i = 0;

i = i + 1;

assert.equal(i, 1);

You can also declare and assign at the same time:

let i = 0;

const

Variables declared via const are immutable. You must always initialize immediately:

const i = 0; // must initialize

assert.throws(

() => { i = i + 1 },

{

name: 'TypeError',

message: 'Assignment to constant variable.',

}

);

const and immutability

In JavaScript, const only means that the binding (the association between variable name and variable value) is immutable. The value itself may be mutable, like obj in the following example.

const obj = { prop: 0 };

// Allowed: changing properties of `obj`

obj.prop = obj.prop + 1;

assert.equal(obj.prop, 1);

// Not allowed: assigning to `obj`

assert.throws(

() => { obj = {} },

{

name: 'TypeError',

message: 'Assignment to constant variable.',

}

);

const and loops

You can use const with for-of loops, where a fresh binding is created for each iteration:

const arr = ['hello', 'world'];

for (const elem of arr) {

console.log(elem);

}

// Output:

// 'hello'

// 'world'

In plain for loops, you must use let, however:

const arr = ['hello', 'world'];

for (let i=0; i<arr.length; i++) {

const elem = arr[i];

console.log(elem);

}

Deciding between const and let

I recommend the following rules to decide between const and let:

- const indicates an immutable binding and that a variable never changes its value. Prefer it.

- let indicates that the value of a variable changes. Use it only when you can’t use const.

Exercise: const

exercises/variables-assignment/const_exrc.mjs

The scope of a variable

The scope of a variable is the region of a program where it can be accessed. Consider the following code.

{ // // Scope A. Accessible: x

const x = 0;

assert.equal(x, 0);

{ // Scope B. Accessible: x, y

const y = 1;

assert.equal(x, 0);

assert.equal(y, 1);

{ // Scope C. Accessible: x, y, z

const z = 2;

assert.equal(x, 0);

assert.equal(y, 1);

assert.equal(z, 2);

}

}

}

// Outside. Not accessible: x, y, z

assert.throws(

() => console.log(x),

{

name: 'ReferenceError',

message: 'x is not defined',

}

);

- Scope A is the (direct) scope of x.

- Scopes B and C are inner scopes of scope A.

- Scope A is an outer scope of scope B and scope C.

Each variable is accessible in its direct scope and all scopes nested within that scope.

The variables declared via const and let are called block-scoped because their scopes are always the innermost surrounding blocks.

Shadowing variables

You can’t declare the same variable twice at the same level:

assert.throws(

() => {

eval('let x = 1; let x = 2;');

},

{

name: 'SyntaxError',

message: "Identifier 'x' has already been declared",

});

Why eval()?

eval() delays parsing (and therefore the SyntaxError), until the callback of assert.throws() is executed. If we didn’t use it, we’d already get an error when this code is parsed and assert.throws() wouldn’t even be executed.

You can, however, nest a block and use the same variable name x that you used outside the block:

const x = 1;

assert.equal(x, 1);

{

const x = 2;

assert.equal(x, 2);

}

assert.equal(x, 1);

Inside the block, the inner x is the only accessible variable with that name. The inner x is said to shadow the outer x. Once you leave the block, you can access the old value again.

Quiz: basic

See quiz app.

(Advanced)

All remaining sections are advanced.

Terminology: static vs. dynamic

These two adjectives describe phenomena in programming languages:

- Static means that something is related to source code and can be determined without executing code.

- Dynamic means at runtime.

Let’s look at examples for these two terms.

Static phenomenon: scopes of variables

Variable scopes are a static phenomenon. Consider the following code:

function f() {

const x = 3;

// ···

}

x is statically (or lexically) scoped. That is, its scope is fixed and doesn’t change at runtime.

Variable scopes form a static tree (via static nesting).

Dynamic phenomenon: function calls

Function calls are a dynamic phenomenon. Consider the following code:

function g(x) {}

function h(y) {

if (Math.random()) g(y); // (A)

}

Whether or not the function call in line A happens, can only be decided at runtime.

Function calls form a dynamic tree (via dynamic calls).

Global variables and the global object

JavaScript’s variable scopes are nested. They form a tree:

- The outermost scope is the root of the tree.

- The scopes directly contained in that scope are the children of the root.

- And so on.

The root is also called the global scope. In web browsers, the only location where one is directly in that scope is at the top level of a script. The variables of the global scope are called global variables and accessible everywhere. There are two kinds of global variables:

- Global declarative variables are normal variables.

- They can only be created while at the top level of a script, via const, let, and class declarations.

- Global object variables are stored in properties of the so-called global object.

- They are created in the top level of a script, via var and function declarations.

- The global object can be accessed via the global variable globalThis. It can be used to create, read, and delete global object variables.

- Other than that, global object variables work like normal variables.

The following HTML fragment demonstrates globalThis and the two kinds of global variables.

<script>

const declarativeVariable = 'd';

var objectVariable = 'o';

</script>

<script>

// All scripts share the same top-level scope:

console.log(declarativeVariable); // 'd'

console.log(objectVariable); // 'o'

// Not all declarations create properties of the global object:

console.log(globalThis.declarativeVariable); // undefined

console.log(globalThis.objectVariable); // 'o'

</script>

Each ECMAScript module has its own scope. Therefore, variables that exist at the top level of a module are not global. Fig. 5 illustrates how the various scopes are related.

Figure 5: The global scope is JavaScript’s outermost scope. It has two kinds of variables: object variables (managed via the global object) and normal declarative variables. Each ECMAScript module has its own scope which is contained in the global scope.

globalThis [ES2020]

The global variable globalThis is the new standard way of accessing the global object. It got its name from the fact that it has the same value as this in global scope.

globalThis does not always directly point to the global object

For example, in browsers, there is an indirection. That indirection is normally not noticable, but it is there and can be observed.

Alternatives to globalThis

Older ways of accessing the global object depend on the platform:

- Global variable window: is the classic way of referring to the global object. But it doesn’t work in Node.js and in Web Workers.

- Global variable self: is available in Web Workers and browsers in general. But it isn’t supported by Node.js.

- Global variable global: is only available in Node.js.

Use cases for globalThis

The global object is now considered a mistake that JavaScript can’t get rid of, due to backward compatibility. It affects performance negatively and is generally confusing.

ECMAScript 6 introduced several features that make it easier to avoid the global object – for example:

- const, let, and class declarations don’t create global object properties when used in global scope.

- Each ECMAScript module has its own local scope.

It is usually better to access global object variables via variables and not via properties of globalThis. The former has always worked the same on all JavaScript platforms.

Tutorials on the web occasionally access global variables globVar via window.globVar. But the prefix “window.” is not necessary and I recommend to omit it:

window.encodeURIComponent(str); // no

encodeURIComponent(str); // yes

Therefore, there are relatively few use cases for globalThis – for example:

- Polyfills that add new features to old JavaScript engines.

- Feature detection, to find out what features a JavaScript engine supports.

Declarations: scope and activation

These are two key aspects of declarations:

- Scope: Where can a declared entity be seen? This is a static trait.

- Activation: When can I access an entity? This is a dynamic trait. Some entities can be accessed as soon as we enter their scopes. For others, we have to wait until execution reaches their declarations.

Tbl. 1 summarizes how various declarations handle these aspects.

Table 1: Aspects of declarations. “Duplicates” describes if a declaration can be used twice with the same name (per scope). “Global prop.” describes if a declaration adds a property to the global object, when it is executed in the global scope of a script. TDZ means temporal dead zone (which is explained later). (*) Function declarations are normally block-scoped, but function-scoped in sloppy mode.

Scope

Activation

Duplicates

Global prop.

const

Block

decl. (TDZ)

✘

✘

let

Block

decl. (TDZ)

✘

✘

function

Block (*)

start

✔

✔

class

Block

decl. (TDZ)

✘

✘

import

Module

same as export

✘

✘

var

Function

start, partially

✔

✔

import is described in §27.5 “ECMAScript modules”. The following sections describe the other constructs in more detail.

const and let: temporal dead zone

For JavaScript, TC39 needed to decide what happens if you access a constant in its direct scope, before its declaration:

{

console.log(x); // What happens here?

const x;

}

Some possible approaches are:

1. The name is resolved in the scope surrounding the current scope.

2. You get undefined.

3. There is an error.

Approach 1 was rejected because there is no precedent in the language for this approach. It would therefore not be intuitive to JavaScript programmers.

Approach 2 was rejected because then x wouldn’t be a constant – it would have different values before and after its declaration.

let uses the same approach 3 as const, so that both work similarly and it’s easy to switch between them.

The time between entering the scope of a variable and executing its declaration is called the temporal dead zone (TDZ) of that variable:

- During this time, the variable is considered to be uninitialized (as if that were a special value it has).

- If you access an uninitialized variable, you get a ReferenceError.

- Once you reach a variable declaration, the variable is set to either the value of the initializer (specified via the assignment symbol) or undefined – if there is no initializer.

The following code illustrates the temporal dead zone:

if (true) { // entering scope of `tmp`, TDZ starts

// `tmp` is uninitialized:

assert.throws(() => (tmp = 'abc'), ReferenceError);

assert.throws(() => console.log(tmp), ReferenceError);

let tmp; // TDZ ends

assert.equal(tmp, undefined);

}

The next example shows that the temporal dead zone is truly temporal (related to time):

if (true) { // entering scope of `myVar`, TDZ starts

const func = () => {

console.log(myVar); // executed later

};

// We are within the TDZ:

// Accessing `myVar` causes `ReferenceError`

let myVar = 3; // TDZ ends

func(); // OK, called outside TDZ

}

Even though func() is located before the declaration of myVar and uses that variable, we can call func(). But we have to wait until the temporal dead zone of myVar is over.

Function declarations and early activation

More information on functions

In this section, we are using functions – before we had a chance to learn them properly. Hopefully, everything still makes sense. Whenever it doesn’t, please see §25 “Callable values”.

A function declaration is always executed when entering its scope, regardless of where it is located within that scope. That enables you to call a function foo() before it is declared:

assert.equal(foo(), 123); // OK

function foo() { return 123; }

The early activation of foo() means that the previous code is equivalent to:

function foo() { return 123; }

assert.equal(foo(), 123);

If you declare a function via const or let, then it is not activated early. In the following example, you can only use bar() after its declaration.

assert.throws(

() => bar(), // before declaration

ReferenceError);

const bar = () => { return 123; };

assert.equal(bar(), 123); // after declaration

Calling ahead without early activation

Even if a function g() is not activated early, it can be called by a preceding function f() (in the same scope) if we adhere to the following rule: f() must be invoked after the declaration of g().

const f = () => g();

const g = () => 123;

// We call f() after g() was declared:

assert.equal(f(), 123);

The functions of a module are usually invoked after its complete body is executed. Therefore, in modules, you rarely need to worry about the order of functions.

Lastly, note how early activation automatically keeps the aforementioned rule: when entering a scope, all function declarations are executed first, before any calls are made.

A pitfall of early activation

If you rely on early activation to call a function before its declaration, then you need to be careful that it doesn’t access data that isn’t activated early.

funcDecl();

const MY_STR = 'abc';

function funcDecl() {

assert.throws(

() => MY_STR,

ReferenceError);

}

The problem goes away if you make the call to funcDecl() after the declaration of MY_STR.

The pros and cons of early activation

We have seen that early activation has a pitfall and that you can get most of its benefits without using it. Therefore, it is better to avoid early activation. But I don’t feel strongly about this and, as mentioned before, often use function declarations because I like their syntax.

Class declarations are not activated early

Even though they are similar to function declarations in some ways, class declarations are not activated early:

assert.throws(

() => new MyClass(),

ReferenceError);

class MyClass {}

assert.equal(new MyClass() instanceof MyClass, true);

Why is that? Consider the following class declaration:

class MyClass extends Object {}

The operand of extends is an expression. Therefore, you can do things like this:

const identity = x => x;

class MyClass extends identity(Object) {}

Evaluating such an expression must be done at the location where it is mentioned. Anything else would be confusing. That explains why class declarations are not activated early.

var: hoisting (partial early activation)

var is an older way of declaring variables that predates const and let (which are preferred now). Consider the following var declaration.

var x = 123;

This declaration has two parts:

- Declaration var x: The scope of a var-declared variable is the innermost surrounding function and not the innermost surrounding block, as for most other declarations. Such a variable is already active at the beginning of its scope and initialized with undefined.

- Assignment x = 123: The assignment is always executed in place.

The following code demonstrates the effects of var:

function f() {

// Partial early activation:

assert.equal(x, undefined);

if (true) {

var x = 123;

// The assignment is executed in place:

assert.equal(x, 123);

}

// Scope is function, not block:

assert.equal(x, 123);

}

Closures

Before we can explore closures, we need to learn about bound variables and free variables.

Bound variables vs. free variables

Per scope, there is a set of variables that are mentioned. Among these variables we distinguish:

- Bound variables are declared within the scope. They are parameters and local variables.

- Free variables are declared externally. They are also called non-local variables.

Consider the following code:

function func(x) {

const y = 123;

console.log(z);

}

In the body of func(), x and y are bound variables. z is a free variable.

What is a closure?

What is a closure then?

A closure is a function plus a connection to the variables that exist at its “birth place”.

What is the point of keeping this connection? It provides the values for the free variables of the function – for example:

function funcFactory(value) {

return () => {

return value;

};

}

const func = funcFactory('abc');

assert.equal(func(), 'abc'); // (A)

funcFactory returns a closure that is assigned to func. Because func has the connection to the variables at its birth place, it can still access the free variable value when it is called in line A (even though it “escaped” its scope).

All functions in JavaScript are closures

Static scoping is supported via closures in JavaScript. Therefore, every function is a closure.

Example: A factory for incrementors

The following function returns incrementors (a name that I just made up). An incrementor is a function that internally stores a number. When it is called, it updates that number by adding the argument to it and returns the new value.

function createInc(startValue) {

return (step) => { // (A)

startValue += step;

return startValue;

};

}

const inc = createInc(5);

assert.equal(inc(2), 7);

We can see that the function created in line A keeps its internal number in the free variable startValue. This time, we don’t just read from the birth scope, we use it to store data that we change and that persists across function calls.

We can create more storage slots in the birth scope, via local variables:

function createInc(startValue) {

let index = -1;

return (step) => {

startValue += step;

index++;

return [index, startValue];

};

}

const inc = createInc(5);

assert.deepEqual(inc(2), [0, 7]);

assert.deepEqual(inc(2), [1, 9]);

assert.deepEqual(inc(2), [2, 11]);

Use cases for closures

What are closures good for?

- For starters, they are simply an implementation of static scoping. As such, they provide context data for callbacks.

- They can also be used by functions to store state that persists across function calls. createInc() is an example of that.

- And they can provide private data for objects (produced via literals or classes). The details of how that works are explained in Exploring ES6.

Quiz: advanced

See quiz app.

Values

- 12.1 What’s a type?

- 12.2 JavaScript’s type hierarchy

- 12.3 The types of the language specification

- 12.4 Primitive values vs. objects

- 12.4.1 Primitive values (short: primitives)

- 12.4.2 Objects

- 12.5 The operators typeof and instanceof: what’s the type of a value?

- 12.5.1 typeof

- 12.5.2 instanceof

- 12.6 Classes and constructor functions

- 12.6.1 Constructor functions associated with primitive types

- 12.7 Converting between types

- 12.7.1 Explicit conversion between types

- 12.7.2 Coercion (automatic conversion between types)

In this chapter, we’ll examine what kinds of values JavaScript has.

Supporting tool: ===

In this chapter, we’ll occasionally use the strict equality operator. a === b evaluates to true if a and b are equal. What exactly that means is explained in §13.4.2 “Strict equality (=== and !==)”.

What’s a type?

For this chapter, I consider types to be sets of values – for example, the type boolean is the set { false, true }.

JavaScript’s type hierarchy

Figure 6: A partial hierarchy of JavaScript’s types. Missing are the classes for errors, the classes associated with primitive types, and more. The diagram hints at the fact that not all objects are instances of Object.

Fig. 6 shows JavaScript’s type hierarchy. What do we learn from that diagram?

- JavaScript distinguishes two kinds of values: primitive values and objects. We’ll see soon what the difference is.

- The diagram differentiates objects and instances of class Object. Each instance of Object is also an object, but not vice versa. However, virtually all objects that you’ll encounter in practice are instances of Object – for example, objects created via object literals. More details on this topic are explained in §29.7.3 “Not all objects are instances of Object”.

The types of the language specification

The ECMAScript specification only knows a total of eight types. The names of those types are (I’m using TypeScript’s names, not the spec’s names):

- undefined with the only element undefined

- null with the only element null

- boolean with the elements false and true

- number the type of all numbers (e.g., -123, 3.141)

- bigint the type of all big integers (e.g., -123n)

- string the type of all strings (e.g., 'abc')

- symbol the type of all symbols (e.g., Symbol('My Symbol'))

- object the type of all objects (different from Object, the type of all instances of class Object and its subclasses)

Primitive values vs. objects

The specification makes an important distinction between values:

- Primitive values are the elements of the types undefined, null, boolean, number, bigint, string, symbol.

- All other values are objects.

In contrast to Java (that inspired JavaScript here), primitive values are not second-class citizens. The difference between them and objects is more subtle. In a nutshell:

- Primitive values: are atomic building blocks of data in JavaScript.

- They are passed by value: when primitive values are assigned to variables or passed to functions, their contents are copied.

- They are compared by value: when comparing two primitive values, their contents are compared.

- Objects: are compound pieces of data.

- They are passed by identity (my term): when objects are assigned to variables or passed to functions, their identities (think pointers) are copied.

- They are compared by identity (my term): when comparing two objects, their identities are compared.

Other than that, primitive values and objects are quite similar: they both have properties (key-value entries) and can be used in the same locations.

Next, we’ll look at primitive values and objects in more depth.

Primitive values (short: primitives)

Primitives are immutable

You can’t change, add, or remove properties of primitives:

const str = 'abc';

assert.equal(str.length, 3);

assert.throws(

() => { str.length = 1 },

/^TypeError: Cannot assign to read only property 'length'/

);

Primitives are passed by value

Primitives are passed by value: variables (including parameters) store the contents of the primitives. When assigning a primitive value to a variable or passing it as an argument to a function, its content is copied.

const x = 123;

const y = x;

// `y` is the same as any other number 123

assert.equal(y, 123);

Observing the difference between passing by value and passing by reference

Due to primitive values being immutable and compared by value (see next subsection), there is no way to observe the difference between passing by value and passing by identity (as used for objects in JavaScript).

Primitives are compared by value

Primitives are compared by value: when comparing two primitive values, we compare their contents.

assert.equal(123 === 123, true);

assert.equal('abc' === 'abc', true);

To see what’s so special about this way of comparing, read on and find out how objects are compared.

Objects

Objects are covered in detail in §28 “Objects” and the following chapter. Here, we mainly focus on how they differ from primitive values.

Let’s first explore two common ways of creating objects:

- Object literal:

const obj = {

first: 'Jane',

last: 'Doe',

};

The object literal starts and ends with curly braces {}. It creates an object with two properties. The first property has the key 'first' (a string) and the value 'Jane'. The second property has the key 'last' and the value 'Doe'. For more information on object literals, consult §28.3.1 “Object literals: properties”.

- Array literal:

const fruits = ['strawberry', 'apple'];

The Array literal starts and ends with square brackets []. It creates an Array with two elements: 'strawberry' and 'apple'. For more information on Array literals, consult [content not included].

Objects are mutable by default

By default, you can freely change, add, and remove the properties of objects:

const obj = {};

obj.count = 2; // add a property

assert.equal(obj.count, 2);

obj.count = 3; // change a property

assert.equal(obj.count, 3);

Objects are passed by identity

Objects are passed by identity (my term): variables (including parameters) store the identities of objects.

The identity of an object is like a pointer (or a transparent reference) to the object’s actual data on the heap (think shared main memory of a JavaScript engine).

When assigning an object to a variable or passing it as an argument to a function, its identity is copied. Each object literal creates a fresh object on the heap and returns its identity.

const a = {}; // fresh empty object

// Pass the identity in `a` to `b`:

const b = a;

// Now `a` and `b` point to the same object

// (they “share” that object):

assert.equal(a === b, true);

// Changing `a` also changes `b`:

a.name = 'Tessa';

assert.equal(b.name, 'Tessa');

JavaScript uses garbage collection to automatically manage memory:

let obj = { prop: 'value' };

obj = {};

Now the old value { prop: 'value' } of obj is garbage (not used anymore). JavaScript will automatically garbage-collect it (remove it from memory), at some point in time (possibly never if there is enough free memory).

Details: passing by identity

“Passing by identity” means that the identity of an object (a transparent reference) is passed by value. This approach is also called “passing by sharing”.

Objects are compared by identity

Objects are compared by identity (my term): two variables are only equal if they contain the same object identity. They are not equal if they refer to different objects with the same content.

const obj = {}; // fresh empty object

assert.equal(obj === obj, true); // same identity

assert.equal({} === {}, false); // different identities, same content

The operators typeof and instanceof: what’s the type of a value?

The two operators typeof and instanceof let you determine what type a given value x has:

if (typeof x === 'string') ···

if (x instanceof Array) ···

How do they differ?

- typeof distinguishes the 7 types of the specification (minus one omission, plus one addition).

- instanceof tests which class created a given value.

Rule of thumb: typeof is for primitive values; instanceof is for objects

typeof

Table 2: The results of the typeof operator.

x

typeof x

undefined

'undefined'

null

'object'

Boolean

'boolean'

Number

'number'

Bigint

'bigint'

String

'string'

Symbol

'symbol'

Function

'function'

All other objects

'object'

Tbl. 2 lists all results of typeof. They roughly correspond to the 7 types of the language specification. Alas, there are two differences, and they are language quirks:

- typeof null returns 'object' and not 'null'. That’s a bug. Unfortunately, it can’t be fixed. TC39 tried to do that, but it broke too much code on the web.

- typeof of a function should be 'object' (functions are objects). Introducing a separate category for functions is confusing.

These are a few examples of using typeof:

> typeof undefined

'undefined'

> typeof 123n

'bigint'

> typeof 'abc'

'string'

> typeof {}

'object'

Exercises: Two exercises on typeof

- exercises/values/typeof_exrc.mjs

- Bonus: exercises/values/is_object_test.mjs

instanceof

This operator answers the question: has a value x been created by a class C?

x instanceof C

For example:

> (function() {}) instanceof Function

true

> ({}) instanceof Object

true

> [] instanceof Array

true

Primitive values are not instances of anything:

> 123 instanceof Number

false

> '' instanceof String

false

> '' instanceof Object

false

Exercise: instanceof

exercises/values/instanceof_exrc.mjs

Classes and constructor functions

JavaScript’s original factories for objects are constructor functions: ordinary functions that return “instances” of themselves if you invoke them via the new operator.

ES6 introduced classes, which are mainly better syntax for constructor functions.

In this book, I’m using the terms constructor function and class interchangeably.

Classes can be seen as partitioning the single type object of the specification into subtypes – they give us more types than the limited 7 ones of the specification. Each class is the type of the objects that were created by it.

Constructor functions associated with primitive types

Each primitive type (except for the spec-internal types for undefined and null) has an associated constructor function (think class):

- The constructor function Boolean is associated with booleans.

- The constructor function Number is associated with numbers.

- The constructor function String is associated with strings.

- The constructor function Symbol is associated with symbols.

Each of these functions plays several roles – for example, Number:

- You can use it as a function and convert values to numbers:

assert.equal(Number('123'), 123);

- Number.prototype provides the properties for numbers – for example, method .toString():

assert.equal((123).toString, Number.prototype.toString);

- Number is a namespace/container object for tool functions for numbers – for example:

assert.equal(Number.isInteger(123), true);

- Lastly, you can also use Number as a class and create number objects. These objects are different from real numbers and should be avoided.

assert.notEqual(new Number(123), 123);

assert.equal(new Number(123).valueOf(), 123);

Wrapping primitive values

The constructor functions related to primitive types are also called wrapper types because they provide the canonical way of converting primitive values to objects. In the process, primitive values are “wrapped” in objects.

const prim = true;

assert.equal(typeof prim, 'boolean');

assert.equal(prim instanceof Boolean, false);

const wrapped = Object(prim);

assert.equal(typeof wrapped, 'object');

assert.equal(wrapped instanceof Boolean, true);

assert.equal(wrapped.valueOf(), prim); // unwrap

Wrapping rarely matters in practice, but it is used internally in the language specification, to give primitives properties.

Converting between types

There are two ways in which values are converted to other types in JavaScript:

- Explicit conversion: via functions such as String().

- Coercion (automatic conversion): happens when an operation receives operands/parameters that it can’t work with.

Explicit conversion between types

The function associated with a primitive type explicitly converts values to that type:

> Boolean(0)

false

> Number('123')

123

> String(123)

'123'

You can also use Object() to convert values to objects:

> typeof Object(123)

'object'

The following table describes in more detail how this conversion works:

x

Object(x)

undefined

{}

null

{}

boolean

new Boolean(x)

number

new Number(x)

bigint

An instance of BigInt (new throws TypeError  )

string

new String(x)

symbol

An instance of Symbol (new throws TypeError  )

object

x

Coercion (automatic conversion between types)

For many operations, JavaScript automatically converts the operands/parameters if their types don’t fit. This kind of automatic conversion is called coercion.

For example, the multiplication operator coerces its operands to numbers:

> '7' * '3'

21

Many built-in functions coerce, too. For example, Number.parseInt() coerces its parameter to a string before parsing it. That explains the following result:

> Number.parseInt(123.45)

123

The number 123.45 is converted to the string '123.45' before it is parsed. Parsing stops before the first non-digit character, which is why the result is 123.

Exercise: Converting values to primitives

exercises/values/conversion_exrc.mjs

Quiz

See quiz app.

Operators

- 13.1 Making sense of operators

- 13.1.1 Operators coerce their operands to appropriate types

- 13.1.2 Most operators only work with primitive values

- 13.2 The plus operator (+)

- 13.3 Assignment operators

- 13.3.1 The plain assignment operator

- 13.3.2 Compound assignment operators

- 13.4 Equality: == vs. ===

- 13.4.1 Loose equality (== and !=)

- 13.4.2 Strict equality (=== and !==)

- 13.4.3 Recommendation: always use strict equality

- 13.4.4 Even stricter than ===: Object.is()

- 13.5 Ordering operators

- 13.6 Various other operators

- 13.6.1 Comma operator

- 13.6.2 void operator

Making sense of operators

JavaScript’s operators may seem quirky. With the following two rules, they are easier to understand:

- Operators coerce their operands to appropriate types

- Most operators only work with primitive values

Operators coerce their operands to appropriate types

If an operator gets operands that don’t have the proper types, it rarely throws an exception. Instead, it coerces (automatically converts) the operands so that it can work with them. Let’s look at two examples.

First, the multiplication operator can only work with numbers. Therefore, it converts strings to numbers before computing its result.

> '7' * '3'

21

Second, the square brackets operator ([ ]) for accessing the properties of an object can only handle strings and symbols. All other values are coerced to string:

const obj = {};

obj['true'] = 123;

// Coerce true to the string 'true'

assert.equal(obj[true], 123);

Most operators only work with primitive values

As mentioned before, most operators only work with primitive values. If an operand is an object, it is usually coerced to a primitive value – for example:

> [1,2,3] + [4,5,6]

'1,2,34,5,6'

Why? The plus operator first coerces its operands to primitive values:

> String([1,2,3])

'1,2,3'

> String([4,5,6])

'4,5,6'

Next, it concatenates the two strings:

> '1,2,3' + '4,5,6'

'1,2,34,5,6'

The plus operator (+)

The plus operator works as follows in JavaScript:

- First, it converts both operands to primitive values. Then it switches to one of two modes:

- String mode: If one of the two primitive values is a string, then it converts the other one to a string, concatenates both strings, and returns the result.

- Number mode: Otherwise, It converts both operands to numbers, adds them, and returns the result.

String mode lets us use + to assemble strings:

> 'There are ' + 3 + ' items'

'There are 3 items'

Number mode means that if neither operand is a string (or an object that becomes a string) then everything is coerced to numbers:

> 4 + true

5

Number(true) is 1.

Assignment operators

The plain assignment operator

The plain assignment operator is used to change storage locations:

x = value; // assign to a previously declared variable

obj.propKey = value; // assign to a property

arr[index] = value; // assign to an Array element

Initializers in variable declarations can also be viewed as a form of assignment:

const x = value;

let y = value;

Compound assignment operators

JavaScript supports the following assignment operators:

- Arithmetic assignment operators: += -= *= /= %= [ES1]

- += can also be used for string concatenation

- Introduced later: **= [ES2016]

- Bitwise assignment operators: &= ^= |= [ES1]

- Bitwise shift assignment operators: <<= >>= >>>= [ES1]

- Logical assignment operators: ||= &&= ??= [ES2021]

Logical assignment operators [ES2021]

Logical assignment operators work differently from other compound assignment operators:

Assignment operator

Equivalent to

Only assigns if a   is

a ||= b

a || (a = b)

Falsy

a &&= b

a && (a = b)

Truthy

a ??= b

a ?? (a = b)

Nullish

Why is a ||= b equivalent to the following expression?

a || (a = b)

Why not to this expression?

a = a || b

The former expression has the benefit of short-circuiting: The assignment is only evaluated if a evaluates to false. Therefore, the assignment is only performed if it’s necessary. In contrast, the latter expression always performs an assignment.

For more on ??=, see §14.4.5 “The nullish coalescing assignment operator (??=) [ES2021]”.

The remaining compound assignment operators

For operators op other than || && ??, the following two ways of assigning are equivalent:

myvar op= value

myvar = myvar op value

If, for example, op is +, then we get the operator += that works as follows.

let str = '';

str += '<b>';

str += 'Hello!';

str += '</b>';

assert.equal(str, '<b>Hello!</b>');

Equality: == vs. ===

JavaScript has two kinds of equality operators: loose equality (==) and strict equality (===). The recommendation is to always use the latter.

Other names for == and ===

- == is also called double equals. Its official name in the language specification is abstract equality comparison.

- === is also called triple equals.

Loose equality (== and !=)

Loose equality is one of JavaScript’s quirks. It often coerces operands. Some of those coercions make sense:

> '123' == 123

true

> false == 0

true

Others less so:

> '' == 0

true

Objects are coerced to primitives if (and only if!) the other operand is primitive:

> [1, 2, 3] == '1,2,3'

true

> ['1', '2', '3'] == '1,2,3'

true

If both operands are objects, they are only equal if they are the same object:

> [1, 2, 3] == ['1', '2', '3']

false

> [1, 2, 3] == [1, 2, 3]

false

> const arr = [1, 2, 3];

> arr == arr

true

Lastly, == considers undefined and null to be equal:

> undefined == null

true

Strict equality (=== and !==)

Strict equality never coerces. Two values are only equal if they have the same type. Let’s revisit our previous interaction with the == operator and see what the === operator does:

> false === 0

false

> '123' === 123

false

An object is only equal to another value if that value is the same object:

> [1, 2, 3] === '1,2,3'

false

> ['1', '2', '3'] === '1,2,3'

false

> [1, 2, 3] === ['1', '2', '3']

false

> [1, 2, 3] === [1, 2, 3]

false

> const arr = [1, 2, 3];

> arr === arr

true

The === operator does not consider undefined and null to be equal:

> undefined === null

false

Recommendation: always use strict equality

I recommend to always use ===. It makes your code easier to understand and spares you from having to think about the quirks of ==.

Let’s look at two use cases for == and what I recommend to do instead.

Use case for ==: comparing with a number or a string

== lets you check if a value x is a number or that number as a string – with a single comparison:

if (x == 123) {

// x is either 123 or '123'

}

I prefer either of the following two alternatives:

if (x === 123 || x === '123') ···

if (Number(x) === 123) ···

You can also convert x to a number when you first encounter it.

Use case for ==: comparing with undefined or null

Another use case for == is to check if a value x is either undefined or null:

if (x == null) {

// x is either null or undefined

}

The problem with this code is that you can’t be sure if someone meant to write it that way or if they made a typo and meant === null.

I prefer either of the following two alternatives:

if (x === undefined || x === null) ···

if (!x) ···

A downside of the second alternative is that it accepts values other than undefined and null, but it is a well-established pattern in JavaScript (to be explained in detail in §15.3 “Truthiness-based existence checks”).

The following three conditions are also roughly equivalent:

if (x != null) ···

if (x !== undefined && x !== null) ···

if (x) ···

Even stricter than ===: Object.is()

Method Object.is() compares two values:

> Object.is(123, 123)

true

> Object.is(123, '123')

false

It is even stricter than ===. For example, it considers NaN, the error value for computations involving numbers, to be equal to itself:

> Object.is(NaN, NaN)

true

> NaN === NaN

false

That is occasionally useful. For example, you can use it to implement an improved version of the Array method .indexOf():

const myIndexOf = (arr, elem) => {

return arr.findIndex(x => Object.is(x, elem));

};

myIndexOf() finds NaN in an Array, while .indexOf() doesn’t:

> myIndexOf([0,NaN,2], NaN)

1

> [0,NaN,2].indexOf(NaN)

-1

The result -1 means that .indexOf() couldn’t find its argument in the Array.

Ordering operators

Table 3: JavaScript’s ordering operators.

Operator

name

<

less than

<=

Less than or equal

>

Greater than

>=

Greater than or equal

JavaScript’s ordering operators (tbl. 3) work for both numbers and strings:

> 5 >= 2

true

> 'bar' < 'foo'

true

<= and >= are based on strict equality.

The ordering operators don’t work well for human languages

The ordering operators don’t work well for comparing text in a human language, e.g., when capitalization or accents are involved. The details are explained in §20.6 “Comparing strings”.

Various other operators

The following operators are covered elsewhere in this book:

- Operators for booleans, numbers, strings, objects

- The nullish coalescing operator (??) for default values

The next two subsections discuss two operators that are rarely used.

Comma operator

The comma operator has two operands, evaluates both of them and returns the second one:

> 'a', 'b'

'b'

For more information on this operator, see Speaking JavaScript.

void operator

The void operator evaluates its operand and returns undefined:

> void (3 + 2)

undefined

For more information on this operator, see Speaking JavaScript.

Quiz

See quiz app.

The non-values undefined and null

- 14.1 undefined vs. null

- 14.2 Occurrences of undefined and null

- 14.2.1 Occurrences of undefined

- 14.2.2 Occurrences of null

- 14.3 Checking for undefined or null

- 14.4 The nullish coalescing operator (??) for default values [ES2020]

- 14.4.1 Example: counting matches

- 14.4.2 Example: specifying a default value for a property

- 14.4.3 Using destructuring for default values

- 14.4.4 Legacy approach: using logical Or (||) for default values

- 14.4.5 The nullish coalescing assignment operator (??=) [ES2021]

- 14.5 undefined and null don’t have properties

- 14.6 The history of undefined and null

Many programming languages have one “non-value” called null. It indicates that a variable does not currently point to an object – for example, when it hasn’t been initialized yet.

In contrast, JavaScript has two of them: undefined and null.

undefined vs. null

Both values are very similar and often used interchangeably. How they differ is therefore subtle. The language itself makes the following distinction:

- undefined means “not initialized” (e.g., a variable) or “not existing” (e.g., a property of an object).

- null means “the intentional absence of any object value” (a quote from the language specification).

Programmers may make the following distinction:

- undefined is the non-value used by the language (when something is uninitialized, etc.).

- null means “explicitly switched off”. That is, it helps implement a type that comprises both meaningful values and a meta-value that stands for “no meaningful value”. Such a type is called option type or maybe type in functional programming.

Occurrences of undefined and null

The following subsections describe where undefined and null appear in the language. We’ll encounter several mechanisms that are explained in more detail later in this book.

Occurrences of undefined

Uninitialized variable myVar:

let myVar;

assert.equal(myVar, undefined);

Parameter x is not provided:

function func(x) {

return x;

}

assert.equal(func(), undefined);

Property .unknownProp is missing:

const obj = {};

assert.equal(obj.unknownProp, undefined);

If we don’t explicitly specify the result of a function via a return statement, JavaScript returns undefined for us:

function func() {}

assert.equal(func(), undefined);

Occurrences of null

The prototype of an object is either an object or, at the end of a chain of prototypes, null. Object.prototype does not have a prototype:

> Object.getPrototypeOf(Object.prototype)

null

If we match a regular expression (such as /a/) against a string (such as 'x'), we either get an object with matching data (if matching was successful) or null (if matching failed):

> /a/.exec('x')

null

The JSON data format does not support undefined, only null:

> JSON.stringify({a: undefined, b: null})

'{"b":null}'

Checking for undefined or null

Checking for either:

if (x === null) ···

if (x === undefined) ···

Does x have a value?

if (x !== undefined && x !== null) {

// ···

}

if (x) { // truthy?

// x is neither: undefined, null, false, 0, NaN, ''

}

Is x either undefined or null?

if (x === undefined || x === null) {

// ···

}

if (!x) { // falsy?

// x is: undefined, null, false, 0, NaN, ''

}

Truthy means “is true if coerced to boolean”. Falsy means “is false if coerced to boolean”. Both concepts are explained properly in §15.2 “Falsy and truthy values”.

The nullish coalescing operator (??) for default values [ES2020]

Sometimes we receive a value and only want to use it if it isn’t either null or undefined. Otherwise, we’d like to use a default value, as a fallback. We can do that via the nullish coalescing operator (??):

const valueToUse = receivedValue ?? defaultValue;

The following two expressions are equivalent:

a ?? b

a !== undefined && a !== null ? a : b

Example: counting matches

The following code shows a real-world example:

function countMatches(regex, str) {

const matchResult = str.match(regex); // null or Array

return (matchResult ?? []).length;

}

assert.equal(

countMatches(/a/g, 'ababa'), 3);

assert.equal(

countMatches(/b/g, 'ababa'), 2);

assert.equal(

countMatches(/x/g, 'ababa'), 0);

If there are one or more matches for regex inside str, then .match() returns an Array. If there are no matches, it unfortunately returns null (and not the empty Array). We fix that via the ?? operator.

We also could have used optional chaining:

return matchResult?.length ?? 0;

Example: specifying a default value for a property

function getTitle(fileDesc) {

return fileDesc.title ?? '(Untitled)';

}

const files = [

{path: 'index.html', title: 'Home'},

{path: 'tmp.html'},

];

assert.deepEqual(

files.map(f => getTitle(f)),

['Home', '(Untitled)']);

Using destructuring for default values

In some cases, destructuring can also be used for default values – for example:

function getTitle(fileDesc) {

const {title = '(Untitled)'} = fileDesc;

return title;

}

Legacy approach: using logical Or (||) for default values

Before ECMAScript 2020 and the nullish coalescing operator, logical Or was used for default values. That has a downside.

|| works as expected for undefined and null:

> undefined || 'default'

'default'

> null || 'default'

'default'

But it also returns the default for all other falsy values – for example:

> false || 'default'

'default'

> 0 || 'default'

'default'

> 0n || 'default'

'default'

> '' || 'default'

'default'

Compare that to how ?? works:

> undefined ?? 'default'

'default'

> null ?? 'default'

'default'

> false ?? 'default'

false

> 0 ?? 'default'

0

> 0n ?? 'default'

0n

> '' ?? 'default'

''

The nullish coalescing assignment operator (??=) [ES2021]

??= is a logical assignment operator. The following two expressions are roughly equivalent:

a ??= b

a ?? (a = b)

That means that ??= is short-circuiting: The assignment is only made if a is undefined or null.

Example: using ??= to add missing properties

const books = [

{

isbn: '123',

},

{

title: 'ECMAScript Language Specification',

isbn: '456',

},

];

// Add property .title where it’s missing

for (const book of books) {

book.title ??= '(Untitled)';

}

assert.deepEqual(

books,

[

{

isbn: '123',

title: '(Untitled)',

},

{

title: 'ECMAScript Language Specification',

isbn: '456',

},

]);

undefined and null don’t have properties

undefined and null are the only two JavaScript values where we get an exception if we try to read a property. To explore this phenomenon, let’s use the following function, which reads (“gets”) property .foo and returns the result.

function getFoo(x) {

return x.foo;

}

If we apply getFoo() to various values, we can see that it only fails for undefined and null:

> getFoo(undefined)

TypeError: Cannot read properties of undefined (reading 'foo')

> getFoo(null)

TypeError: Cannot read properties of null (reading 'foo')

> getFoo(true)

undefined

> getFoo({})

undefined

The history of undefined and null

In Java (which inspired many aspects of JavaScript), initialization values depend on the static type of a variable:

- Variables with object types are initialized with null.

- Each primitive type has its own initialization value. For example, int variables are initialized with 0.

In JavaScript, each variable can hold both object values and primitive values. Therefore, if null means “not an object”, JavaScript also needs an initialization value that means “neither an object nor a primitive value”. That initialization value is undefined.

Quiz

See quiz app.

Booleans

- 15.1 Converting to boolean

- 15.2 Falsy and truthy values

- 15.2.1 Checking for truthiness or falsiness

- 15.3 Truthiness-based existence checks

- 15.3.1 Pitfall: truthiness-based existence checks are imprecise

- 15.3.2 Use case: was a parameter provided?

- 15.3.3 Use case: does a property exist?

- 15.4 Conditional operator (? :)

- 15.5 Binary logical operators: And (x && y), Or (x || y)

- 15.5.1 Value-preservation

- 15.5.2 Short-circuiting

- 15.5.3 Logical And (x && y)

- 15.5.4 Logical Or (||)

- 15.6 Logical Not (!)

The primitive type boolean comprises two values – false and true:

> typeof false

'boolean'

> typeof true

'boolean'

Converting to boolean

The meaning of “converting to [type]”

“Converting to [type]” is short for “Converting arbitrary values to values of type [type]”.

These are three ways in which you can convert an arbitrary value x to a boolean.

- Boolean(x)

Most descriptive; recommended.

- x ? true : false

Uses the conditional operator (explained later in this chapter).

- !!x

Uses the logical Not operator (!). This operator coerces its operand to boolean. It is applied a second time to get a non-negated result.

Tbl. 4 describes how various values are converted to boolean.

Table 4: Converting values to booleans.

x

Boolean(x)

undefined

false

null

false

boolean

x   (no change)

number

0 → false, NaN → false

Other numbers → true

bigint

0 → false

Other numbers → true

string

'' → false

Other strings → true

symbol

true

object

Always true

Falsy and truthy values

When checking the condition of an if statement, a while loop, or a do-while loop, JavaScript works differently than you may expect. Take, for example, the following condition:

if (value) {}

In many programming languages, this condition is equivalent to:

if (value === true) {}

However, in JavaScript, it is equivalent to:

if (Boolean(value) === true) {}

That is, JavaScript checks if value is true when converted to boolean. This kind of check is so common that the following names were introduced:

- A value is called truthy if it is true when converted to boolean.

- A value is called falsy if it is false when converted to boolean.

Each value is either truthy or falsy. Consulting tbl. 4, we can make an exhaustive list of falsy values:

- undefined

- null

- Boolean: false

- Numbers: 0, NaN

- Bigint: 0n

- String: ''

All other values (including all objects) are truthy:

> Boolean('abc')

true

> Boolean([])

true

> Boolean({})

true

Checking for truthiness or falsiness

if (x) {

// x is truthy

}

if (!x) {

// x is falsy

}

if (x) {

// x is truthy

} else {

// x is falsy

}

const result = x ? 'truthy' : 'falsy';

The conditional operator that is used in the last line, is explained later in this chapter.

Exercise: Truthiness

exercises/booleans/truthiness_exrc.mjs

Truthiness-based existence checks

In JavaScript, if you read something that doesn’t exist (e.g., a missing parameter or a missing property), you usually get undefined as a result. In these cases, an existence check amounts to comparing a value with undefined. For example, the following code checks if object obj has the property .prop:

if (obj.prop !== undefined) {

// obj has property .prop

}

Due to undefined being falsy, we can shorten this check to:

if (obj.prop) {

// obj has property .prop

}

Pitfall: truthiness-based existence checks are imprecise

Truthiness-based existence checks have one pitfall: they are not very precise. Consider this previous example:

if (obj.prop) {

// obj has property .prop

}

The body of the if statement is skipped if:

- obj.prop is missing (in which case, JavaScript returns undefined).

However, it is also skipped if:

- obj.prop is undefined.

- obj.prop is any other falsy value (null, 0, '', etc.).

In practice, this rarely causes problems, but you have to be aware of this pitfall.

Use case: was a parameter provided?

A truthiness check is often used to determine if the caller of a function provided a parameter:

function func(x) {

if (!x) {

throw new Error('Missing parameter x');

}

// ···

}

On the plus side, this pattern is established and short. It correctly throws errors for undefined and null.

On the minus side, there is the previously mentioned pitfall: the code also throws errors for all other falsy values.

An alternative is to check for undefined:

if (x === undefined) {

throw new Error('Missing parameter x');

}

Use case: does a property exist?

Truthiness checks are also often used to determine if a property exists:

function readFile(fileDesc) {

if (!fileDesc.path) {

throw new Error('Missing property: .path');

}

// ···

}

readFile({ path: 'foo.txt' }); // no error

This pattern is also established and has the usual caveat: it not only throws if the property is missing, but also if it exists and has any of the falsy values.

If you truly want to check if the property exists, you have to use the in operator:

if (! ('path' in fileDesc)) {

throw new Error('Missing property: .path');

}

Conditional operator (? :)

The conditional operator is the expression version of the if statement. Its syntax is:

«condition» ? «thenExpression» : «elseExpression»

It is evaluated as follows:

- If condition is truthy, evaluate and return thenExpression.

- Otherwise, evaluate and return elseExpression.

The conditional operator is also called ternary operator because it has three operands.

Examples:

> true ? 'yes' : 'no'

'yes'

> false ? 'yes' : 'no'

'no'

> '' ? 'yes' : 'no'

'no'

The following code demonstrates that whichever of the two branches “then” and “else” is chosen via the condition, only that branch is evaluated. The other branch isn’t.

const x = (true ? console.log('then') : console.log('else'));

// Output:

// 'then'

Binary logical operators: And (x && y), Or (x || y)

The binary logical operators && and || are value-preserving and short-circuiting.

Value-preservation

Value-preservation means that operands are interpreted as booleans but returned unchanged:

> 12 || 'hello'

12

> 0 || 'hello'

'hello'

Short-circuiting

Short-circuiting means if the first operand already determines the result, then the second operand is not evaluated. The only other operator that delays evaluating its operands is the conditional operator. Usually, all operands are evaluated before performing an operation.

For example, logical And (&&) does not evaluate its second operand if the first one is falsy:

const x = false && console.log('hello');

// No output

If the first operand is truthy, console.log() is executed:

const x = true && console.log('hello');

// Output:

// 'hello'

Logical And (x && y)

The expression a && b (“a And b”) is evaluated as follows:

1. Evaluate a.

2. Is the result falsy? Return it.

3. Otherwise, evaluate b and return the result.

In other words, the following two expressions are roughly equivalent:

a && b

!a ? a : b

Examples:

> false && true

false

> false && 'abc'

false

> true && false

false

> true && 'abc'

'abc'

> '' && 'abc'

''

Logical Or (||)

The expression a || b (“a Or b”) is evaluated as follows:

1. Evaluate a.

2. Is the result truthy? Return it.

3. Otherwise, evaluate b and return the result.

In other words, the following two expressions are roughly equivalent:

a || b

a ? a : b

Examples:

> true || false

true

> true || 'abc'

true

> false || true

true

> false || 'abc'

'abc'

> 'abc' || 'def'

'abc'

Legacy use case for logical Or (||): providing default values

ECMAScript 2020 introduced the nullish coalescing operator (??) for default values. Before that, logical Or was used for this purpose:

const valueToUse = receivedValue || defaultValue;

See §14.4 “The nullish coalescing operator (??) for default values [ES2020]” for more information on ?? and the downsides of || in this case.

Legacy exercise: Default values via the Or operator (||)

exercises/booleans/default_via_or_exrc.mjs

Logical Not (!)

The expression !x (“Not x”) is evaluated as follows:

1. Evaluate x.

2. Is it truthy? Return false.

3. Otherwise, return true.

Examples:

> !false

true

> !true

false

> !0

true

> !123

false

> !''

true

> !'abc'

false

Quiz

See quiz app.

Numbers

- 16.1 Numbers are used for both floating point numbers and integers

- 16.2 Number literals

- 16.2.1 Integer literals

- 16.2.2 Floating point literals

- 16.2.3 Syntactic pitfall: properties of integer literals

- 16.2.4 Underscores (_) as separators in number literals [ES2021]

- 16.3 Arithmetic operators

- 16.3.1 Binary arithmetic operators

- 16.3.2 Unary plus (+) and negation (-)

- 16.3.3 Incrementing (++) and decrementing (--)

- 16.4 Converting to number

- 16.5 Error values

- 16.5.1 Error value: NaN

- 16.5.2 Error value: Infinity

- 16.6 The precision of numbers: careful with decimal fractions

- 16.7 (Advanced)

- 16.8 Background: floating point precision

- 16.8.1 A simplified representation of floating point numbers

- 16.9 Integer numbers in JavaScript

- 16.9.1 Converting to integer

- 16.9.2 Ranges of integer numbers in JavaScript

- 16.9.3 Safe integers

- 16.10 Bitwise operators

- 16.10.1 Internally, bitwise operators work with 32-bit integers

- 16.10.2 Bitwise Not

- 16.10.3 Binary bitwise operators

- 16.10.4 Bitwise shift operators

- 16.10.5 b32(): displaying unsigned 32-bit integers in binary notation

- 16.11 Quick reference: numbers

- 16.11.1 Global functions for numbers

- 16.11.2 Static properties of Number

- 16.11.3 Static methods of Number

- 16.11.4 Methods of Number.prototype

- 16.11.5 Sources

JavaScript has two kinds of numeric values:

- Numbers are 64-bit floating point numbers and are also used for smaller integers (within a range of plus/minus 53 bits).

- Bigints represent integers with an arbitrary precision.

This chapter covers numbers. Bigints are covered later in this book.

Numbers are used for both floating point numbers and integers

The type number is used for both integers and floating point numbers in JavaScript:

98

123.45

However, all numbers are doubles, 64-bit floating point numbers implemented according to the IEEE Standard for Floating-Point Arithmetic (IEEE 754).

Integer numbers are simply floating point numbers without a decimal fraction:

> 98 === 98.0

true

Note that, under the hood, most JavaScript engines are often able to use real integers, with all associated performance and storage size benefits.

Number literals

Let’s examine literals for numbers.

Integer literals

Several integer literals let us express integers with various bases:

// Binary (base 2)

assert.equal(0b11, 3); // ES6

// Octal (base 8)

assert.equal(0o10, 8); // ES6

// Decimal (base 10)

assert.equal(35, 35);

// Hexadecimal (base 16)

assert.equal(0xE7, 231);

Floating point literals

Floating point numbers can only be expressed in base 10.

Fractions:

> 35.0

35

Exponent: eN means ×10N

> 3e2

300

> 3e-2

0.03

> 0.3e2

30

Syntactic pitfall: properties of integer literals

Accessing a property of an integer literal entails a pitfall: If the integer literal is immediately followed by a dot, then that dot is interpreted as a decimal dot:

7.toString(); // syntax error

There are four ways to work around this pitfall:

7.0.toString()

(7).toString()

7..toString()

7 .toString()  // space before dot

Underscores (_) as separators in number literals [ES2021]

Grouping digits to make long numbers more readable has a long tradition. For example:

- In 1825, London had 1,335,000 inhabitants.

- The distance between Earth and Sun is 149,600,000 km.

Since ES2021, we can use underscores as separators in number literals:

const inhabitantsOfLondon = 1_335_000;

const distanceEarthSunInKm = 149_600_000;

With other bases, grouping is important, too:

const fileSystemPermission = 0b111_111_000;

const bytes = 0b1111_10101011_11110000_00001101;

const words = 0xFAB_F00D;

We can also use the separator in fractions and exponents:

const massOfElectronInKg = 9.109_383_56e-31;

const trillionInShortScale = 1e1_2;

Where can we put separators?

The locations of separators are restricted in two ways:

- We can only put underscores between two digits. Therefore, all of the following number literals are illegal:

3_.141

3._141

1_e12

1e_12

_1464301  // valid variable name!

1464301_

0_b111111000

0b_111111000

- We can’t use more than one underscore in a row:

123__456 // two underscores – not allowed

The motivation behind these restrictions is to keep parsing simple and to avoid strange edge cases.

Parsing numbers with separators

The following functions for parsing numbers do not support separators:

- Number()

- Number.parseInt()

- Number.parseFloat()

For example:

> Number('123_456')

NaN

> Number.parseInt('123_456')

123

The rationale is that numeric separators are for code. Other kinds of input should be processed differently.

Arithmetic operators

Binary arithmetic operators

Tbl. 5 lists JavaScript’s binary arithmetic operators.

Table 5: Binary arithmetic operators.

Operator

Name

Example

n + m

Addition

ES1

3 + 4 → 7

n - m

Subtraction

ES1

9 - 1 → 8

n * m

Multiplication

ES1

3 * 2.25 → 6.75

n / m

Division

ES1

5.625 / 5 → 1.125

n % m

Remainder

ES1

8 % 5 → 3

-8 % 5 → -3

n ** m

Exponentiation

ES2016

4 ** 2 → 16

% is a remainder operator

% is a remainder operator, not a modulo operator. Its result has the sign of the first operand:

> 5 % 3

2

> -5 % 3

-2

For more information on the difference between remainder and modulo, see the blog post “Remainder operator vs. modulo operator (with JavaScript code)” on 2ality.

Unary plus (+) and negation (-)

Tbl. 6 summarizes the two operators unary plus (+) and negation (-).

Table 6: The operators unary plus (+) and negation (-).

Operator

Name

Example

+n

Unary plus

ES1

+(-7) → -7

-n

Unary negation

ES1

-(-7) → 7

Both operators coerce their operands to numbers:

> +'5'

5

> +'-12'

-12

> -'9'

-9

Thus, unary plus lets us convert arbitrary values to numbers.

Incrementing (++) and decrementing (--)

The incrementation operator ++ exists in a prefix version and a suffix version. In both versions, it destructively adds one to its operand. Therefore, its operand must be a storage location that can be changed.

The decrementation operator -- works the same, but subtracts one from its operand. The next two examples explain the difference between the prefix and the suffix version.

Tbl. 7 summarizes the incrementation and decrementation operators.

Table 7: Incrementation operators and decrementation operators.

Operator

Name

Example

v++

Increment

ES1

let v=0; [v++, v] → [0, 1]

++v

Increment

ES1

let v=0; [++v, v] → [1, 1]

v--

Decrement

ES1

let v=1; [v--, v] → [1, 0]

--v

Decrement

ES1

let v=1; [--v, v] → [0, 0]

Next, we’ll look at examples of these operators in use.

Prefix ++ and prefix -- change their operands and then return them.

let foo = 3;

assert.equal(++foo, 4);

assert.equal(foo, 4);

let bar = 3;

assert.equal(--bar, 2);

assert.equal(bar, 2);

Suffix ++ and suffix -- return their operands and then change them.

let foo = 3;

assert.equal(foo++, 3);

assert.equal(foo, 4);

let bar = 3;

assert.equal(bar--, 3);

assert.equal(bar, 2);

Operands: not just variables

We can also apply these operators to property values:

const obj = { a: 1 };

++obj.a;

assert.equal(obj.a, 2);

And to Array elements:

const arr = [ 4 ];

arr[0]++;

assert.deepEqual(arr, [5]);

Exercise: Number operators

exercises/numbers-math/is_odd_test.mjs

Converting to number

These are three ways of converting values to numbers:

- Number(value)

- +value

- parseFloat(value) (avoid; different than the other two!)

Recommendation: use the descriptive Number(). Tbl. 8 summarizes how it works.

Table 8: Converting values to numbers.

x

Number(x)

undefined

NaN

null

0

boolean

false → 0, true → 1

number

x   (no change)

bigint

-1n → -1, 1n → 1  , etc.

string

'' → 0

Other →   parsed number, ignoring leading/trailing whitespace

symbol

Throws TypeError

object

Configurable (e.g. via .valueOf()  )

Examples:

assert.equal(Number(123.45), 123.45);

assert.equal(Number(''), 0);

assert.equal(Number('\n 123.45 \t'), 123.45);

assert.equal(Number('xyz'), NaN);

assert.equal(Number(-123n), -123);

How objects are converted to numbers can be configured – for example, by overriding .valueOf():

> Number({ valueOf() { return 123 } })

123

Exercise: Converting to number

exercises/numbers-math/parse_number_test.mjs

Error values

Two number values are returned when errors happen:

- NaN

- Infinity

Error value: NaN

NaN is an abbreviation of “not a number”. Ironically, JavaScript considers it to be a number:

> typeof NaN

'number'

When is NaN returned?

NaN is returned if a number can’t be parsed:

> Number('$$$')

NaN

> Number(undefined)

NaN

NaN is returned if an operation can’t be performed:

> Math.log(-1)

NaN

> Math.sqrt(-1)

NaN

NaN is returned if an operand or argument is NaN (to propagate errors):

> NaN - 3

NaN

> 7 ** NaN

NaN

Checking for NaN

NaN is the only JavaScript value that is not strictly equal to itself:

const n = NaN;

assert.equal(n === n, false);

These are several ways of checking if a value x is NaN:

const x = NaN;

assert.equal(Number.isNaN(x), true); // preferred

assert.equal(Object.is(x, NaN), true);

assert.equal(x !== x, true);

In the last line, we use the comparison quirk to detect NaN.

Finding NaN in Arrays

Some Array methods can’t find NaN:

> [NaN].indexOf(NaN)

-1

Others can:

> [NaN].includes(NaN)

true

> [NaN].findIndex(x => Number.isNaN(x))

0

> [NaN].find(x => Number.isNaN(x))

NaN

Alas, there is no simple rule of thumb. We have to check for each method how it handles NaN.

Error value: Infinity

When is the error value Infinity returned?

Infinity is returned if a number is too large:

> Math.pow(2, 1023)

8.98846567431158e+307

> Math.pow(2, 1024)

Infinity

Infinity is returned if there is a division by zero:

> 5 / 0

Infinity

> -5 / 0

-Infinity

Infinity as a default value

Infinity is larger than all other numbers (except NaN), making it a good default value:

function findMinimum(numbers) {

let min = Infinity;

for (const n of numbers) {

if (n < min) min = n;

}

return min;

}

assert.equal(findMinimum([5, -1, 2]), -1);

assert.equal(findMinimum([]), Infinity);

Checking for Infinity

These are two common ways of checking if a value x is Infinity:

const x = Infinity;

assert.equal(x === Infinity, true);

assert.equal(Number.isFinite(x), false);

Exercise: Comparing numbers

exercises/numbers-math/find_max_test.mjs

The precision of numbers: careful with decimal fractions

Internally, JavaScript floating point numbers are represented with base 2 (according to the IEEE 754 standard). That means that decimal fractions (base 10) can’t always be represented precisely:

> 0.1 + 0.2

0.30000000000000004

> 1.3 * 3

3.9000000000000004

> 1.4 * 100000000000000

139999999999999.98

We therefore need to take rounding errors into consideration when performing arithmetic in JavaScript.

Read on for an explanation of this phenomenon.

Quiz: basic

See quiz app.

(Advanced)

All remaining sections of this chapter are advanced.

Background: floating point precision

In JavaScript, computations with numbers don’t always produce correct results – for example:

> 0.1 + 0.2

0.30000000000000004

To understand why, we need to explore how JavaScript represents floating point numbers internally. It uses three integers to do so, which take up a total of 64 bits of storage (double precision):

Component

Size

Integer range

Sign

1 bit

[0, 1]

Fraction

52 bits

[0, 2^52  −1]

Exponent

11 bits

[−1023, 1024]

The floating point number represented by these integers is computed as follows:

(–1)sign × 0b1.fraction × 2exponent

This representation can’t encode a zero because its second component (involving the fraction) always has a leading 1. Therefore, a zero is encoded via the special exponent −1023 and a fraction 0.

A simplified representation of floating point numbers

To make further discussions easier, we simplify the previous representation:

- Instead of base 2 (binary), we use base 10 (decimal) because that’s what most people are more familiar with.

- The fraction is a natural number that is interpreted as a fraction (digits after a point). We switch to a mantissa, an integer that is interpreted as itself. As a consequence, the exponent is used differently, but its fundamental role doesn’t change.

- As the mantissa is an integer (with its own sign), we don’t need a separate sign, anymore.

The new representation works like this:

mantissa × 10exponent

Let’s try out this representation for a few floating point numbers.

- For the integer −123, we mainly need the mantissa:

> -123 * (10 ** 0)

-123

- For the number 1.5, we imagine there being a point after the mantissa. We use a negative exponent to move that point one digit to the left:

> 15 * (10 ** -1)

1.5

- For the number 0.25, we move the point two digits to the left:

> 25 * (10 ** -2)

0.25

Representations with negative exponents can also be written as fractions with positive exponents in the denominators:

> 15 * (10 ** -1) === 15 / (10 ** 1)

true

> 25 * (10 ** -2) === 25 / (10 ** 2)

true

These fractions help with understanding why there are numbers that our encoding cannot represent:

- 1/10 can be represented. It already has the required format: a power of 10 in the denominator.

- 1/2 can be represented as 5/10. We turned the 2 in the denominator into a power of 10 by multiplying the numerator and denominator by 5.

- 1/4 can be represented as 25/100. We turned the 4 in the denominator into a power of 10 by multiplying the numerator and denominator by 25.

- 1/3 cannot be represented. There is no way to turn the denominator into a power of 10. (The prime factors of 10 are 2 and 5. Therefore, any denominator that only has these prime factors can be converted to a power of 10, by multiplying both the numerator and denominator by enough twos and fives. If a denominator has a different prime factor, then there’s nothing we can do.)

To conclude our excursion, we switch back to base 2:

- 0.5 = 1/2 can be represented with base 2 because the denominator is already a power of 2.

- 0.25 = 1/4 can be represented with base 2 because the denominator is already a power of 2.

- 0.1 = 1/10 cannot be represented because the denominator cannot be converted to a power of 2.

- 0.2 = 2/10 cannot be represented because the denominator cannot be converted to a power of 2.

Now we can see why 0.1 + 0.2 doesn’t produce a correct result: internally, neither of the two operands can be represented precisely.

The only way to compute precisely with decimal fractions is by internally switching to base 10. For many programming languages, base 2 is the default and base 10 an option. For example, Java has the class BigDecimal and Python has the module decimal. There are plans to add something similar to JavaScript: the ECMAScript proposal “Decimal”.

Integer numbers in JavaScript

Integer numbers are normal (floating point) numbers without decimal fractions:

> 1 === 1.0

true

> Number.isInteger(1.0)

true

In this section, we’ll look at a few tools for working with these pseudo-integers. JavaScript also supports bigints, which are real integers.

Converting to integer

The recommended way of converting numbers to integers is to use one of the rounding methods of the Math object:

- Math.floor(n): returns the largest integer i ≤ n

> Math.floor(2.1)

2

> Math.floor(2.9)

2

- Math.ceil(n): returns the smallest integer i ≥ n

> Math.ceil(2.1)

3

> Math.ceil(2.9)

3

- Math.round(n): returns the integer that is “closest” to n with __.5 being rounded up – for example:

> Math.round(2.4)

2

> Math.round(2.5)

3

- Math.trunc(n): removes any decimal fraction (after the point) that n has, therefore turning it into an integer.

> Math.trunc(2.1)

2

> Math.trunc(2.9)

2

For more information on rounding, consult §17.3 “Rounding”.

Ranges of integer numbers in JavaScript

These are important ranges of integer numbers in JavaScript:

- Safe integers: can be represented “safely” by JavaScript (more on what that means in the next subsection)

- Precision: 53 bits plus sign

- Range: (−2^53, 2^53)

- Array indices

- Precision: 32 bits, unsigned

- Range: [0, 2^32−1) (excluding the maximum length)

- Typed Arrays have a larger range of 53 bits (safe and unsigned)

- Bitwise operators (bitwise Or, etc.)

- Precision: 32 bits

- Range of unsigned right shift (>>>): unsigned, [0, 2^32)

- Range of all other bitwise operators: signed, [−2^31, 2^31)

Safe integers

This is the range of integer numbers that are safe in JavaScript (53 bits plus a sign):

[–(2^53)+1, 2^53–1]

An integer is safe if it is represented by exactly one JavaScript number. Given that JavaScript numbers are encoded as a fraction multiplied by 2 to the power of an exponent, higher integers can also be represented, but then there are gaps between them.

For example (18014398509481984 is 2^54):

> 18014398509481984

18014398509481984

> 18014398509481985

18014398509481984

> 18014398509481986

18014398509481984

> 18014398509481987

18014398509481988

The following properties of Number help determine if an integer is safe:

assert.equal(Number.MAX_SAFE_INTEGER, (2 ** 53) - 1);

assert.equal(Number.MIN_SAFE_INTEGER, -Number.MAX_SAFE_INTEGER);

assert.equal(Number.isSafeInteger(5), true);

assert.equal(Number.isSafeInteger('5'), false);

assert.equal(Number.isSafeInteger(5.1), false);

assert.equal(Number.isSafeInteger(Number.MAX_SAFE_INTEGER), true);

assert.equal(Number.isSafeInteger(Number.MAX_SAFE_INTEGER+1), false);

Exercise: Detecting safe integers

exercises/numbers-math/is_safe_integer_test.mjs

Safe computations

Let’s look at computations involving unsafe integers.

The following result is incorrect and unsafe, even though both of its operands are safe:

> 9007199254740990 + 3

9007199254740992

The following result is safe, but incorrect. The first operand is unsafe; the second operand is safe:

> 9007199254740995 - 10

9007199254740986

Therefore, the result of an expression a op b is correct if and only if:

isSafeInteger(a) && isSafeInteger(b) && isSafeInteger(a op b)

That is, both operands and the result must be safe.

Bitwise operators

Internally, bitwise operators work with 32-bit integers

Internally, JavaScript’s bitwise operators work with 32-bit integers. They produce their results in the following steps:

- Input (JavaScript numbers): The 1–2 operands are first converted to JavaScript numbers (64-bit floating point numbers) and then to 32-bit integers.

- Computation (32-bit integers): The actual operation processes 32-bit integers and produces a 32-bit integer.

- Output (JavaScript number): Before returning the result, it is converted back to a JavaScript number.

The types of operands and results

For each bitwise operator, this book mentions the types of its operands and its result. Each type is always one of the following two:

Type

Description

Size

Range

Int32

signed 32-bit integer

32 bits incl. sign

[−2^31, 2^31  )

Uint32

unsigned 32-bit integer

32 bits

[0, 2^32  )

Considering the previously mentioned steps, I recommend to pretend that bitwise operators internally work with unsigned 32-bit integers (step “computation”) and that Int32 and Uint32 only affect how JavaScript numbers are converted to and from integers (steps “input” and “output”).

Displaying JavaScript numbers as unsigned 32-bit integers

While exploring the bitwise operators, it occasionally helps to display JavaScript numbers as unsigned 32-bit integers in binary notation. That’s what b32() does (whose implementation is shown later):

assert.equal(

b32(-1),

'11111111111111111111111111111111');

assert.equal(

b32(1),

'00000000000000000000000000000001');

assert.equal(

b32(2 ** 31),

'10000000000000000000000000000000');

Bitwise Not

Table 9: The bitwise Not operator.

Operation

Name

Type signature

~num

Bitwise Not, ones’ complement

Int32 →   Int32

ES1

The bitwise Not operator (tbl. 9) inverts each binary digit of its operand:

> b32(~0b100)

'11111111111111111111111111111011'

This so-called ones’ complement is similar to a negative for some arithmetic operations. For example, adding an integer to its ones’ complement is always -1:

> 4 + ~4

-1

> -11 + ~-11

-1

Binary bitwise operators

Table 10: Binary bitwise operators.

Operation

Name

Type signature

num1 & num2

Bitwise And

Int32 × Int32 →   Int32

ES1

num1 ¦ num2

Bitwise Or

Int32 × Int32 →   Int32

ES1

num1 ^ num2

Bitwise Xor

Int32 × Int32 →   Int32

ES1

The binary bitwise operators (tbl. 10) combine the bits of their operands to produce their results:

> (0b1010 & 0b0011).toString(2).padStart(4, '0')

'0010'

> (0b1010 | 0b0011).toString(2).padStart(4, '0')

'1011'

> (0b1010 ^ 0b0011).toString(2).padStart(4, '0')

'1001'

Bitwise shift operators

Table 11: Bitwise shift operators.

Operation

Name

Type signature

num << count

Left shift

Int32 × Uint32 →   Int32

ES1

num >> count

Signed right shift

Int32 × Uint32 →   Int32

ES1

num >>> count

Unsigned right shift

Uint32 × Uint32 →   Uint32

ES1

The shift operators (tbl. 11) move binary digits to the left or to the right:

> (0b10 << 1).toString(2)

'100'

>> preserves highest bit, >>> doesn’t:

> b32(0b10000000000000000000000000000010 >> 1)

'11000000000000000000000000000001'

> b32(0b10000000000000000000000000000010 >>> 1)

'01000000000000000000000000000001'

b32(): displaying unsigned 32-bit integers in binary notation

We have now used b32() a few times. The following code is an implementation of it:

/**

* Return a string representing n as a 32-bit unsigned integer,

* in binary notation.

*/

function b32(n) {

// >>> ensures highest bit isn’t interpreted as a sign

return (n >>> 0).toString(2).padStart(32, '0');

}

assert.equal(

b32(6),

'00000000000000000000000000000110');

n >>> 0 means that we are shifting n zero bits to the right. Therefore, in principle, the >>> operator does nothing, but it still coerces n to an unsigned 32-bit integer:

> 12 >>> 0

12

> -12 >>> 0

4294967284

> (2**32 + 1) >>> 0

1

Quick reference: numbers

Global functions for numbers

JavaScript has the following four global functions for numbers:

- isFinite()

- isNaN()

- parseFloat()

- parseInt()

However, it is better to use the corresponding methods of Number (Number.isFinite(), etc.), which have fewer pitfalls. They were introduced with ES6 and are discussed below.

Static properties of Number

- .EPSILON: number [ES6]

The difference between 1 and the next representable floating point number. In general, a machine epsilon provides an upper bound for rounding errors in floating point arithmetic.

- Approximately: 2.2204460492503130808472633361816 × 10-16

- .MAX_SAFE_INTEGER: number [ES6]

The largest integer that JavaScript can represent unambiguously (2^53−1).

- .MAX_VALUE: number [ES1]

The largest positive finite JavaScript number.

- Approximately: 1.7976931348623157 × 10^308

- .MIN_SAFE_INTEGER: number [ES6]

The smallest integer that JavaScript can represent unambiguously (−2^53+1).

- .MIN_VALUE: number [ES1]

The smallest positive JavaScript number. Approximately 5 × 10−324.

- .NaN: number [ES1]

The same as the global variable NaN.

- .NEGATIVE_INFINITY: number [ES1]

The same as -Number.POSITIVE_INFINITY.

- .POSITIVE_INFINITY: number [ES1]

The same as the global variable Infinity.

Static methods of Number

- .isFinite(num: number): boolean [ES6]

Returns true if num is an actual number (neither Infinity nor -Infinity nor NaN).

> Number.isFinite(Infinity)

false

> Number.isFinite(-Infinity)

false

> Number.isFinite(NaN)

false

> Number.isFinite(123)

true

- .isInteger(num: number): boolean [ES6]

Returns true if num is a number and does not have a decimal fraction.

> Number.isInteger(-17)

true

> Number.isInteger(33)

true

> Number.isInteger(33.1)

false

> Number.isInteger('33')

false

> Number.isInteger(NaN)

false

> Number.isInteger(Infinity)

false

- .isNaN(num: number): boolean [ES6]

Returns true if num is the value NaN:

> Number.isNaN(NaN)

true

> Number.isNaN(123)

false

> Number.isNaN('abc')

false

- .isSafeInteger(num: number): boolean [ES6]

Returns true if num is a number and unambiguously represents an integer.

- .parseFloat(str: string): number [ES6]

Coerces its parameter to string and parses it as a floating point number. For converting strings to numbers, Number() (which ignores leading and trailing whitespace) is usually a better choice than Number.parseFloat() (which ignores leading whitespace and illegal trailing characters and can hide problems).

> Number.parseFloat(' 123.4#')

123.4

> Number(' 123.4#')

NaN

- .parseInt(str: string, radix=10): number [ES6]

Coerces its parameter to string and parses it as an integer, ignoring leading whitespace and illegal trailing characters:

> Number.parseInt('  123#')

123

The parameter radix specifies the base of the number to be parsed:

> Number.parseInt('101', 2)

5

> Number.parseInt('FF', 16)

255

Do not use this method to convert numbers to integers: coercing to string is inefficient. And stopping before the first non-digit is not a good algorithm for removing the fraction of a number. Here is an example where it goes wrong:

> Number.parseInt(1e21, 10) // wrong

1

It is better to use one of the rounding functions of Math to convert a number to an integer:

> Math.trunc(1e21) // correct

1e+21

Methods of Number.prototype

(Number.prototype is where the methods of numbers are stored.)

- .toExponential(fractionDigits?: number): string [ES3]

Returns a string that represents the number via exponential notation. With fractionDigits, we can specify, how many digits should be shown of the number that is multiplied with the exponent (the default is to show as many digits as necessary).

Example: number too small to get a positive exponent via .toString().

> 1234..toString()

'1234'

> 1234..toExponential() // 3 fraction digits

'1.234e+3'

> 1234..toExponential(5)

'1.23400e+3'

> 1234..toExponential(1)

'1.2e+3'

Example: fraction not small enough to get a negative exponent via .toString().

> 0.003.toString()

'0.003'

> 0.003.toExponential()

'3e-3'

- .toFixed(fractionDigits=0): string [ES3]

Returns an exponent-free representation of the number, rounded to fractionDigits digits.

> 0.00000012.toString() // with exponent

'1.2e-7'

> 0.00000012.toFixed(10) // no exponent

'0.0000001200'

> 0.00000012.toFixed()

'0'

If the number is 10^21 or greater, even .toFixed() uses an exponent:

> (10 ** 21).toFixed()

'1e+21'

- .toPrecision(precision?: number): string [ES3]

Works like .toString(), but precision specifies how many digits should be shown. If precision is missing, .toString() is used.

> 1234..toPrecision(3)  // requires exponential notation

'1.23e+3'

> 1234..toPrecision(4)

'1234'

> 1234..toPrecision(5)

'1234.0'

> 1.234.toPrecision(3)

'1.23'

- .toString(radix=10): string [ES1]

Returns a string representation of the number.

By default, we get a base 10 numeral as a result:

> 123.456.toString()

'123.456'

If we want the numeral to have a different base, we can specify it via radix:

> 4..toString(2) // binary (base 2)

'100'

> 4.5.toString(2)

'100.1'

> 255..toString(16) // hexadecimal (base 16)

'ff'

> 255.66796875.toString(16)

'ff.ab'

> 1234567890..toString(36)

'kf12oi'

Number.parseInt() provides the inverse operation: it converts a string that contains an integer (no fraction!) numeral with a given base, to a number.

> Number.parseInt('kf12oi', 36)

1234567890

Sources

- Wikipedia

- TypeScript’s built-in typings

- MDN web docs for JavaScript

- ECMAScript language specification

Quiz: advanced

See quiz app.

Math

- 17.1 Data properties

- 17.2 Exponents, roots, logarithms

- 17.3 Rounding

- 17.4 Trigonometric Functions

- 17.5 Various other functions

- 17.6 Sources

Math is an object with data properties and methods for processing numbers. You can see it as a poor man’s module: It was created long before JavaScript had modules.

Data properties

- Math.E: number [ES1]

Euler’s number, base of the natural logarithms, approximately 2.7182818284590452354.

- Math.LN10: number [ES1]

The natural logarithm of 10, approximately 2.302585092994046.

- Math.LN2: number [ES1]

The natural logarithm of 2, approximately 0.6931471805599453.

- Math.LOG10E: number [ES1]

The logarithm of e to base 10, approximately 0.4342944819032518.

- Math.LOG2E: number [ES1]

The logarithm of e to base 2, approximately 1.4426950408889634.

- Math.PI: number [ES1]

The mathematical constant π, ratio of a circle’s circumference to its diameter, approximately 3.1415926535897932.

- Math.SQRT1_2: number [ES1]

The square root of 1/2, approximately 0.7071067811865476.

- Math.SQRT2: number [ES1]

The square root of 2, approximately 1.4142135623730951.

Exponents, roots, logarithms

- Math.cbrt(x: number): number [ES6]

Returns the cube root of x.

> Math.cbrt(8)

2

- Math.exp(x: number): number [ES1]

Returns ex (e being Euler’s number). The inverse of Math.log().

> Math.exp(0)

1

> Math.exp(1) === Math.E

true

- Math.expm1(x: number): number [ES6]

Returns Math.exp(x)-1. The inverse of Math.log1p(). Very small numbers (fractions close to 0) are represented with a higher precision. Therefore, this function returns more precise values whenever .exp() returns values close to 1.

- Math.log(x: number): number [ES1]

Returns the natural logarithm of x (to base e, Euler’s number). The inverse of Math.exp().

> Math.log(1)

0

> Math.log(Math.E)

1

> Math.log(Math.E ** 2)

2

- Math.log1p(x: number): number [ES6]

Returns Math.log(1 + x). The inverse of Math.expm1(). Very small numbers (fractions close to 0) are represented with a higher precision. Therefore, you can provide this function with a more precise argument whenever the argument for .log() is close to 1.

- Math.log10(x: number): number [ES6]

Returns the logarithm of x to base 10. The inverse of 10 ** x.

> Math.log10(1)

0

> Math.log10(10)

1

> Math.log10(100)

2

- Math.log2(x: number): number [ES6]

Returns the logarithm of x to base 2. The inverse of 2 ** x.

> Math.log2(1)

0

> Math.log2(2)

1

> Math.log2(4)

2

- Math.pow(x: number, y: number): number [ES1]

Returns xy, x to the power of y. The same as x ** y.

> Math.pow(2, 3)

8

> Math.pow(25, 0.5)

5

- Math.sqrt(x: number): number [ES1]

Returns the square root of x. The inverse of x ** 2.

> Math.sqrt(9)

3

Rounding

Rounding means converting an arbitrary number to an integer (a number without a decimal fraction). The following functions implement different approaches to rounding.

- Math.ceil(x: number): number [ES1]

Returns the smallest (closest to −∞) integer i with x ≤ i.

> Math.ceil(2.1)

3

> Math.ceil(2.9)

3

- Math.floor(x: number): number [ES1]

Returns the largest (closest to +∞) integer i with i ≤ x.

> Math.floor(2.1)

2

> Math.floor(2.9)

2

- Math.round(x: number): number [ES1]

Returns the integer that is closest to x. If the decimal fraction of x is .5 then .round() rounds up (to the integer closer to positive infinity):

> Math.round(2.4)

2

> Math.round(2.5)

3

- Math.trunc(x: number): number [ES6]

Removes the decimal fraction of x and returns the resulting integer.

> Math.trunc(2.1)

2

> Math.trunc(2.9)

2

Tbl. 12 shows the results of the rounding functions for a few representative inputs.

Table 12: Rounding functions of Math. Note how things change with negative numbers because “larger” always means “closer to positive infinity”.

-2.9

-2.5

-2.1

2.1

2.5

2.9

Math.floor

-3

-3

-3

2

2

2

Math.ceil

-2

-2

-2

3

3

3

Math.round

-3

-2

-2

2

3

3

Math.trunc

-2

-2

-2

2

2

2

Trigonometric Functions

All angles are specified in radians. Use the following two functions to convert between degrees and radians.

function degreesToRadians(degrees) {

return degrees / 180 * Math.PI;

}

assert.equal(degreesToRadians(90), Math.PI/2);

function radiansToDegrees(radians) {

return radians / Math.PI * 180;

}

assert.equal(radiansToDegrees(Math.PI), 180);

- Math.acos(x: number): number [ES1]

Returns the arc cosine (inverse cosine) of x.

> Math.acos(0)

1.5707963267948966

> Math.acos(1)

0

- Math.acosh(x: number): number [ES6]

Returns the inverse hyperbolic cosine of x.

- Math.asin(x: number): number [ES1]

Returns the arc sine (inverse sine) of x.

> Math.asin(0)

0

> Math.asin(1)

1.5707963267948966

- Math.asinh(x: number): number [ES6]

Returns the inverse hyperbolic sine of x.

- Math.atan(x: number): number [ES1]

Returns the arc tangent (inverse tangent) of x.

- Math.atanh(x: number): number [ES6]

Returns the inverse hyperbolic tangent of x.

- Math.atan2(y: number, x: number): number [ES1]

Returns the arc tangent of the quotient y/x.

- Math.cos(x: number): number [ES1]

Returns the cosine of x.

> Math.cos(0)

1

> Math.cos(Math.PI)

-1

- Math.cosh(x: number): number [ES6]

Returns the hyperbolic cosine of x.

- Math.hypot(...values: number[]): number [ES6]

Returns the square root of the sum of the squares of values (Pythagoras’ theorem):

> Math.hypot(3, 4)

5

- Math.sin(x: number): number [ES1]

Returns the sine of x.

> Math.sin(0)

0

> Math.sin(Math.PI / 2)

1

- Math.sinh(x: number): number [ES6]

Returns the hyperbolic sine of x.

- Math.tan(x: number): number [ES1]

Returns the tangent of x.

> Math.tan(0)

0

> Math.tan(1)

1.5574077246549023

- Math.tanh(x: number): number; [ES6]

Returns the hyperbolic tangent of x.

Various other functions

- Math.abs(x: number): number [ES1]

Returns the absolute value of x.

> Math.abs(3)

3

> Math.abs(-3)

3

> Math.abs(0)

0

- Math.clz32(x: number): number [ES6]

Counts the leading zero bits in the 32-bit integer x. Used in DSP algorithms.

> Math.clz32(0b01000000000000000000000000000000)

1

> Math.clz32(0b00100000000000000000000000000000)

2

> Math.clz32(2)

30

> Math.clz32(1)

31

- Math.max(...values: number[]): number [ES1]

Converts values to numbers and returns the largest one.

> Math.max(3, -5, 24)

24

- Math.min(...values: number[]): number [ES1]

Converts values to numbers and returns the smallest one.

> Math.min(3, -5, 24)

-5

- Math.random(): number [ES1]

Returns a pseudo-random number n where 0 ≤ n < 1.

/** Returns a random integer i with 0 <= i < max */

function getRandomInteger(max) {

return Math.floor(Math.random() * max);

}

- Math.sign(x: number): number [ES6]

Returns the sign of a number:

> Math.sign(-8)

-1

> Math.sign(0)

0

> Math.sign(3)

1

Sources

- Wikipedia

- TypeScript’s built-in typings

- MDN web docs for JavaScript

- ECMAScript language specification

Bigints – arbitrary-precision integers [ES2020] (advanced)

- 18.1 Why bigints?

- 18.2 Bigints

- 18.2.1 Going beyond 53 bits for integers

- 18.2.2 Example: using bigints

- 18.3 Bigint literals

- 18.3.1 Underscores (_) as separators in bigint literals [ES2021]

- 18.4 Reusing number operators for bigints (overloading)

- 18.4.1 Arithmetic operators

- 18.4.2 Ordering operators

- 18.4.3 Bitwise operators

- 18.4.4 Loose equality (==) and inequality (!=)

- 18.4.5 Strict equality (===) and inequality (!==)

- 18.5 The wrapper constructor BigInt

- 18.5.1 BigInt as a constructor and as a function

- 18.5.2 BigInt.prototype.* methods

- 18.5.3 BigInt.* methods

- 18.5.4 Casting and 64-bit integers

- 18.6 Coercing bigints to other primitive types

- 18.7 TypedArrays and DataView operations for 64-bit values

- 18.8 Bigints and JSON

- 18.8.1 Stringifying bigints

- 18.8.2 Parsing bigints

- 18.9 FAQ: Bigints

- 18.9.1 How do I decide when to use numbers and when to use bigints?

- 18.9.2 Why not just increase the precision of numbers in the same manner as is done for bigints?

In this chapter, we take a look at bigints, JavaScript’s integers whose storage space grows and shrinks as needed.

Why bigints?

Before ECMAScript 2020, JavaScript handled integers as follows:

- There only was a single type for floating point numbers and integers: 64-bit floating point numbers (IEEE 754 double precision).

- Under the hood, most JavaScript engines transparently supported integers: If a number has no decimal digits and is within a certain range, it can internally be stored as a genuine integer. This representation is called small integer and usually fits into 32 bits. For example, the range of small integers on the 64-bit version of the V8 engine is from −2^31 to 2^31−1 (source).

- JavaScript numbers could also represent integers beyond the small integer range, as floating point numbers. Here, the safe range is plus/minus 53 bits. For more information on this topic, see §16.9.3 “Safe integers”.

Sometimes, we need more than signed 53 bits – for example:

- Twitter uses 64-bit integers as IDs for tweets (source). In JavaScript, these IDs had to be stored in strings.

- Financial technology uses so-called big integers (integers with arbitrary precision) to represent amounts of money. Internally, the amounts are multiplied so that the decimal numbers disappear. For example, USD amounts are multiplied by 100 so that the cents disappear.

Bigints

Bigint is a new primitive data type for integers. Bigints don’t have a fixed storage size in bits; their sizes adapt to the integers they represent:

- Small integers are represented with fewer bits than large integers.

- There is no negative lower limit or positive upper limit for the integers that can be represented.

A bigint literal is a sequence of one or more digits, suffixed with an n – for example:

123n

Operators such as - and * are overloaded and work with bigints:

> 123n * 456n

56088n

Bigints are primitive values. typeof returns a new result for them:

> typeof 123n

'bigint'

Going beyond 53 bits for integers

JavaScript numbers are internally represented as a fraction multiplied by an exponent (see §16.8 “Background: floating point precision” for details). As a consequence, if we go beyond the highest safe integer 2^53−1, there are still some integers that can be represented, but with gaps between them:

> 2**53 - 2 // safe

9007199254740990

> 2**53 - 1 // safe

9007199254740991

> 2**53 // unsafe, same as next integer

9007199254740992

> 2**53 + 1

9007199254740992

> 2**53 + 2

9007199254740994

> 2**53 + 3

9007199254740996

> 2**53 + 4

9007199254740996

> 2**53 + 5

9007199254740996

Bigints enable us to go beyond 53 bits:

> 2n**53n

9007199254740992n

> 2n**53n + 1n

9007199254740993n

> 2n**53n + 2n

9007199254740994n

Example: using bigints

This is what using bigints looks like (code based on an example in the proposal):

/**

* Takes a bigint as an argument and returns a bigint

*/

function nthPrime(nth) {

if (typeof nth !== 'bigint') {

throw new TypeError();

}

function isPrime(p) {

for (let i = 2n; i < p; i++) {

if (p % i === 0n) return false;

}

return true;

}

for (let i = 2n; ; i++) {

if (isPrime(i)) {

if (--nth === 0n) return i;

}

}

}

assert.deepEqual(

[1n, 2n, 3n, 4n, 5n].map(nth => nthPrime(nth)),

[2n, 3n, 5n, 7n, 11n]

);

Bigint literals

Like number literals, bigint literals support several bases:

- Decimal: 123n

- Hexadecimal: 0xFFn

- Binary: 0b1101n

- Octal: 0o777n

Negative bigints are produced by prefixing the unary minus operator: -0123n

Underscores (_) as separators in bigint literals [ES2021]

Just like in number literals, we can use underscores (_) as separators in bigint literals:

const massOfEarthInKg = 6_000_000_000_000_000_000_000_000n;

Bigints are often used to represent money in the financial technical sector. Separators can help here, too:

const priceInCents = 123_000_00n; // 123 thousand dollars

As with number literals, two restrictions apply:

- We can only put an underscore between two digits.

- We can use at most one underscore in a row.

Reusing number operators for bigints (overloading)

With most operators, we are not allowed to mix bigints and numbers. If we do, exceptions are thrown:

> 2n + 1

TypeError: Cannot mix BigInt and other types, use explicit conversions

The reason for this rule is that there is no general way of coercing a number and a bigint to a common type: numbers can’t represent bigints beyond 53 bits, bigints can’t represent fractions. Therefore, the exceptions warn us about typos that may lead to unexpected results.

For example, should the result of the following expression be 9007199254740993n or 9007199254740992?

2**53 + 1n

It is also not clear what the result of the following expression should be:

2n**53n * 3.3

Arithmetic operators

Binary +, binary -, *, ** work as expected:

> 7n * 3n

21n

It is OK to mix bigints and strings:

> 6n + ' apples'

'6 apples'

/, % round towards zero (like Math.trunc()):

> 1n / 2n

0n

Unary - works as expected:

> -(-64n)

64n

Unary + is not supported for bigints because much code relies on it coercing its operand to number:

> +23n

TypeError: Cannot convert a BigInt value to a number

Ordering operators

Ordering operators <, >, >=, <= work as expected:

> 17n <= 17n

true

> 3n > -1n

true

Comparing bigints and numbers does not pose any risks. Therefore, we can mix bigints and numbers:

> 3n > -1

true

Bitwise operators

Bitwise operators for numbers

Bitwise operators interpret numbers as 32-bit integers. These integers are either unsigned or signed. If they are signed, the negative of an integer is its two’s complement (adding an integer to its two’s complement – while ignoring overflow – produces zero):

> 2**32-1 >> 0

-1

Due to these integers having a fixed size, their highest bits indicate their signs:

> 2**31 >> 0 // highest bit is 1

-2147483648

> 2**31 - 1 >> 0 // highest bit is 0

2147483647

Bitwise operators for bigints

For bigints, bitwise operators interpret a negative sign as an infinite two’s complement – for example:

- -1 is ···111111 (ones extend infinitely to the left)

- -2 is ···111110

- -3 is ···111101

- -4 is ···111100

That is, a negative sign is more of an external flag and not represented as an actual bit.

Bitwise Not (~)

Bitwise Not (~) inverts all bits:

> ~0b10n

-3n

> ~0n

-1n

> ~-2n

1n

Binary bitwise operators (&, |, ^)

Applying binary bitwise operators to bigints works analogously to applying them to numbers:

> (0b1010n |  0b0111n).toString(2)

'1111'

> (0b1010n &  0b0111n).toString(2)

'10'

> (0b1010n | -1n).toString(2)

'-1'

> (0b1010n & -1n).toString(2)

'1010'

Bitwise signed shift operators (<< and >>)

The signed shift operators for bigints preserve the sign of a number:

> 2n << 1n

4n

> -2n << 1n

-4n

> 2n >> 1n

1n

> -2n >> 1n

-1n

Recall that -1n is a sequence of ones that extends infinitely to the left. That’s why shifting it left doesn’t change it:

> -1n >> 20n

-1n

Bitwise unsigned right shift operator (>>>)

There is no unsigned right shift operator for bigints:

> 2n >>> 1n

TypeError: BigInts have no unsigned right shift, use >> instead

Why? The idea behind unsigned right shifting is that a zero is shifted in “from the left”. In other words, the assumption is that there is a finite amount of binary digits.

However, with bigints, there is no “left”, their binary digits extend infinitely. This is especially important with negative numbers.

Signed right shift works even with an infinite number of digits because the highest digit is preserved. Therefore, it can be adapted to bigints.

Loose equality (==) and inequality (!=)

Loose equality (==) and inequality (!=) coerce values:

> 0n == false

true

> 1n == true

true

> 123n == 123

true

> 123n == '123'

true

Strict equality (===) and inequality (!==)

Strict equality (===) and inequality (!==) only consider values to be equal if they have the same type:

> 123n === 123

false

> 123n === 123n

true

The wrapper constructor BigInt

Analogously to numbers, bigints have the associated wrapper constructor BigInt.

BigInt as a constructor and as a function

- new BigInt(): throws a TypeError.

- BigInt(x) converts arbitrary values x to bigint. This works similarly to Number(), with several differences which are summarized in tbl. 13 and explained in more detail in the following subsections.

Table 13: Converting values to bigints.

x

BigInt(x)

undefined

Throws TypeError

null

Throws TypeError

boolean

false → 0n, true → 1n

number

Example: 123 → 123n

Non-integer → throws RangeError

bigint

x   (no change)

string

Example: '123' → 123n

Unparsable → throws SyntaxError

symbol

Throws TypeError

object

Configurable (e.g. via .valueOf()  )

Converting undefined and null

A TypeError is thrown if x is either undefined or null:

> BigInt(undefined)

TypeError: Cannot convert undefined to a BigInt

> BigInt(null)

TypeError: Cannot convert null to a BigInt

Converting strings

If a string does not represent an integer, BigInt() throws a SyntaxError (whereas Number() returns the error value NaN):

> BigInt('abc')

SyntaxError: Cannot convert abc to a BigInt

The suffix 'n' is not allowed:

> BigInt('123n')

SyntaxError: Cannot convert 123n to a BigInt

All bases of bigint literals are allowed:

> BigInt('123')

123n

> BigInt('0xFF')

255n

> BigInt('0b1101')

13n

> BigInt('0o777')

511n

Non-integer numbers produce exceptions

> BigInt(123.45)

RangeError: The number 123.45 cannot be converted to a BigInt because

it is not an integer

> BigInt(123)

123n

Converting objects

How objects are converted to bigints can be configured – for example, by overriding .valueOf():

> BigInt({valueOf() {return 123n}})

123n

BigInt.prototype.* methods

BigInt.prototype holds the methods “inherited” by primitive bigints:

- BigInt.prototype.toLocaleString(locales?, options?)

- BigInt.prototype.toString(radix?)

- BigInt.prototype.valueOf()

BigInt.* methods

- BigInt.asIntN(width, theInt)

Casts theInt to width bits (signed). This influences how the value is represented internally.

- BigInt.asUintN(width, theInt)

Casts theInt to width bits (unsigned).

Casting and 64-bit integers

Casting allows us to create integer values with a specific number of bits. If we want to restrict ourselves to just 64-bit integers, we have to always cast:

const uint64a = BigInt.asUintN(64, 12345n);

const uint64b = BigInt.asUintN(64, 67890n);

const result = BigInt.asUintN(64, uint64a * uint64b);

Coercing bigints to other primitive types

This table show what happens if we convert bigints to other primitive types:

Convert to

Explicit conversion

Coercion (implicit conversion)

boolean

Boolean(0n) → false

!0n → true

Boolean(int) → true

!int → false

number

Number(7n) → 7   (example)

+int → TypeError   (1)

string

String(7n) → '7'   (example)

''+7n → '7'   (example)

Footnote:

- (1) Unary + is not supported for bigints, because much code relies on it coercing its operand to number.

TypedArrays and DataView operations for 64-bit values

Thanks to bigints, Typed Arrays and DataViews can support 64-bit values:

- Typed Array constructors:

- BigInt64Array

- BigUint64Array

- DataView methods:

- DataView.prototype.getBigInt64()

- DataView.prototype.setBigInt64()

- DataView.prototype.getBigUint64()

- DataView.prototype.setBigUint64()

Bigints and JSON

The JSON standard is fixed and won’t change. The upside is that old JSON parsing code will never be outdated. The downside is that JSON can’t be extended to contain bigints.

Stringifying bigints throws exceptions:

> JSON.stringify(123n)

TypeError: Do not know how to serialize a BigInt

> JSON.stringify([123n])

TypeError: Do not know how to serialize a BigInt

Stringifying bigints

Therefore, our best option is to store bigints in strings:

const bigintPrefix = '[[bigint]]';

function bigintReplacer(_key, value) {

if (typeof value === 'bigint') {

return bigintPrefix + value;

}

return value;

}

const data = { value: 9007199254740993n };

assert.equal(

JSON.stringify(data, bigintReplacer),

'{"value":"[[bigint]]9007199254740993"}'

);

Parsing bigints

The following code shows how to parse strings such as the one that we have produced in the previous example.

function bigintReviver(_key, value) {

if (typeof value === 'string' && value.startsWith(bigintPrefix)) {

return BigInt(value.slice(bigintPrefix.length));

}

return value;

}

const str = '{"value":"[[bigint]]9007199254740993"}';

assert.deepEqual(

JSON.parse(str, bigintReviver),

{ value: 9007199254740993n }

);

FAQ: Bigints

How do I decide when to use numbers and when to use bigints?

My recommendations:

- Use numbers for up to 53 bits and for Array indices. Rationale: They already appear everywhere and are handled efficiently by most engines (especially if they fit into 31 bits). Appearances include:

- Array.prototype.forEach()

- Array.prototype.entries()

- Use bigints for large numeric values: If your fraction-less values don’t fit into 53 bits, you have no choice but to move to bigints.

All existing web APIs return and accept only numbers and will only upgrade to bigint on a case-by-case basis.

Why not just increase the precision of numbers in the same manner as is done for bigints?

One could conceivably split number into integer and double, but that would add many new complexities to the language (several integer-only operators etc.). I’ve sketched the consequences in a Gist.

Acknowledgements:

- Thanks to Daniel Ehrenberg for reviewing an earlier version of this content.

- Thanks to Dan Callahan for reviewing an earlier version of this content.

Unicode – a brief introduction (advanced)

- 19.1 Code points vs. code units

- 19.1.1 Code points

- 19.1.2 Encoding Unicode code points: UTF-32, UTF-16, UTF-8

- 19.2 Encodings used in web development: UTF-16 and UTF-8

- 19.2.1 Source code internally: UTF-16

- 19.2.2 Strings: UTF-16

- 19.2.3 Source code in files: UTF-8

- 19.3 Grapheme clusters – the real characters

- 19.3.1 Grapheme clusters vs. glyphs

Unicode is a standard for representing and managing text in most of the world’s writing systems. Virtually all modern software that works with text, supports Unicode. The standard is maintained by the Unicode Consortium. A new version of the standard is published every year (with new emojis, etc.). Unicode version 1.0.0 was published in October 1991.

Code points vs. code units

Two concepts are crucial for understanding Unicode:

- Code points are numbers that represent the atomic parts of Unicode text. Most of them represent visible symbols but they can also have other meanings such as specifying an aspect of a symbol (the accent of a letter, the skin tone of an emoji, etc.).

- Code units are numbers that encode code points, to store or transmit Unicode text. One or more code units encode a single code point. Each code unit has the same size, which depends on the encoding format that is used. The most popular format, UTF-8, has 8-bit code units.

Code points

The first version of Unicode had 16-bit code points. Since then, the number of characters has grown considerably and the size of code points was extended to 21 bits. These 21 bits are partitioned in 17 planes, with 16 bits each:

- Plane 0: Basic Multilingual Plane (BMP), 0x0000–0xFFFF

- Contains characters for almost all modern languages (Latin characters, Asian characters, etc.) and many symbols.

- Plane 1: Supplementary Multilingual Plane (SMP), 0x10000–0x1FFFF

- Supports historic writing systems (e.g., Egyptian hieroglyphs and cuneiform) and additional modern writing systems.

- Supports emojis and many other symbols.

- Plane 2: Supplementary Ideographic Plane (SIP), 0x20000–0x2FFFF

- Contains additional CJK (Chinese, Japanese, Korean) ideographs.

- Plane 3–13: Unassigned

- Plane 14: Supplementary Special-Purpose Plane (SSP), 0xE0000–0xEFFFF

- Contains non-graphical characters such as tag characters and glyph variation selectors.

- Plane 15–16: Supplementary Private Use Area (S PUA A/B), 0x0F0000–0x10FFFF

- Available for character assignment by parties outside the ISO and the Unicode Consortium. Not standardized.

Planes 1-16 are called supplementary planes or astral planes.

Let’s check the code points of a few characters:

> 'A'.codePointAt(0).toString(16)

'41'

> 'ü'.codePointAt(0).toString(16)

'fc'

> 'π'.codePointAt(0).toString(16)

'3c0'

> '🙂'.codePointAt(0).toString(16)

'1f642'

The hexadecimal numbers of the code points tell us that the first three characters reside in plane 0 (within 16 bits), while the emoji resides in plane 1.

Encoding Unicode code points: UTF-32, UTF-16, UTF-8

The main ways of encoding code points are three Unicode Transformation Formats (UTFs): UTF-32, UTF-16, UTF-8. The number at the end of each format indicates the size (in bits) of its code units.

UTF-32 (Unicode Transformation Format 32)

UTF-32 uses 32 bits to store code units, resulting in one code unit per code point. This format is the only one with fixed-length encoding; all others use a varying number of code units to encode a single code point.

UTF-16 (Unicode Transformation Format 16)

UTF-16 uses 16-bit code units. It encodes code points as follows:

- The BMP (first 16 bits of Unicode) is stored in single code units.

- Astral planes: The BMP comprises 0x10_000 code points. Given that Unicode has a total of 0x110_000 code points, we still need to encode the remaining 0x100_000 code points (20 bits). The BMP has two ranges of unassigned code points that provide the necessary storage:

- Most significant 10 bits (leading surrogate): 0xD800-0xDBFF

- Least significant 10 bits (trailing surrogate): 0xDC00-0xDFFF

In other words, the two hexadecimal digits at the end contribute 8 bits. But we can only use those 8 bits if a BMP starts with one of the following 2-digit pairs:

- D8, D9, DA, DB

- DC, DD, DE, DF

Per surrogate, we have a choice between 4 pairs, which is where the remaining 2 bits come from.

As a consequence, each UTF-16 code unit is always either a leading surrogate, a trailing surrogate, or encodes a BMP code point.

These are two examples of UTF-16-encoded code points:

- Code point 0x03C0 (π) is in the BMP and can therefore be represented by a single UTF-16 code unit: 0x03C0.

- Code point 0x1F642 (🙂) is in an astral plane and represented by two code units: 0xD83D and 0xDE42.

UTF-8 (Unicode Transformation Format 8)

UTF-8 has 8-bit code units. It uses 1–4 code units to encode a code point:

Code points

Code units

0000–007F

0bbbbbbb (7 bits)

0080–07FF

110bbbbb, 10bbbbbb (5+6 bits)

0800–FFFF

1110bbbb, 10bbbbbb, 10bbbbbb (4+6+6 bits)

10000–1FFFFF

11110bbb, 10bbbbbb, 10bbbbbb, 10bbbbbb (3+6+6+6 bits)

Notes:

- The bit prefix of each code unit tells us:

- Is it first in a series of code units? If yes, how many code units will follow?

- Is it second or later in a series of code units?

- The character mappings in the 0000–007F range are the same as ASCII, which leads to a degree of backward compatibility with older software.

Three examples:

Character

Code point

Code units

A

0x0041

01000001

π

0x03C0

11001111, 10000000

🙂

0x1F642

11110000, 10011111, 10011001, 10000010

Encodings used in web development: UTF-16 and UTF-8

The Unicode encoding formats that are used in web development are: UTF-16 and UTF-8.

Source code internally: UTF-16

The ECMAScript specification internally represents source code as UTF-16.

Strings: UTF-16

The characters in JavaScript strings are based on UTF-16 code units:

> const smiley = '🙂';

> smiley.length

2

> smiley === '\uD83D\uDE42' // code units

true

For more information on Unicode and strings, consult §20.7 “Atoms of text: code points, JavaScript characters, grapheme clusters”.

Source code in files: UTF-8

HTML and JavaScript are almost always encoded as UTF-8 these days.

For example, this is how HTML files usually start now:

<!doctype html>

<html>

<head>

<meta charset="UTF-8">

···

For HTML modules loaded in web browsers, the standard encoding is also UTF-8.

Grapheme clusters – the real characters

The concept of a character becomes remarkably complex once we consider the various writing systems of the world. That’s why there are several different Unicode terms that all mean “character” in some way: code point, grapheme cluster, glyph, etc.

In Unicode, a code point is an atomic part of text.

However, a grapheme cluster corresponds most closely to a symbol displayed on screen or paper. It is defined as “a horizontally segmentable unit of text”. Therefore, official Unicode documents also call it a user-perceived character. One or more code points are needed to encode a grapheme cluster.

For example, the Devanagari kshi is encoded by 4 code points. We use Array.from() to split a string into an Array with code points (for details, consult §20.7.1 “Working with code points”):

Flag emojis are also grapheme clusters and composed of two code points – for example, the flag of Japan:

Grapheme clusters vs. glyphs

A symbol is an abstract concept and part of written language:

- It is represented in computer memory by a grapheme cluster – a sequence of one or more numbers (code points).

- It is drawn on screen via glyphs. A glyph is an image and usually stored in a font. More than one glyph may be used to draw a single symbol – for example, the symbol “é” may be drawn by combining the glyph “e” with the glyph “´”.

The distinction between a concept and its representation is subtle and can blur when talking about Unicode.

More information on grapheme clusters

For more information, consult “Let’s Stop Ascribing Meaning to Code Points” by Manish Goregaokar.

Quiz

See quiz app.

Strings

- 20.1 Cheat sheet: strings

- 20.1.1 Working with strings

- 20.1.2 JavaScript characters vs. code points vs. grapheme clusters

- 20.1.3 String methods

- 20.2 Plain string literals

- 20.2.1 Escaping

- 20.3 Accessing JavaScript characters

- 20.4 String concatenation via +

- 20.5 Converting to string

- 20.5.1 Stringifying objects

- 20.5.2 Customizing the stringification of objects

- 20.5.3 An alternate way of stringifying values

- 20.6 Comparing strings

- 20.7 Atoms of text: code points, JavaScript characters, grapheme clusters

- 20.7.1 Working with code points

- 20.7.2 Working with code units (char codes)

- 20.7.3 ASCII escapes

- 20.7.4 Caveat: grapheme clusters

- 20.8 Quick reference: Strings

- 20.8.1 Converting to string

- 20.8.2 Numeric values of text atoms

- 20.8.3 String.prototype: finding and matching

- 20.8.4 String.prototype: extracting

- 20.8.5 String.prototype: combining

- 20.8.6 String.prototype: transforming

- 20.8.7 Sources

Cheat sheet: strings

Strings are primitive values in JavaScript and immutable. That is, string-related operations always produce new strings and never change existing strings.

Working with strings

Literals for strings:

const str1 = 'Don\'t say "goodbye"'; // string literal

const str2 = "Don't say \"goodbye\""; // string literals

assert.equal(

`As easy as ${123}!`, // template literal

'As easy as 123!',

);

Backslashes are used to:

- Escape literal delimiters (first 2 lines of previous example)

- Represent special characters:

- \\ represents a backslash

- \n represents a newline

- \r represents a carriage return

- \t represents a tab

Inside a String.raw tagged template (line A), backslashes are treated as normal characters:

assert.equal(

String.raw`\ \n\t`, // (A)

'\\ \\n\\t',

);

Convertings values to strings:

> String(undefined)

'undefined'

> String(null)

'null'

> String(123.45)

'123.45'

> String(true)

'true'

Copying parts of a string

// There is no type for characters;

// reading characters produces strings:

const str3 = 'abc';

assert.equal(

str3[2], 'c' // no negative indices allowed

);

assert.equal(

str3.at(-1), 'c' // negative indices allowed

);

// Copying more than one character:

assert.equal(

'abc'.slice(0, 2), 'ab'

);

Concatenating strings:

assert.equal(

'I bought ' + 3 + ' apples',

'I bought 3 apples',

);

let str = '';

str += 'I bought ';

str += 3;

str += ' apples';

assert.equal(

str, 'I bought 3 apples',

);

JavaScript characters vs. code points vs. grapheme clusters

JavaScript characters are 16 bits in size. They are what is indexed in strings and what .length counts.

Code points are the atomic parts of Unicode text. Most of them fit into one JavaScript character, some of them occupy two (especially emojis):

assert.equal(

'A'.length, 1

);

assert.equal(

'🙂'.length, 2

);

Grapheme clusters (user-perceived characters) represent written symbols. Each one comprises one or more code points.

Due to these facts, we shouldn’t split text into JavaScript characters, we should split it into graphemes. For more information on how to handle text, see §20.7 “Atoms of text: code points, JavaScript characters, grapheme clusters”.

String methods

This subsection gives a brief overview of the string API. There is a more comprehensive quick reference at the end of this chapter.

Finding substrings:

> 'abca'.includes('a')

true

> 'abca'.startsWith('ab')

true

> 'abca'.endsWith('ca')

true

> 'abca'.indexOf('a')

0

> 'abca'.lastIndexOf('a')

3

Splitting and joining:

assert.deepEqual(

'a, b,c'.split(/, ?/),

['a', 'b', 'c']

);

assert.equal(

['a', 'b', 'c'].join(', '),

'a, b, c'

);

Padding and trimming:

> '7'.padStart(3, '0')

'007'

> 'yes'.padEnd(6, '!')

'yes!!!'

> '\t abc\n '.trim()

'abc'

> '\t abc\n '.trimStart()

'abc\n '

> '\t abc\n '.trimEnd()

'\t abc'

Repeating and changing case:

> '*'.repeat(5)

'*****'

> '= b2b ='.toUpperCase()

'= B2B ='

> 'ΑΒΓ'.toLowerCase()

'αβγ'

Plain string literals

Plain string literals are delimited by either single quotes or double quotes:

const str1 = 'abc';

const str2 = "abc";

assert.equal(str1, str2);

Single quotes are used more often because it makes it easier to mention HTML, where double quotes are preferred.

The next chapter covers template literals, which give us:

- String interpolation

- Multiple lines

- Raw string literals (backslash has no special meaning)

Escaping

The backslash lets us create special characters:

- Unix line break: '\n'

- Windows line break: '\r\n'

- Tab: '\t'

- Backslash: '\\'

The backslash also lets us use the delimiter of a string literal inside that literal:

assert.equal(

'She said: "Let\'s go!"',

"She said: \"Let's go!\"");

Accessing JavaScript characters

JavaScript has no extra data type for characters – characters are always represented as strings.

const str = 'abc';

// Reading a JavaScript character at a given index

assert.equal(str[1], 'b');

// Counting the JavaScript characters in a string:

assert.equal(str.length, 3);

The characters we see on screen are called grapheme clusters. Most of them are represented by single JavaScript characters. However, there are also grapheme clusters (especially emojis) that are represented by multiple JavaScript characters:

> '🙂'.length

2

How that works is explained in §20.7 “Atoms of text: code points, JavaScript characters, grapheme clusters”.

String concatenation via +

If at least one operand is a string, the plus operator (+) converts any non-strings to strings and concatenates the result:

assert.equal(3 + ' times ' + 4, '3 times 4');

The assignment operator += is useful if we want to assemble a string, piece by piece:

let str = ''; // must be `let`!

str += 'Say it';

str += ' one more';

str += ' time';

assert.equal(str, 'Say it one more time');

Concatenating via + is efficient

Using + to assemble strings is quite efficient because most JavaScript engines internally optimize it.

Exercise: Concatenating strings

exercises/strings/concat_string_array_test.mjs

Converting to string

These are three ways of converting a value x to a string:

- String(x)

- ''+x

- x.toString() (does not work for undefined and null)

Recommendation: use the descriptive and safe String().

Examples:

assert.equal(String(undefined), 'undefined');

assert.equal(String(null), 'null');

assert.equal(String(false), 'false');

assert.equal(String(true), 'true');

assert.equal(String(123.45), '123.45');

Pitfall for booleans: If we convert a boolean to a string via String(), we generally can’t convert it back via Boolean():

> String(false)

'false'

> Boolean('false')

true

The only string for which Boolean() returns false, is the empty string.

Stringifying objects

Plain objects have a default string representation that is not very useful:

> String({a: 1})

'[object Object]'

Arrays have a better string representation, but it still hides much information:

> String(['a', 'b'])

'a,b'

> String(['a', ['b']])

'a,b'

> String([1, 2])

'1,2'

> String(['1', '2'])

'1,2'

> String([true])

'true'

> String(['true'])

'true'

> String(true)

'true'

Stringifying functions, returns their source code:

> String(function f() {return 4})

'function f() {return 4}'

Customizing the stringification of objects

We can override the built-in way of stringifying objects by implementing the method toString():

const obj = {

toString() {

return 'hello';

}

};

assert.equal(String(obj), 'hello');

An alternate way of stringifying values

The JSON data format is a text representation of JavaScript values. Therefore, JSON.stringify() can also be used to convert values to strings:

> JSON.stringify({a: 1})

'{"a":1}'

> JSON.stringify(['a', ['b']])

'["a",["b"]]'

The caveat is that JSON only supports null, booleans, numbers, strings, Arrays, and objects (which it always treats as if they were created by object literals).

Tip: The third parameter lets us switch on multiline output and specify how much to indent – for example:

console.log(JSON.stringify({first: 'Jane', last: 'Doe'}, null, 2));

This statement produces the following output:

{

"first": "Jane",

"last": "Doe"

}

Comparing strings

Strings can be compared via the following operators:

< <= > >=

There is one important caveat to consider: These operators compare based on the numeric values of JavaScript characters. That means that the order that JavaScript uses for strings is different from the one used in dictionaries and phone books:

> 'A' < 'B' // ok

true

> 'a' < 'B' // not ok

false

> 'ä' < 'b' // not ok

false

Properly comparing text is beyond the scope of this book. It is supported via the ECMAScript Internationalization API (Intl).

Atoms of text: code points, JavaScript characters, grapheme clusters

Quick recap of §19 “Unicode – a brief introduction”:

- Code points are the atomic parts of Unicode text. Each code point is 21 bits in size.

- JavaScript strings implement Unicode via the encoding format UTF-16. It uses one or two 16-bit code units to encode a single code point.

- Each JavaScript character (as indexed in strings) is a code unit. In the JavaScript standard library, code units are also called char codes.

- Grapheme clusters (user-perceived characters) represent written symbols, as displayed on screen or paper. One or more code points are needed to encode a single grapheme cluster.

The following code demonstrates that a single code point comprises one or two JavaScript characters. We count the latter via .length:

// 3 code points, 3 JavaScript characters:

assert.equal('abc'.length, 3);

// 1 code point, 2 JavaScript characters:

assert.equal('🙂'.length, 2);

The following table summarizes the concepts we have just explored:

Entity

Size

Encoded via

JavaScript character (UTF-16 code unit)

16 bits

–

Unicode code point

21 bits

1–2 code units

Unicode grapheme cluster

1+ code points

Working with code points

Let’s explore JavaScript’s tools for working with code points.

A Unicode code point escape lets us specify a code point hexadecimally (1–5 digits). It produces one or two JavaScript characters.

> '\u{1F642}'

'🙂'

Unicode escape sequences

In the ECMAScript language specification, Unicode code point escapes and Unicode code unit escapes (which we’ll encounter later) are called Unicode escape sequences.

String.fromCodePoint() converts a single code point to 1–2 JavaScript characters:

> String.fromCodePoint(0x1F642)

'🙂'

.codePointAt() converts 1–2 JavaScript characters to a single code point:

> '🙂'.codePointAt(0).toString(16)

'1f642'

We can iterate over a string, which visits code points (not JavaScript characters). Iteration is described later in this book. One way of iterating is via a for-of loop:

const str = '🙂a';

assert.equal(str.length, 3);

for (const codePointChar of str) {

console.log(codePointChar);

}

// Output:

// '🙂'

// 'a'

Array.from() is also based on iteration and visits code points:

> Array.from('🙂a')

[ '🙂', 'a' ]

That makes it a good tool for counting code points:

> Array.from('🙂a').length

2

> '🙂a'.length

3

Working with code units (char codes)

Indices and lengths of strings are based on JavaScript characters (as represented by UTF-16 code units).

To specify a code unit hexadecimally, we can use a Unicode code unit escape with exactly four hexadecimal digits:

> '\uD83D\uDE42'

'🙂'

And we can use String.fromCharCode(). Char code is the standard library’s name for code unit:

> String.fromCharCode(0xD83D) + String.fromCharCode(0xDE42)

'🙂'

To get the char code of a character, use .charCodeAt():

> '🙂'.charCodeAt(0).toString(16)

'd83d'

ASCII escapes

If the code point of a character is below 256, we can refer to it via a ASCII escape with exactly two hexadecimal digits:

> 'He\x6C\x6Co'

'Hello'

(The official name of ASCII escapes is Hexadecimal escape sequences – it was the first escape that used hexadecimal numbers.)

Caveat: grapheme clusters

When working with text that may be written in any human language, it’s best to split at the boundaries of grapheme clusters, not at the boundaries of code points.

TC39 is working on Intl.Segmenter, a proposal for the ECMAScript Internationalization API to support Unicode segmentation (along grapheme cluster boundaries, word boundaries, sentence boundaries, etc.).

Until that proposal becomes a standard, we can use one of several libraries that are available (do a web search for “JavaScript grapheme”).

Quick reference: Strings

Converting to string

Tbl. 14 describes how various values are converted to strings.

Table 14: Converting values to strings.

x

String(x)

undefined

'undefined'

null

'null'

boolean

false → 'false', true → 'true'

number

Example: 123 → '123'

bigint

Example: 123n → '123'

string

x   (input, unchanged)

symbol

Example: Symbol('abc') → 'Symbol(abc)'

object

Configurable via, e.g., toString()

Numeric values of text atoms

- Char code: number representing a JavaScript character. JavaScript’s name for Unicode code unit.

- Size: 16 bits, unsigned

- Convert number to string: String.fromCharCode() [ES1]

- Convert string to number: string method .charCodeAt() [ES1]

- Code point: number representing an atomic part of Unicode text.

- Size: 21 bits, unsigned (17 planes, 16 bits each)

- Convert number to string: String.fromCodePoint() [ES6]

- Convert string to number: string method .codePointAt() [ES6]

String.prototype: finding and matching

(String.prototype is where the methods of strings are stored.)

- .endsWith(searchString: string, endPos=this.length): boolean [ES6]

Returns true if the string would end with searchString if its length were endPos. Returns false otherwise.

> 'foo.txt'.endsWith('.txt')

true

> 'abcde'.endsWith('cd', 4)

true

- .includes(searchString: string, startPos=0): boolean [ES6]

Returns true if the string contains the searchString and false otherwise. The search starts at startPos.

> 'abc'.includes('b')

true

> 'abc'.includes('b', 2)

false

- .indexOf(searchString: string, minIndex=0): number [ES1]

Returns the lowest index at which searchString appears within the string or -1, otherwise. Any returned index will beminIndex` or higher.

> 'abab'.indexOf('a')

0

> 'abab'.indexOf('a', 1)

2

> 'abab'.indexOf('c')

-1

- .lastIndexOf(searchString: string, maxIndex=Infinity): number [ES1]

Returns the highest index at which searchString appears within the string or -1, otherwise. Any returned index will bemaxIndex` or lower.

> 'abab'.lastIndexOf('ab', 2)

2

> 'abab'.lastIndexOf('ab', 1)

0

> 'abab'.lastIndexOf('ab')

2

- [1 of 2] .match(regExp: string | RegExp): RegExpMatchArray | null [ES3]

If regExp is a regular expression with flag /g not set, then .match() returns the first match for regExp within the string. Or null if there is no match. If regExp is a string, it is used to create a regular expression (think parameter of new RegExp()) before performing the previously mentioned steps.

The result has the following type:

interface RegExpMatchArray extends Array<string> {

index: number;

input: string;

groups: undefined | {

[key: string]: string

};

}

Numbered capture groups become Array indices (which is why this type extends Array). Named capture groups (ES2018) become properties of .groups. In this mode, .match() works like RegExp.prototype.exec().

Examples:

> 'ababb'.match(/a(b+)/)

{ 0: 'ab', 1: 'b', index: 0, input: 'ababb', groups: undefined }

> 'ababb'.match(/a(?<foo>b+)/)

{ 0: 'ab', 1: 'b', index: 0, input: 'ababb', groups: { foo: 'b' } }

> 'abab'.match(/x/)

null

- [2 of 2] .match(regExp: RegExp): string[] | null [ES3]

If flag /g of regExp is set, .match() returns either an Array with all matches or null if there was no match.

> 'ababb'.match(/a(b+)/g)

[ 'ab', 'abb' ]

> 'ababb'.match(/a(?<foo>b+)/g)

[ 'ab', 'abb' ]

> 'abab'.match(/x/g)

null

- .search(regExp: string | RegExp): number [ES3]

Returns the index at which regExp occurs within the string. If regExp is a string, it is used to create a regular expression (think parameter of new RegExp()).

> 'a2b'.search(/[0-9]/)

1

> 'a2b'.search('[0-9]')

1

- .startsWith(searchString: string, startPos=0): boolean [ES6]

Returns true if searchString occurs in the string at index startPos. Returns false otherwise.

> '.gitignore'.startsWith('.')

true

> 'abcde'.startsWith('bc', 1)

true

String.prototype: extracting

- .slice(start=0, end=this.length): string [ES3]

Returns the substring of the string that starts at (including) index start and ends at (excluding) index end. If an index is negative, it is added to .length before it is used (-1 becomes this.length-1, etc.).

> 'abc'.slice(1, 3)

'bc'

> 'abc'.slice(1)

'bc'

> 'abc'.slice(-2)

'bc'

- .at(index: number): string | undefined [ES2022]

Returns the JavaScript character at index as a string. If index is negative, it is added to .length before it is used (-1 becomes this.length-1, etc.).

> 'abc'.at(0)

'a'

> 'abc'.at(-1)

'c'

- .split(separator: string | RegExp, limit?: number): string[] [ES3]

Splits the string into an Array of substrings – the strings that occur between the separators. The separator can be a string:

> 'a | b | c'.split('|')

[ 'a ', ' b ', ' c' ]

It can also be a regular expression:

> 'a : b : c'.split(/ *: */)

[ 'a', 'b', 'c' ]

> 'a : b : c'.split(/( *):( *)/)

[ 'a', ' ', ' ', 'b', ' ', ' ', 'c' ]

The last invocation demonstrates that captures made by groups in the regular expression become elements of the returned Array.

Warning: .split('') splits a string into JavaScript characters. That doesn’t work well when dealing with astral code points (which are encoded as two JavaScript characters). For example, emojis are astral:

> '🙂X🙂'.split('')

[ '\uD83D', '\uDE42', 'X', '\uD83D', '\uDE42' ]

Instead, it is better to use Array.from() (or spreading):

> Array.from('🙂X🙂')

[ '🙂', 'X', '🙂' ]

- .substring(start: number, end=this.length): string [ES1]

Use .slice() instead of this method. .substring() wasn’t implemented consistently in older engines and doesn’t support negative indices.

String.prototype: combining

- .concat(...strings: string[]): string [ES3]

Returns the concatenation of the string and strings. 'a'.concat('b') is equivalent to 'a'+'b'. The latter is much more popular.

> 'ab'.concat('cd', 'ef', 'gh')

'abcdefgh'

- .padEnd(len: number, fillString=' '): string [ES2017]

Appends (fragments of) fillString to the string until it has the desired length len. If it already has or exceeds len, then it is returned without any changes.

> '#'.padEnd(2)

'# '

> 'abc'.padEnd(2)

'abc'

> '#'.padEnd(5, 'abc')

'#abca'

- .padStart(len: number, fillString=' '): string [ES2017]

Prepends (fragments of) fillString to the string until it has the desired length len. If it already has or exceeds len, then it is returned without any changes.

> '#'.padStart(2)

' #'

> 'abc'.padStart(2)

'abc'

> '#'.padStart(5, 'abc')

'abca#'

- .repeat(count=0): string [ES6]

Returns the string, concatenated count times.

> '*'.repeat()

''

> '*'.repeat(3)

'***'

String.prototype: transforming

- .normalize(form: 'NFC'|'NFD'|'NFKC'|'NFKD' = 'NFC'): string [ES6]

Normalizes the string according to the Unicode Normalization Forms.

- [1 of 2] .replaceAll(searchValue: string | RegExp, replaceValue: string): string [ES2021]

What to do if you can’t use .replaceAll()

If .replaceAll() isn’t available on your targeted platform, you can use .replace() instead. How is explained in [content not included].

Replaces all matches of searchValue with replaceValue. If searchValue is a regular expression without flag /g, a TypeError is thrown.

> 'x.x.'.replaceAll('.', '#')

'x#x#'

> 'x.x.'.replaceAll(/./g, '#')

'####'

> 'x.x.'.replaceAll(/./, '#')

TypeError: String.prototype.replaceAll called with

a non-global RegExp argument

Special characters in replaceValue are:

- $$: becomes $

- $n: becomes the capture of numbered group n (alas, $0 stands for the string '$0', it does not refer to the complete match)

- $&: becomes the complete match

- $`: becomes everything before the match

- $': becomes everything after the match

Examples:

> 'a 1995-12 b'.replaceAll(/([0-9]{4})-([0-9]{2})/g, '|$2|')

'a |12| b'

> 'a 1995-12 b'.replaceAll(/([0-9]{4})-([0-9]{2})/g, '|$&|')

'a |1995-12| b'

> 'a 1995-12 b'.replaceAll(/([0-9]{4})-([0-9]{2})/g, '|$`|')

'a |a | b'

Named capture groups (ES2018) are supported, too:

- $<name> becomes the capture of named group name

Example:

assert.equal(

'a 1995-12 b'.replaceAll(

/(?<year>[0-9]{4})-(?<month>[0-9]{2})/g, '|$<month>|'),

'a |12| b');

- [2 of 2] .replaceAll(searchValue: string | RegExp, replacer: (...args: any[]) => string): string [ES2021]

If the second parameter is a function, occurrences are replaced with the strings it returns. Its parameters args are:

- matched: string. The complete match

- g1: string|undefined. The capture of numbered group 1

- g2: string|undefined. The capture of numbered group 2

- (Etc.)

- offset: number. Where was the match found in the input string?

- input: string. The whole input string

const regexp = /([0-9]{4})-([0-9]{2})/g;

const replacer = (all, year, month) => '|' + all + '|';

assert.equal(

'a 1995-12 b'.replaceAll(regexp, replacer),

'a |1995-12| b');

Named capture groups (ES2018) are supported, too. If there are any, an argument is added at the end with an object whose properties contain the captures:

const regexp = /(?<year>[0-9]{4})-(?<month>[0-9]{2})/g;

const replacer = (...args) => {

const groups=args.pop();

return '|' + groups.month + '|';

};

assert.equal(

'a 1995-12 b'.replaceAll(regexp, replacer),

'a |12| b');

- .replace(searchValue: string | RegExp, replaceValue: string): string [ES3]

- .replace(searchValue: string | RegExp, replacer: (...args: any[]) => string): string [ES3]

.replace() works like .replaceAll(), but only replaces the first occurrence if searchValue is a string or a regular expression without /g:

> 'x.x.'.replace('.', '#')

'x#x.'

> 'x.x.'.replace(/./, '#')

'#.x.'

For more information on this method, see [content not included].

- .toUpperCase(): string [ES1]

Returns a copy of the string in which all lowercase alphabetic characters are converted to uppercase. How well that works for various alphabets, depends on the JavaScript engine.

> '-a2b-'.toUpperCase()

'-A2B-'

> 'αβγ'.toUpperCase()

'ΑΒΓ'

- .toLowerCase(): string [ES1]

Returns a copy of the string in which all uppercase alphabetic characters are converted to lowercase. How well that works for various alphabets, depends on the JavaScript engine.

> '-A2B-'.toLowerCase()

'-a2b-'

> 'ΑΒΓ'.toLowerCase()

'αβγ'

- .trim(): string [ES5]

Returns a copy of the string in which all leading and trailing whitespace (spaces, tabs, line terminators, etc.) is gone.

> '\r\n#\t  '.trim()

'#'

> '  abc  '.trim()

'abc'

- .trimEnd(): string [ES2019]

Similar to .trim() but only the end of the string is trimmed:

> '  abc  '.trimEnd()

'  abc'

- .trimStart(): string [ES2019]

Similar to .trim() but only the beginning of the string is trimmed:

> '  abc  '.trimStart()

'abc  '

Sources

- TypeScript’s built-in typings

- MDN web docs for JavaScript

- ECMAScript language specification

Exercise: Using string methods

exercises/strings/remove_extension_test.mjs

Quiz

See quiz app.

Using template literals and tagged templates

- 21.1 Disambiguation: “template”

- 21.2 Template literals

- 21.3 Tagged templates

- 21.3.1 Cooked vs. raw template strings (advanced)

- 21.4 Examples of tagged templates (as provided via libraries)

- 21.4.1 Tag function library: lit-html

- 21.4.2 Tag function library: re-template-tag

- 21.4.3 Tag function library: graphql-tag

- 21.5 Raw string literals

- 21.6 (Advanced)

- 21.7 Multiline template literals and indentation

- 21.7.1 Fix: template tag for dedenting

- 21.7.2 Fix: .trim()

- 21.8 Simple templating via template literals

- 21.8.1 A more complex example

- 21.8.2 Simple HTML-escaping

Before we dig into the two features template literal and tagged template, let’s first examine the multiple meanings of the term template.

Disambiguation: “template”

The following three things are significantly different despite all having template in their names and despite all of them looking similar:

- A text template is a function from data to text. It is frequently used in web development and often defined via text files. For example, the following text defines a template for the library Handlebars:

<div class="entry">

<h1>{{title}}</h1>

<div class="body">

{{body}}

</div>

</div>

This template has two blanks to be filled in: title and body. It is used like this:

// First step: retrieve the template text, e.g. from a text file.

const tmplFunc = Handlebars.compile(TMPL_TEXT); // compile string

const data = {title: 'My page', body: 'Welcome to my page!'};

const html = tmplFunc(data);

- A template literal is similar to a string literal, but has additional features – for example, interpolation. It is delimited by backticks:

const num = 5;

assert.equal(`Count: ${num}!`, 'Count: 5!');

- Syntactically, a tagged template is a template literal that follows a function (or rather, an expression that evaluates to a function). That leads to the function being called. Its arguments are derived from the contents of the template literal.

const getArgs = (...args) => args;

assert.deepEqual(

getArgs`Count: ${5}!`,

[['Count: ', '!'], 5] );

Note that getArgs() receives both the text of the literal and the data interpolated via ${}.

Template literals

A template literal has two new features compared to a normal string literal.

First, it supports string interpolation: if we put a dynamically computed value inside a ${}, it is converted to a string and inserted into the string returned by the literal.

const MAX = 100;

function doSomeWork(x) {

if (x > MAX) {

throw new Error(`At most ${MAX} allowed: ${x}!`);

}

// ···

}

assert.throws(

() => doSomeWork(101),

{message: 'At most 100 allowed: 101!'});

Second, template literals can span multiple lines:

const str = `this is

a text with

multiple lines`;

Template literals always produce strings.

Tagged templates

The expression in line A is a tagged template. It is equivalent to invoking tagFunc() with the arguments listed in the Array in line B.

function tagFunc(...args) {

return args;

}

const setting = 'dark mode';

const value = true;

assert.deepEqual(

tagFunc`Setting ${setting} is ${value}!`, // (A)

[['Setting ', ' is ', '!'], 'dark mode', true] // (B)

);

The function tagFunc before the first backtick is called a tag function. Its arguments are:

- Template strings (first argument): an Array with the text fragments surrounding the interpolations ${}.

- In the example: ['Setting ', ' is ', '!']

- Substitutions (remaining arguments): the interpolated values.

- In the example: 'dark mode' and true

The static (fixed) parts of the literal (the template strings) are kept separate from the dynamic parts (the substitutions).

A tag function can return arbitrary values.

Cooked vs. raw template strings (advanced)

So far, we have only seen the cooked interpretation of template strings. But tag functions actually get two interpretations:

- A cooked interpretation where backslashes have special meaning. For example, \t produces a tab character. This interpretation of the template strings is stored as an Array in the first argument.

- A raw interpretation where backslashes do not have special meaning. For example, \t produces two characters – a backslash and a t. This interpretation of the template strings is stored in property .raw of the first argument (an Array).

The raw interpretation enables raw string literals via String.raw (described later) and similar applications.

The following tag function cookedRaw uses both interpretations:

function cookedRaw(templateStrings, ...substitutions) {

return {

cooked: Array.from(templateStrings), // copy only Array elements

raw: templateStrings.raw,

substitutions,

};

}

assert.deepEqual(

cookedRaw`\tab${'subst'}\newline\\`,

{

cooked: ['\tab', '\newline\\'],

raw:    ['\\tab', '\\newline\\\\'],

substitutions: ['subst'],

});

We can also use Unicode code point escapes (\u{1F642}), Unicode code unit escapes (\u03A9), and ASCII escapes (\x52) in tagged templates:

assert.deepEqual(

cookedRaw`\u{54}\u0065\x78t`,

{

cooked: ['Text'],

raw:    ['\\u{54}\\u0065\\x78t'],

substitutions: [],

});

If the syntax of one of these escapes isn’t correct, the corresponding cooked template string is undefined, while the raw version is still verbatim:

assert.deepEqual(

cookedRaw`\uu\xx ${1} after`,

{

cooked: [undefined, ' after'],

raw:    ['\\uu\\xx ', ' after'],

substitutions: [1],

});

Incorrect escapes produce syntax errors in template literals and string literals. Before ES2018, they even produced errors in tagged templates. Why was that changed? We can now use tagged templates for text that was previously illegal – for example:

windowsPath`C:\uuu\xxx\111`

latex`\unicode`

Examples of tagged templates (as provided via libraries)

Tagged templates are great for supporting small embedded languages (so-called domain-specific languages). We’ll continue with a few examples.

Tag function library: lit-html

lit-html is a templating library that is based on tagged templates and used by the frontend framework Polymer:

import {html, render} from 'lit-html';

const template = (items) => html`

<ul>

${

repeat(items,

(item) => item.id,

(item, index) => html`<li>${index}. ${item.name}</li>`

)

}

</ul>

`;

repeat() is a custom function for looping. Its 2nd parameter produces unique keys for the values returned by the 3rd parameter. Note the nested tagged template used by that parameter.

Tag function library: re-template-tag

re-template-tag is a simple library for composing regular expressions. Templates tagged with re produce regular expressions. The main benefit is that we can interpolate regular expressions and plain text via ${} (line A):

const RE_YEAR = re`(?<year>[0-9]{4})`;

const RE_MONTH = re`(?<month>[0-9]{2})`;

const RE_DAY = re`(?<day>[0-9]{2})`;

const RE_DATE = re`/${RE_YEAR}-${RE_MONTH}-${RE_DAY}/u`; // (A)

const match = RE_DATE.exec('2017-01-27');

assert.equal(match.groups.year, '2017');

Tag function library: graphql-tag

The library graphql-tag lets us create GraphQL queries via tagged templates:

import gql from 'graphql-tag';

const query = gql`

{

user(id: 5) {

firstName

lastName

}

}

`;

Additionally, there are plugins for pre-compiling such queries in Babel, TypeScript, etc.

Raw string literals

Raw string literals are implemented via the tag function String.raw. They are string literals where backslashes don’t do anything special (such as escaping characters, etc.):

assert.equal(String.raw`\back`, '\\back');

This helps whenever data contains backslashes – for example, strings with regular expressions:

const regex1 = /^\./;

const regex2 = new RegExp('^\\.');

const regex3 = new RegExp(String.raw`^\.`);

All three regular expressions are equivalent. With a normal string literal, we have to write the backslash twice, to escape it for that literal. With a raw string literal, we don’t have to do that.

Raw string literals are also useful for specifying Windows filename paths:

const WIN_PATH = String.raw`C:\foo\bar`;

assert.equal(WIN_PATH, 'C:\\foo\\bar');

(Advanced)

All remaining sections are advanced

Multiline template literals and indentation

If we put multiline text in template literals, two goals are in conflict: On one hand, the template literal should be indented to fit inside the source code. On the other hand, the lines of its content should start in the leftmost column.

For example:

function div(text) {

return `

<div>

${text}

</div>

`;

}

console.log('Output:');

console.log(

div('Hello!')

// Replace spaces with mid-dots:

.replace(/ /g, '·')

// Replace \n with #\n:

.replace(/\n/g, '#\n')

);

Due to the indentation, the template literal fits well into the source code. Alas, the output is also indented. And we don’t want the return at the beginning and the return plus two spaces at the end.

Output:

#

····<div>#

······Hello!#

····</div>#

··

There are two ways to fix this: via a tagged template or by trimming the result of the template literal.

Fix: template tag for dedenting

The first fix is to use a custom template tag that removes the unwanted whitespace. It uses the first line after the initial line break to determine in which column the text starts and shortens the indentation everywhere. It also removes the line break at the very beginning and the indentation at the very end. One such template tag is dedent by Desmond Brand:

import dedent from 'dedent';

function divDedented(text) {

return dedent`

<div>

${text}

</div>

`.replace(/\n/g, '#\n');

}

console.log('Output:');

console.log(divDedented('Hello!'));

This time, the output is not indented:

Output:

<div>#

Hello!#

</div>

Fix: .trim()

The second fix is quicker, but also dirtier:

function divDedented(text) {

return `

<div>

${text}

</div>

`.trim().replace(/\n/g, '#\n');

}

console.log('Output:');

console.log(divDedented('Hello!'));

The string method .trim() removes the superfluous whitespace at the beginning and at the end, but the content itself must start in the leftmost column. The advantage of this solution is that we don’t need a custom tag function. The downside is that it looks ugly.

The output is the same as with dedent:

Output:

<div>#

Hello!#

</div>

Simple templating via template literals

While template literals look like text templates, it is not immediately obvious how to use them for (text) templating: A text template gets its data from an object, while a template literal gets its data from variables. The solution is to use a template literal in the body of a function whose parameter receives the templating data – for example:

const tmpl = (data) => `Hello ${data.name}!`;

assert.equal(tmpl({name: 'Jane'}), 'Hello Jane!');

A more complex example

As a more complex example, we’d like to take an Array of addresses and produce an HTML table. This is the Array:

const addresses = [

{ first: '<Jane>', last: 'Bond' },

{ first: 'Lars', last: '<Croft>' },

];

The function tmpl() that produces the HTML table looks as follows:

const tmpl = (addrs) => `

<table>

${addrs.map(

(addr) => `

<tr>

<td>${escapeHtml(addr.first)}</td>

<td>${escapeHtml(addr.last)}</td>

</tr>

`.trim()

).join('')}

</table>

`.trim();

This code contains two templating functions:

- The first one (line 1) takes addrs, an Array with addresses, and returns a string with a table.

- The second one (line 4) takes addr, an object containing an address, and returns a string with a table row. Note the .trim() at the end, which removes unnecessary whitespace.

The first templating function produces its result by wrapping a table element around an Array that it joins into a string (line 10). That Array is produced by mapping the second templating function to each element of addrs (line 3). It therefore contains strings with table rows.

The helper function escapeHtml() is used to escape special HTML characters (line 6 and line 7). Its implementation is shown in the next subsection.

Let us call tmpl() with the addresses and log the result:

console.log(tmpl(addresses));

The output is:

<table>

<tr>

<td>&lt;Jane&gt;</td>

<td>Bond</td>

</tr><tr>

<td>Lars</td>

<td>&lt;Croft&gt;</td>

</tr>

</table>

Simple HTML-escaping

The following function escapes plain text so that it is displayed verbatim in HTML:

function escapeHtml(str) {

return str

.replace(/&/g, '&amp;') // first!

.replace(/>/g, '&gt;')

.replace(/</g, '&lt;')

.replace(/"/g, '&quot;')

.replace(/'/g, '&#39;')

.replace(/`/g, '&#96;')

;

}

assert.equal(

escapeHtml('Rock & Roll'), 'Rock &amp; Roll');

assert.equal(

escapeHtml('<blank>'), '&lt;blank&gt;');

Exercise: HTML templating

Exercise with bonus challenge: exercises/template-literals/templating_test.mjs

Quiz

See quiz app.

Symbols

- 22.1 Symbols are primitives that are also like objects

- 22.1.1 Symbols are primitive values

- 22.1.2 Symbols are also like objects

- 22.2 The descriptions of symbols

- 22.3 Use cases for symbols

- 22.3.1 Symbols as values for constants

- 22.3.2 Symbols as unique property keys

- 22.4 Publicly known symbols

- 22.5 Converting symbols

Symbols are primitives that are also like objects

Symbols are primitive values that are created via the factory function Symbol():

const mySymbol = Symbol('mySymbol');

The parameter is optional and provides a description, which is mainly useful for debugging.

Symbols are primitive values

Symbols are primitive values:

- They have to be categorized via typeof:

const sym = Symbol();

assert.equal(typeof sym, 'symbol');

- They can be property keys in objects:

const obj = {

[sym]: 123,

};

Symbols are also like objects

Even though symbols are primitives, they are also like objects in that each value created by Symbol() is unique and not compared by value:

> Symbol() === Symbol()

false

Prior to symbols, objects were the best choice if we needed values that were unique (only equal to themselves):

const string1 = 'abc';

const string2 = 'abc';

assert.equal(

string1 === string2, true); // not unique

const object1 = {};

const object2 = {};

assert.equal(

object1 === object2, false); // unique

const symbol1 = Symbol();

const symbol2 = Symbol();

assert.equal(

symbol1 === symbol2, false); // unique

The descriptions of symbols

The parameter we pass to the symbol factory function provides a description for the created symbol:

const mySymbol = Symbol('mySymbol');

The description can be accessed in two ways.

First, it is part of the string returned by .toString():

assert.equal(mySymbol.toString(), 'Symbol(mySymbol)');

Second, since ES2019, we can retrieve the description via the property .description:

assert.equal(mySymbol.description, 'mySymbol');

Use cases for symbols

The main use cases for symbols, are:

- Values for constants

- Unique property keys

Symbols as values for constants

Let’s assume you want to create constants representing the colors red, orange, yellow, green, blue, and violet. One simple way of doing so would be to use strings:

const COLOR_BLUE = 'Blue';

On the plus side, logging that constant produces helpful output. On the minus side, there is a risk of mistaking an unrelated value for a color because two strings with the same content are considered equal:

const MOOD_BLUE = 'Blue';

assert.equal(COLOR_BLUE, MOOD_BLUE);

We can fix that problem via symbols:

const COLOR_BLUE = Symbol('Blue');

const MOOD_BLUE = Symbol('Blue');

assert.notEqual(COLOR_BLUE, MOOD_BLUE);

Let’s use symbol-valued constants to implement a function:

const COLOR_RED    = Symbol('Red');

const COLOR_ORANGE = Symbol('Orange');

const COLOR_YELLOW = Symbol('Yellow');

const COLOR_GREEN  = Symbol('Green');

const COLOR_BLUE   = Symbol('Blue');

const COLOR_VIOLET = Symbol('Violet');

function getComplement(color) {

switch (color) {

case COLOR_RED:

return COLOR_GREEN;

case COLOR_ORANGE:

return COLOR_BLUE;

case COLOR_YELLOW:

return COLOR_VIOLET;

case COLOR_GREEN:

return COLOR_RED;

case COLOR_BLUE:

return COLOR_ORANGE;

case COLOR_VIOLET:

return COLOR_YELLOW;

default:

throw new Exception('Unknown color: '+color);

}

}

assert.equal(getComplement(COLOR_YELLOW), COLOR_VIOLET);

Symbols as unique property keys

The keys of properties (fields) in objects are used at two levels:

- The program operates at a base level. The keys at that level reflect the problem domain – the area in which a program solves a problem – for example:

- If a program manages employees, the property keys may be about job titles, salary categories, department IDs, etc.

- If the program is a chess app, the property keys may be about chess pieces, chess boards, player colors, etc.

- ECMAScript and many libraries operate at a meta-level. They manage data and provide services that are not part of the problem domain – for example:

- The standard method .toString() is used by ECMAScript when creating a string representation of an object (line A):

const point = {

x: 7,

y: 4,

toString() {

return `(${this.x}, ${this.y})`;

},

};

assert.equal(

String(point), '(7, 4)'); // (A)

.x and .y are base-level properties – they are used to solve the problem of computing with points. .toString() is a meta-level property – it doesn’t have anything to do with the problem domain.

- The standard ECMAScript method .toJSON()

const point = {

x: 7,

y: 4,

toJSON() {

return [this.x, this.y];

},

};

assert.equal(

JSON.stringify(point), '[7,4]');

.x and .y are base-level properties, .toJSON() is a meta-level property.

The base level and the meta-level of a program must be independent: Base-level property keys should not be in conflict with meta-level property keys.

If we use names (strings) as property keys, we are facing two challenges:

- When a language is first created, it can use any meta-level names it wants. Base-level code is forced to avoid those names. Later, however, when much base-level code already exists, meta-level names can’t be chosen freely, anymore.

- We could introduce naming rules to separate base level and meta-level. For example, Python brackets meta-level names with two underscores: __init__, __iter__, __hash__, etc. However, the meta-level names of the language and the meta-level names of libraries would still exist in the same namespace and can clash.

These are two examples of where the latter was an issue for JavaScript:

- In May 2018, the Array method .flatten() had to be renamed to .flat() because the former name was already used by libraries (source).

- In November 2020, the Array method .item() had to be renamed to .at() because the former name was already used by library (source).

Symbols, used as property keys, help us here: Each symbol is unique and a symbol key never clashes with any other string or symbol key.

Example: a library with a meta-level method

As an example, let’s assume we are writing a library that treats objects differently if they implement a special method. This is what defining a property key for such a method and implementing it for an object would look like:

const specialMethod = Symbol('specialMethod');

const obj = {

_id: 'kf12oi',

[specialMethod]() { // (A)

return this._id;

}

};

assert.equal(obj[specialMethod](), 'kf12oi');

The square brackets in line A enable us to specify that the method must have the key specialMethod. More details are explained in §28.7.2 “Computed keys in object literals”.

Publicly known symbols

Symbols that play special roles within ECMAScript are called publicly known symbols. Examples include:

- Symbol.iterator: makes an object iterable. It’s the key of a method that returns an iterator. For more information on this topic, see [content not included].

- Symbol.hasInstance: customizes how instanceof works. If an object implements a method with that key, it can be used at the right-hand side of that operator. For example:

const PrimitiveNull = {

[Symbol.hasInstance](x) {

return x === null;

}

};

assert.equal(null instanceof PrimitiveNull, true);

- Symbol.toStringTag: influences the default .toString() method.

> String({})

'[object Object]'

> String({ [Symbol.toStringTag]: 'is no money' })

'[object is no money]'

Note: It’s usually better to override .toString().

Exercises: Publicly known symbols

- Symbol.toStringTag: exercises/symbols/to_string_tag_test.mjs

- Symbol.hasInstance: exercises/symbols/has_instance_test.mjs

Converting symbols

What happens if we convert a symbol sym to another primitive type? Tbl. 15 has the answers.

Table 15: The results of converting symbols to other primitive types.

Convert to

Explicit conversion

Coercion (implicit conv.)

boolean

Boolean(sym) →   OK

!sym →   OK

number

Number(sym) → TypeError

sym*2 → TypeError

string

String(sym) →   OK

''+sym → TypeError

sym.toString() →   OK

`${sym}` → TypeError

One key pitfall with symbols is how often exceptions are thrown when converting them to something else. What is the thinking behind that? First, conversion to number never makes sense and should be warned about. Second, converting a symbol to a string is indeed useful for diagnostic output. But it also makes sense to warn about accidentally turning a symbol into a string (which is a different kind of property key):

const obj = {};

const sym = Symbol();

assert.throws(

() => { obj['__'+sym+'__'] = true },

{ message: 'Cannot convert a Symbol value to a string' });

The downside is that the exceptions make working with symbols more complicated. You have to explicitly convert symbols when assembling strings via the plus operator:

> const mySymbol = Symbol('mySymbol');

> 'Symbol I used: ' + mySymbol

TypeError: Cannot convert a Symbol value to a string

> 'Symbol I used: ' + String(mySymbol)

'Symbol I used: Symbol(mySymbol)'

Quiz

See quiz app.

Control flow statements

- 23.1 Controlling loops: break and continue

- 23.1.1 break

- 23.1.2 break plus label: leaving any labeled statement

- 23.1.3 continue

- 23.2 Conditions of control flow statements

- 23.3 if statements [ES1]

- 23.3.1 The syntax of if statements

- 23.4 switch statements [ES3]

- 23.4.1 A first example of a switch statement

- 23.4.2 Don’t forget to return or break!

- 23.4.3 Empty case clauses

- 23.4.4 Checking for illegal values via a default clause

- 23.5 while loops [ES1]

- 23.5.1 Examples of while loops

- 23.6 do-while loops [ES3]

- 23.7 for loops [ES1]

- 23.7.1 Examples of for loops

- 23.8 for-of loops [ES6]

- 23.8.1 const: for-of vs. for

- 23.8.2 Iterating over iterables

- 23.8.3 Iterating over [index, element] pairs of Arrays

- 23.9 for-await-of loops [ES2018]

- 23.10 for-in loops (avoid) [ES1]

- 23.11 Recomendations for looping

This chapter covers the following control flow statements:

- if statement [ES1]

- switch statement [ES3]

- while loop [ES1]

- do-while loop [ES3]

- for loop [ES1]

- for-of loop [ES6]

- for-await-of loop [ES2018]

- for-in loop [ES1]

Controlling loops: break and continue

The two operators break and continue can be used to control loops and other statements while we are inside them.

break

There are two versions of break: one with an operand and one without an operand. The latter version works inside the following statements: while, do-while, for, for-of, for-await-of, for-in and switch. It immediately leaves the current statement:

for (const x of ['a', 'b', 'c']) {

console.log(x);

if (x === 'b') break;

console.log('---')

}

// Output:

// 'a'

// '---'

// 'b'

break plus label: leaving any labeled statement

break with an operand works everywhere. Its operand is a label. Labels can be put in front of any statement, including blocks. break my_label leaves the statement whose label is my_label:

my_label: { // label

if (condition) break my_label; // labeled break

// ···

}

In the following example, the search can either:

- Fail: The loop finishes without finding a result. That is handled directly after the loop (line B).

- Succeed: While looping, we find a result. Then we use break plus label (line A) to skip the code that handles failure.

function findSuffix(stringArray, suffix) {

let result;

search_block: {

for (const str of stringArray) {

if (str.endsWith(suffix)) {

// Success:

result = str;

break search_block; // (A)

}

} // for

// Failure:

result = '(Untitled)'; // (B)

} // search_block

return { suffix, result };

// Same as: {suffix: suffix, result: result}

}

assert.deepEqual(

findSuffix(['notes.txt', 'index.html'], '.html'),

{ suffix: '.html', result: 'index.html' }

);

assert.deepEqual(

findSuffix(['notes.txt', 'index.html'], '.mjs'),

{ suffix: '.mjs', result: '(Untitled)' }

);

continue

continue only works inside while, do-while, for, for-of, for-await-of, and for-in. It immediately leaves the current loop iteration and continues with the next one – for example:

const lines = [

'Normal line',

'# Comment',

'Another normal line',

];

for (const line of lines) {

if (line.startsWith('#')) continue;

console.log(line);

}

// Output:

// 'Normal line'

// 'Another normal line'

Conditions of control flow statements

if, while, and do-while have conditions that are, in principle, boolean. However, a condition only has to be truthy (true if coerced to boolean) in order to be accepted. In other words, the following two control flow statements are equivalent:

if (value) {}

if (Boolean(value) === true) {}

This is a list of all falsy values:

- undefined, null

- false

- 0, NaN

- 0n

- ''

All other values are truthy. For more information, see §15.2 “Falsy and truthy values”.

if statements [ES1]

These are two simple if statements: one with just a “then” branch and one with both a “then” branch and an “else” branch:

if (cond) {

// then branch

}

if (cond) {

// then branch

} else {

// else branch

}

Instead of the block, else can also be followed by another if statement:

if (cond1) {

// ···

} else if (cond2) {

// ···

}

if (cond1) {

// ···

} else if (cond2) {

// ···

} else {

// ···

}

You can continue this chain with more else ifs.

The syntax of if statements

The general syntax of if statements is:

if (cond) «then_statement»

else «else_statement»

So far, the then_statement has always been a block, but we can use any statement. That statement must be terminated with a semicolon:

if (true) console.log('Yes'); else console.log('No');

That means that else if is not its own construct; it’s simply an if statement whose else_statement is another if statement.

switch statements [ES3]

A switch statement looks as follows:

switch («switch_expression») {

«switch_body»

}

The body of switch consists of zero or more case clauses:

case «case_expression»:

«statements»

And, optionally, a default clause:

default:

«statements»

A switch is executed as follows:

- It evaluates the switch expression.

- It jumps to the first case clause whose expression has the same result as the switch expression.

- Otherwise, if there is no such clause, it jumps to the default clause.

- Otherwise, if there is no default clause, it does nothing.

A first example of a switch statement

Let’s look at an example: The following function converts a number from 1–7 to the name of a weekday.

function dayOfTheWeek(num) {

switch (num) {

case 1:

return 'Monday';

case 2:

return 'Tuesday';

case 3:

return 'Wednesday';

case 4:

return 'Thursday';

case 5:

return 'Friday';

case 6:

return 'Saturday';

case 7:

return 'Sunday';

}

}

assert.equal(dayOfTheWeek(5), 'Friday');

Don’t forget to return or break!

At the end of a case clause, execution continues with the next case clause, unless we return or break – for example:

function englishToFrench(english) {

let french;

switch (english) {

case 'hello':

french = 'bonjour';

case 'goodbye':

french = 'au revoir';

}

return french;

}

// The result should be 'bonjour'!

assert.equal(englishToFrench('hello'), 'au revoir');

That is, our implementation of dayOfTheWeek() only worked because we used return. We can fix englishToFrench() by using break:

function englishToFrench(english) {

let french;

switch (english) {

case 'hello':

french = 'bonjour';

break;

case 'goodbye':

french = 'au revoir';

break;

}

return french;

}

assert.equal(englishToFrench('hello'), 'bonjour'); // ok

Empty case clauses

The statements of a case clause can be omitted, which effectively gives us multiple case expressions per case clause:

function isWeekDay(name) {

switch (name) {

case 'Monday':

case 'Tuesday':

case 'Wednesday':

case 'Thursday':

case 'Friday':

return true;

case 'Saturday':

case 'Sunday':

return false;

}

}

assert.equal(isWeekDay('Wednesday'), true);

assert.equal(isWeekDay('Sunday'), false);

Checking for illegal values via a default clause

A default clause is jumped to if the switch expression has no other match. That makes it useful for error checking:

function isWeekDay(name) {

switch (name) {

case 'Monday':

case 'Tuesday':

case 'Wednesday':

case 'Thursday':

case 'Friday':

return true;

case 'Saturday':

case 'Sunday':

return false;

default:

throw new Error('Illegal value: '+name);

}

}

assert.throws(

() => isWeekDay('January'),

{message: 'Illegal value: January'});

Exercises: switch

- exercises/control-flow/number_to_month_test.mjs

- Bonus: exercises/control-flow/is_object_via_switch_test.mjs

while loops [ES1]

A while loop has the following syntax:

while («condition») {

«statements»

}

Before each loop iteration, while evaluates condition:

- If the result is falsy, the loop is finished.

- If the result is truthy, the while body is executed one more time.

Examples of while loops

The following code uses a while loop. In each loop iteration, it removes the first element of arr via .shift() and logs it.

const arr = ['a', 'b', 'c'];

while (arr.length > 0) {

const elem = arr.shift(); // remove first element

console.log(elem);

}

// Output:

// 'a'

// 'b'

// 'c'

If the condition always evaluates to true, then while is an infinite loop:

while (true) {

if (Math.random() === 0) break;

}

do-while loops [ES3]

The do-while loop works much like while, but it checks its condition after each loop iteration, not before.

let input;

do {

input = prompt('Enter text:');

console.log(input);

} while (input !== ':q');

do-while can also be viewed as a while loop that runs at least once.

prompt() is a global function that is available in web browsers. It prompts the user to input text and returns it.

for loops [ES1]

A for loop has the following syntax:

for («initialization»; «condition»; «post_iteration») {

«statements»

}

The first line is the head of the loop and controls how often the body (the remainder of the loop) is executed. It has three parts and each of them is optional:

- initialization: sets up variables, etc. for the loop. Variables declared here via let or const only exist inside the loop.

- condition: This condition is checked before each loop iteration. If it is falsy, the loop stops.

- post_iteration: This code is executed after each loop iteration.

A for loop is therefore roughly equivalent to the following while loop:

«initialization»

while («condition») {

«statements»

«post_iteration»

}

Examples of for loops

As an example, this is how to count from zero to two via a for loop:

for (let i=0; i<3; i++) {

console.log(i);

}

// Output:

// 0

// 1

// 2

This is how to log the contents of an Array via a for loop:

const arr = ['a', 'b', 'c'];

for (let i=0; i<arr.length; i++) {

console.log(arr[i]);

}

// Output:

// 'a'

// 'b'

// 'c'

If we omit all three parts of the head, we get an infinite loop:

for (;;) {

if (Math.random() === 0) break;

}

for-of loops [ES6]

A for-of loop iterates over any iterable – a data container that supports the iteration protocol. Each iterated value is stored in a variable, as specified in the head:

for («iteration_variable» of «iterable») {

«statements»

}

The iteration variable is usually created via a variable declaration:

const iterable = ['hello', 'world'];

for (const elem of iterable) {

console.log(elem);

}

// Output:

// 'hello'

// 'world'

But we can also use a (mutable) variable that already exists:

const iterable = ['hello', 'world'];

let elem;

for (elem of iterable) {

console.log(elem);

}

const: for-of vs. for

Note that in for-of loops we can use const. The iteration variable can still be different for each iteration (it just can’t change during the iteration). Think of it as a new const declaration being executed each time in a fresh scope.

In contrast, in for loops we must declare variables via let or var if their values change.

Iterating over iterables

As mentioned before, for-of works with any iterable object, not just with Arrays – for example, with Sets:

const set = new Set(['hello', 'world']);

for (const elem of set) {

console.log(elem);

}

Iterating over [index, element] pairs of Arrays

Lastly, we can also use for-of to iterate over the [index, element] entries of Arrays:

const arr = ['a', 'b', 'c'];

for (const [index, elem] of arr.entries()) {

console.log(`${index} -> ${elem}`);

}

// Output:

// '0 -> a'

// '1 -> b'

// '2 -> c'

With [index, element], we are using destructuring to access Array elements.

Exercise: for-of

exercises/control-flow/array_to_string_test.mjs

for-await-of loops [ES2018]

for-await-of is like for-of, but it works with asynchronous iterables instead of synchronous ones. And it can only be used inside async functions and async generators.

for await (const item of asyncIterable) {

// ···

}

for-await-of is described in detail in the chapter on asynchronous iteration.

for-in loops (avoid) [ES1]

The for-in loop visits all (own and inherited) enumerable property keys of an object. When looping over an Array, it is rarely a good choice:

- It visits property keys, not values.

- As property keys, the indices of Array elements are strings, not numbers (more information on how Array elements work).

- It visits all enumerable property keys (both own and inherited ones), not just those of Array elements.

The following code demonstrates these points:

const arr = ['a', 'b', 'c'];

arr.propKey = 'property value';

for (const key in arr) {

console.log(key);

}

// Output:

// '0'

// '1'

// '2'

// 'propKey'

Recomendations for looping

- If you want to loop over an asynchronous iterable (in ES2018+), you must use for-await-of.

- For looping over a synchronous iterable (in ES6+), you must use for-of. Note that Arrays are iterables.

- For looping over an Array in ES5+, you can use the Array method .forEach().

- Before ES5, you can use a plain for loop to loop over an Array.

- Don’t use for-in to loop over an Array.

Quiz

See quiz app.

Exception handling

- 24.1 Motivation: throwing and catching exceptions

- 24.2 throw

- 24.2.1 What values should we throw?

- 24.3 The try statement

- 24.3.1 The try block

- 24.3.2 The catch clause

- 24.3.3 The finally clause

- 24.4 Error and its subclasses

- 24.4.1 Class Error

- 24.4.2 The built-in subclasses of Error

- 24.4.3 Subclassing Error

- 24.5 Chaining errors

- 24.5.1 Why would we want to chain errors?

- 24.5.2 Chaining errors via error.cause [ES2022]

- 24.5.3 An alternative to .cause: a custom error class

This chapter covers how JavaScript handles exceptions.

Why doesn’t JavaScript throw exceptions more often?

JavaScript didn’t support exceptions until ES3. That explains why they are used sparingly by the language and its standard library.

Motivation: throwing and catching exceptions

Consider the following code. It reads profiles stored in files into an Array with instances of class Profile:

function readProfiles(filePaths) {

const profiles = [];

for (const filePath of filePaths) {

try {

const profile = readOneProfile(filePath);

profiles.push(profile);

} catch (err) { // (A)

console.log('Error in: '+filePath, err);

}

}

}

function readOneProfile(filePath) {

const profile = new Profile();

const file = openFile(filePath);

// ··· (Read the data in `file` into `profile`)

return profile;

}

function openFile(filePath) {

if (!fs.existsSync(filePath)) {

throw new Error('Could not find file '+filePath); // (B)

}

// ··· (Open the file whose path is `filePath`)

}

Let’s examine what happens in line B: An error occurred, but the best place to handle the problem is not the current location, it’s line A. There, we can skip the current file and move on to the next one.

Therefore:

- In line B, we use a throw statement to indicate that there was a problem.

- In line A, we use a try-catch statement to handle the problem.

When we throw, the following constructs are active:

readProfiles(···)

for (const filePath of filePaths)

try

readOneProfile(···)

openFile(···)

if (!fs.existsSync(filePath))

throw

One by one, throw exits the nested constructs, until it encounters a try statement. Execution continues in the catch clause of that try statement.

throw

This is the syntax of the throw statement:

throw «value»;

What values should we throw?

Any value can be thrown in JavaScript. However, it’s best to use instances of Error or a subclass because they support additional features such as stack traces and error chaining (see §24.4 “Error and its subclasses”).

That leaves us with the following options:

- Using class Error directly. That is less limiting in JavaScript than in a more static language because we can add our own properties to instances:

const err = new Error('Could not find the file');

err.filePath = filePath;

throw err;

- Using one of the subclasses of Error.

- Subclassing Error (more details are explained later):

class MyError extends Error {

}

function func() {

throw new MyError('Problem!');

}

assert.throws(

() => func(),

MyError);

The try statement

The maximal version of the try statement looks as follows:

try {

«try_statements»

} catch (error) {

«catch_statements»

} finally {

«finally_statements»

}

We can combine these clauses as follows:

- try-catch

- try-finally

- try-catch-finally

The try block

The try block can be considered the body of the statement. This is where we execute the regular code.

The catch clause

If an exception reaches the try block, then it is assigned to the parameter of the catch clause and the code in that clause is executed. Next, execution normally continues after the try statement. That may change if:

- There is a return, break, or throw inside the catch block.

- There is a finally clause (which is always executed before the try statement ends).

The following code demonstrates that the value that is thrown in line A is indeed caught in line B.

const errorObject = new Error();

function func() {

throw errorObject; // (A)

}

try {

func();

} catch (err) { // (B)

assert.equal(err, errorObject);

}

Omitting the catch binding [ES2019]

We can omit the catch parameter if we are not interested in the value that was thrown:

try {

// ···

} catch {

// ···

}

That may occasionally be useful. For example, Node.js has the API function assert.throws(func) that checks whether an error is thrown inside func. It could be implemented as follows.

function throws(func) {

try {

func();

} catch {

return; // everything OK

}

throw new Error('Function didn’t throw an exception!');

}

However, a more complete implementation of this function would have a catch parameter and would, for example, check that its type is as expected.

The finally clause

The code inside the finally clause is always executed at the end of a try statement – no matter what happens in the try block or the catch clause.

Let’s look at a common use case for finally: We have created a resource and want to always destroy it when we are done with it, no matter what happens while working with it. We would implement that as follows:

const resource = createResource();

try {

// Work with `resource`. Errors may be thrown.

} finally {

resource.destroy();

}

finally is always executed

The finally clause is always executed, even if an error is thrown (line A):

let finallyWasExecuted = false;

assert.throws(

() => {

try {

throw new Error(); // (A)

} finally {

finallyWasExecuted = true;

}

},

Error

);

assert.equal(finallyWasExecuted, true);

And even if there is a return statement (line A):

let finallyWasExecuted = false;

function func() {

try {

return; // (A)

} finally {

finallyWasExecuted = true;

}

}

func();

assert.equal(finallyWasExecuted, true);

Error and its subclasses

Error is the common superclass of all built-in error classes.

Class Error

This is what Error’s instance properties and constructor look like:

class Error {

// Instance properties

message: string;

cause?: any; // ES2022

stack: string; // non-standard but widely supported

constructor(

message: string = '',

options?: ErrorOptions // ES2022

);

}

interface ErrorOptions {

cause?: any; // ES2022

}

The constructor has two parameters:

- message specifies an error message.

- options was introduced in ECMAScript 2022. It contains an object where one property is currently supported:

- .cause specifies which exception (if any) caused the current error.

The subsections after the next one explain the instance properties .message, .cause and .stack in more detail.

Error.prototype.name

Each built-in error class E has a property E.prototype.name:

> Error.prototype.name

'Error'

> RangeError.prototype.name

'RangeError'

Therefore, there are two ways to get the name of the class of a built-in error object:

> new RangeError().name

'RangeError'

> new RangeError().constructor.name

'RangeError'

Error instance property .message

.message contains just the error message:

const err = new Error('Hello!');

assert.equal(String(err), 'Error: Hello!');

assert.equal(err.message, 'Hello!');

If we omit the message then the empty string is used as a default value (inherited from Error.prototype.message):

If we omit the message, it is the empty string:

assert.equal(new Error().message, '');

Error instance property .stack

The instance property .stack is not an ECMAScript feature, but it is widely supported by JavaScript engines. It is usually a string, but its exact structure is not standardized and varies between engines.

This is what it looks like on the JavaScript engine V8:

const err = new Error('Hello!');

assert.equal(

err.stack,

`

Error: Hello!

at file://ch_exception-handling.mjs:1:13

`.trim());

Error instance property .cause [ES2022]

The instance property .cause is created via the options object in the second parameter of new Error(). It specifies which other error caused the current one.

const err = new Error('msg', {cause: 'the cause'});

assert.equal(err.cause, 'the cause');

For information on how to use this property see §24.5 “Chaining errors”.

The built-in subclasses of Error

Error has the following subclasses – quoting the ECMAScript specification:

- AggregateError [ES2021] represents multiple errors at once. In the standard library, only Promise.any() uses it.

- RangeError indicates a value that is not in the set or range of allowable values.

- ReferenceError indicates that an invalid reference value has been detected.

- SyntaxError indicates that a parsing error has occurred.

- TypeError is used to indicate an unsuccessful operation when none of the other NativeError objects are an appropriate indication of the failure cause.

- URIError indicates that one of the global URI handling functions was used in a way that is incompatible with its definition.

Subclassing Error

Since ECMAScript 2022, the Error constructor accepts two parameters (see previous subsection). Therefore, we have two choices when subclassing it: We can either omit the constructor in our subclass or we can invoke super() like this:

class MyCustomError extends Error {

constructor(message, options) {

super(message, options);

// ···

}

}

Chaining errors

Why would we want to chain errors?

Sometimes, we catch errors that are thrown during a more deeply nested function call and would like to attach more information to it:

function readFiles(filePaths) {

return filePaths.map(

(filePath) => {

try {

const text = readText(filePath);

const json = JSON.parse(text);

return processJson(json);

} catch (error) {

// (A)

}

});

}

The statements inside the try clause may throw all kinds of errors. In most cases, an error won’t be aware of the path of the file that caused it. That‘s why we would like to attach that information in line A.

Chaining errors via error.cause [ES2022]

Since ECMAScript 2022, new Error() lets us specify what caused it:

function readFiles(filePaths) {

return filePaths.map(

(filePath) => {

try {

// ···

} catch (error) {

throw new Error(

`While processing ${filePath}`,

{cause: error}

);

}

});

}

An alternative to .cause: a custom error class

The following custom error class supports chaining. It is forward compatible with .cause.

/**

* An error class that supports error chaining.

* If there is built-in support for .cause, it uses it.

* Otherwise, it creates this property itself.

*

* @see https://github.com/tc39/proposal-error-cause

*/

class CausedError extends Error {

constructor(message, options) {

super(message, options);

if (

(isObject(options) && 'cause' in options)

&& !('cause' in this)

) {

// .cause was specified but the superconstructor

// did not create an instance property.

const cause = options.cause;

this.cause = cause;

if ('stack' in cause) {

this.stack = this.stack + '\nCAUSE: ' + cause.stack;

}

}

}

}

function isObject(value) {

return value !== null && typeof value === 'object';

}

Exercise: Exception handling

exercises/exception-handling/call_function_test.mjs

Quiz

See quiz app.

Callable values

- 25.1 Kinds of functions

- 25.2 Ordinary functions

- 25.2.1 Named function expressions (advanced)

- 25.2.2 Terminology: function definitions and function expressions

- 25.2.3 Parts of a function declaration

- 25.2.4 Roles played by ordinary functions

- 25.2.5 Terminology: entity vs. syntax vs. role (advanced)

- 25.3 Specialized functions

- 25.3.1 Specialized functions are still functions

- 25.3.2 Arrow functions

- 25.3.3 The special variable this in methods, ordinary functions and arrow functions

- 25.3.4 Recommendation: prefer specialized functions over ordinary functions

- 25.4 Summary: kinds of callable values

- 25.5 Returning values from functions and methods

- 25.6 Parameter handling

- 25.6.1 Terminology: parameters vs. arguments

- 25.6.2 Terminology: callback

- 25.6.3 Too many or not enough arguments

- 25.6.4 Parameter default values

- 25.6.5 Rest parameters

- 25.6.6 Named parameters

- 25.6.7 Simulating named parameters

- 25.6.8 Spreading (...) into function calls

- 25.7 Methods of functions: .call(), .apply(), .bind()

- 25.7.1 The function method .call()

- 25.7.2 The function method .apply()

- 25.7.3 The function method .bind()

In this chapter, we look at JavaScript values that can be invoked: functions, methods, and classes.

Kinds of functions

JavaScript has two categories of functions:

- An ordinary function can play several roles:

- Real function

- Method

- Constructor function

- A specialized function can only play one of those roles – for example:

- An arrow function can only be a real function.

- A method can only be a method.

- A class can only be a constructor function.

Specialized functions were added to the language in ECMAScript 6.

Read on to find out what all of those things mean.

Ordinary functions

The following code shows two ways of doing (roughly) the same thing: creating an ordinary function.

// Function declaration (a statement)

function ordinary1(a, b, c) {

// ···

}

// const plus anonymous (nameless) function expression

const ordinary2 = function (a, b, c) {

// ···

};

Inside a scope, function declarations are activated early (see §11.8 “Declarations: scope and activation”) and can be called before they are declared. That is occasionally useful.

Variable declarations, such as the one for ordinary2, are not activated early.

Named function expressions (advanced)

So far, we have only seen anonymous function expressions – which don’t have names:

const anonFuncExpr = function (a, b, c) {

// ···

};

But there are also named function expressions:

const namedFuncExpr = function myName(a, b, c) {

// `myName` is only accessible in here

};

myName is only accessible inside the body of the function. The function can use it to refer to itself (for self-recursion, etc.) – independently of which variable it is assigned to:

const func = function funcExpr() { return funcExpr };

assert.equal(func(), func);

// The name `funcExpr` only exists inside the function body:

assert.throws(() => funcExpr(), ReferenceError);

Even if they are not assigned to variables, named function expressions have names (line A):

function getNameOfCallback(callback) {

return callback.name;

}

assert.equal(

getNameOfCallback(function () {}), ''); // anonymous

assert.equal(

getNameOfCallback(function named() {}), 'named'); // (A)

Note that functions created via function declarations or variable declarations always have names:

function funcDecl() {}

assert.equal(

getNameOfCallback(funcDecl), 'funcDecl');

const funcExpr = function () {};

assert.equal(

getNameOfCallback(funcExpr), 'funcExpr');

One benefit of functions having names is that those names show up in error stack traces.

Terminology: function definitions and function expressions

A function definition is syntax that creates functions:

- A function declaration (a statement)

- A function expression

Function declarations always produce ordinary functions. Function expressions produce either ordinary functions or specialized functions:

- Ordinary function expressions (which we have already encountered):

- Anonymous function expressions

- Named function expressions

- Specialized function expressions (which we’ll look at later):

- Arrow functions (which are always expressions)

While function declarations are still popular in JavaScript, function expressions are almost always arrow functions in modern code.

Parts of a function declaration

Let’s examine the parts of a function declaration via the following example. Most of the terms also apply to function expressions.

function add(x, y) {

return x + y;

}

- add is the name of the function declaration.

- add(x, y) is the head of the function declaration.

- x and y are the parameters.

- The curly braces ({ and }) and everything between them are the body of the function declaration.

- The return statement explicitly returns a value from the function.

Trailing commas in parameter lists

JavaScript has always allowed and ignored trailing commas in Array literals. Since ES5, they are also allowed in object literals. Since ES2017, we can add trailing commas to parameter lists (declarations and invocations):

// Declaration

function retrieveData(

contentText,

keyword,

{unique, ignoreCase, pageSize}, // trailing comma

) {

// ···

}

// Invocation

retrieveData(

'',

null,

{ignoreCase: true, pageSize: 10}, // trailing comma

);

Roles played by ordinary functions

Consider the following function declaration from the previous section:

function add(x, y) {

return x + y;

}

This function declaration creates an ordinary function whose name is add. As an ordinary function, add() can play three roles:

- Real function: invoked via a function call.

assert.equal(add(2, 1), 3);

- Method: stored in a property, invoked via a method call.

const obj = { addAsMethod: add };

assert.equal(obj.addAsMethod(2, 4), 6); // (A)

In line A, obj is called the receiver of the method call.

- Constructor function: invoked via new.

const inst = new add();

assert.equal(inst instanceof add, true);

As an aside, the names of constructor functions (incl. classes) normally start with capital letters.

Terminology: entity vs. syntax vs. role (advanced)

The distinction between the concepts syntax, entity, and role is subtle and often doesn’t matter. But I’d like to sharpen your eye for it:

- An entity is a JavaScript feature as it “lives” in RAM. An ordinary function is an entity.

- Entities include: ordinary functions, arrow functions, methods, and classes.

- Syntax is the code that we use to create entities. Function declarations and anonymous function expressions are syntax. They both create entities that are called ordinary functions.

- Syntax includes: function declarations and anonymous function expressions. The syntax that produces arrow functions is also called arrow functions. The same is true for methods and classes.

- A role describes how we use entities. The entity ordinary function can play the role real function, or the role method, or the role class. The entity arrow function can also play the role real function.

- The roles of functions are: real function, method, and constructor function.

Many other programming languages only have a single entity that plays the role real function. Then they can use the name function for both role and entity.

Specialized functions

Specialized functions are single-purpose versions of ordinary functions. Each one of them specializes in a single role:

- The purpose of an arrow function is to be a real function:

const arrow = () => {

return 123;

};

assert.equal(arrow(), 123);

- The purpose of a method is to be a method:

const obj = {

myMethod() {

return 'abc';

}

};

assert.equal(obj.myMethod(), 'abc');

- The purpose of a class is to be a constructor function:

class MyClass {

/* ··· */

}

const inst = new MyClass();

Apart from nicer syntax, each kind of specialized function also supports new features, making them better at their jobs than ordinary functions.

- Arrow functions are explained soon.

- Methods are explained in the chapter on single objects.

- Classes are explained in the chapter on classes.

Tbl. 16 lists the capabilities of ordinary and specialized functions.

Table 16: Capabilities of four kinds of functions. If a cell value is in parentheses, that implies some kind of limitation. The special variable this is explained in §25.3.3 “The special variable this in methods, ordinary functions and arrow functions”.

Function call

Method call

Constructor call

Ordinary function

(this === undefined  )

✔

✔

Arrow function

✔

(lexical this  )

✘

Method

(this === undefined  )

✔

✘

Class

✘

✘

✔

Specialized functions are still functions

It’s important to note that arrow functions, methods, and classes are still categorized as functions:

> (() => {}) instanceof Function

true

> ({ method() {} }.method) instanceof Function

true

> (class SomeClass {}) instanceof Function

true

Arrow functions

Arrow functions were added to JavaScript for two reasons:

1. To provide a more concise way for creating functions.

2. They work better as real functions inside methods: Methods can refer to the object that received a method call via the special variable this. Arrow functions can access the this of a surrounding method, ordinary functions can’t (because they have their own this).

We’ll first examine the syntax of arrow functions and then how this works in various functions.

The syntax of arrow functions

Let’s review the syntax of an anonymous function expression:

const f = function (x, y, z) { return 123 };

The (roughly) equivalent arrow function looks as follows. Arrow functions are expressions.

const f = (x, y, z) => { return 123 };

Here, the body of the arrow function is a block. But it can also be an expression. The following arrow function works exactly like the previous one.

const f = (x, y, z) => 123;

If an arrow function has only a single parameter and that parameter is an identifier (not a destructuring pattern) then you can omit the parentheses around the parameter:

const id = x => x;

That is convenient when passing arrow functions as parameters to other functions or methods:

> [1,2,3].map(x => x+1)

[ 2, 3, 4 ]

This previous example demonstrates one benefit of arrow functions – conciseness. If we perform the same task with a function expression, our code is more verbose:

[1,2,3].map(function (x) { return x+1 });

Syntax pitfall: returning an object literal from an arrow function

If you want the expression body of an arrow function to be an object literal, you must put the literal in parentheses:

const func1 = () => ({a: 1});

assert.deepEqual(func1(), { a: 1 });

If you don’t, JavaScript thinks, the arrow function has a block body (that doesn’t return anything):

const func2 = () => {a: 1};

assert.deepEqual(func2(), undefined);

{a: 1} is interpreted as a block with the label a: and the expression statement 1. Without an explicit return statement, the block body returns undefined.

This pitfall is caused by syntactic ambiguity: object literals and code blocks have the same syntax. We use the parentheses to tell JavaScript that the body is an expression (an object literal) and not a statement (a block).

The special variable this in methods, ordinary functions and arrow functions

The special variable this is an object-oriented feature

We are taking a quick look at the special variable this here, in order to understand why arrow functions are better real functions than ordinary functions.

But this feature only matters in object-oriented programming and is covered in more depth in §28.5 “Methods and the special variable this”. Therefore, don’t worry if you don’t fully understand it yet.

Inside methods, the special variable this lets us access the receiver – the object which received the method call:

const obj = {

myMethod() {

assert.equal(this, obj);

}

};

obj.myMethod();

Ordinary functions can be methods and therefore also have the implicit parameter this:

const obj = {

myMethod: function () {

assert.equal(this, obj);

}

};

obj.myMethod();

this is even an implicit parameter when we use an ordinary function as a real function. Then its value is undefined (if strict mode is active, which it almost always is):

function ordinaryFunc() {

assert.equal(this, undefined);

}

ordinaryFunc();

That means that an ordinary function, used as a real function, can’t access the this of a surrounding method (line A). In contrast, arrow functions don’t have this as an implicit parameter. They treat it like any other variable and can therefore access the this of a surrounding method (line B):

const jill = {

name: 'Jill',

someMethod() {

function ordinaryFunc() {

assert.throws(

() => this.name, // (A)

/^TypeError: Cannot read properties of undefined \(reading 'name'\)$/);

}

ordinaryFunc();

const arrowFunc = () => {

assert.equal(this.name, 'Jill'); // (B)

};

arrowFunc();

},

};

jill.someMethod();

In this code, we can observe two ways of handling this:

- Dynamic this: In line A, we try to access the this of .someMethod() from an ordinary function. There, it is shadowed by the function’s own this, which is undefined (as filled in by the function call). Given that ordinary functions receive their this via (dynamic) function or method calls, their this is called dynamic.

- Lexical this: In line B, we again try to access the this of .someMethod(). This time, we succeed because the arrow function does not have its own this. this is resolved lexically, just like any other variable. That’s why the this of arrow functions is called lexical.

Recommendation: prefer specialized functions over ordinary functions

Normally, you should prefer specialized functions over ordinary functions, especially classes and methods.

When it comes to real functions, the choice between an arrow function and an ordinary function is less clear-cut, though:

- For anonymous inline function expressions, arrow functions are clear winners, due to their compact syntax and them not having this as an implicit parameter:

const twiceOrdinary = [1, 2, 3].map(function (x) {return x * 2});

const twiceArrow = [1, 2, 3].map(x => x * 2);

- For stand-alone named function declarations, arrow functions still benefit from lexical this. But function declarations (which produce ordinary functions) have nice syntax and early activation is also occasionally useful (see §11.8 “Declarations: scope and activation”). If this doesn’t appear in the body of an ordinary function, there is no downside to using it as a real function. The static checking tool ESLint can warn us during development when we do this wrong via a built-in rule.

function timesOrdinary(x, y) {

return x * y;

}

const timesArrow = (x, y) => {

return x * y;

};

Summary: kinds of callable values

This section refers to upcoming content

This section mainly serves as a reference for the current and upcoming chapters. Don’t worry if you don’t understand everything.

So far, all (real) functions and methods, that we have seen, were:

- Single-result

- Synchronous

Later chapters will cover other modes of programming:

- Iteration treats objects as containers of data (so-called iterables) and provides a standardized way for retrieving what is inside them. If a function or a method returns an iterable, it returns multiple values.

- Asynchronous programming deals with handling a long-running computation. You are notified when the computation is finished and can do something else in between. The standard pattern for asynchronously delivering single results is called Promise.

These modes can be combined – for example, there are synchronous iterables and asynchronous iterables.

Several new kinds of functions and methods help with some of the mode combinations:

- Async functions help implement functions that return Promises. There are also async methods.

- Synchronous generator functions help implement functions that return synchronous iterables. There are also synchronous generator methods.

- Asynchronous generator functions help implement functions that return asynchronous iterables. There are also asynchronous generator methods.

That leaves us with 4 kinds (2 × 2) of functions and methods:

- Synchronous vs. asynchronous

- Generator vs. single-result

Tbl. 17 gives an overview of the syntax for creating these 4 kinds of functions and methods.

Table 17: Syntax for creating functions and methods. The last column specifies how many values are produced by an entity.

Result

#

Sync function

Sync method

function f() {}

{ m() {} }

value

1

f = function () {}

f = () => {}

Sync generator function

Sync gen. method

function* f() {}

{ * m() {} }

iterable

0+

f = function* () {}

Async function

Async method

async function f() {}

{ async m() {} }

Promise

1

f = async function () {}

f = async () => {}

Async generator function

Async gen. method

async function* f() {}

{ async * m() {} }

async iterable

0+

f = async function* () {}

Returning values from functions and methods

(Everything mentioned in this section applies to both functions and methods.)

The return statement explicitly returns a value from a function:

function func() {

return 123;

}

assert.equal(func(), 123);

Another example:

function boolToYesNo(bool) {

if (bool) {

return 'Yes';

} else {

return 'No';

}

}

assert.equal(boolToYesNo(true), 'Yes');

assert.equal(boolToYesNo(false), 'No');

If, at the end of a function, you haven’t returned anything explicitly, JavaScript returns undefined for you:

function noReturn() {

// No explicit return

}

assert.equal(noReturn(), undefined);

Parameter handling

Once again, I am only mentioning functions in this section, but everything also applies to methods.

Terminology: parameters vs. arguments

The term parameter and the term argument basically mean the same thing. If you want to, you can make the following distinction:

- Parameters are part of a function definition. They are also called formal parameters and formal arguments.

- Arguments are part of a function call. They are also called actual parameters and actual arguments.

Terminology: callback

A callback or callback function is a function that is an argument of a function or method call.

The following is an example of a callback:

const myArray = ['a', 'b'];

const callback = (x) => console.log(x);

myArray.forEach(callback);

// Output:

// 'a'

// 'b'

Too many or not enough arguments

JavaScript does not complain if a function call provides a different number of arguments than expected by the function definition:

- Extra arguments are ignored.

- Missing parameters are set to undefined.

For example:

function foo(x, y) {

return [x, y];

}

// Too many arguments:

assert.deepEqual(foo('a', 'b', 'c'), ['a', 'b']);

// The expected number of arguments:

assert.deepEqual(foo('a', 'b'), ['a', 'b']);

// Not enough arguments:

assert.deepEqual(foo('a'), ['a', undefined]);

Parameter default values

Parameter default values specify the value to use if a parameter has not been provided – for example:

function f(x, y=0) {

return [x, y];

}

assert.deepEqual(f(1), [1, 0]);

assert.deepEqual(f(), [undefined, 0]);

undefined also triggers the default value:

assert.deepEqual(

f(undefined, undefined),

[undefined, 0]);

Rest parameters

A rest parameter is declared by prefixing an identifier with three dots (...). During a function or method call, it receives an Array with all remaining arguments. If there are no extra arguments at the end, it is an empty Array – for example:

function f(x, ...y) {

return [x, y];

}

assert.deepEqual(

f('a', 'b', 'c'), ['a', ['b', 'c']]

);

assert.deepEqual(

f(), [undefined, []]

);

There are two restrictions related to how we can use rest parameters:

- We cannot use more than one rest parameter per function definition.

assert.throws(

() => eval('function f(...x, ...y) {}'),

/^SyntaxError: Rest parameter must be last formal parameter$/

);

- A rest parameter must always come last. As a consequence, we can’t access the last parameter like this:

assert.throws(

() => eval('function f(...restParams, lastParam) {}'),

/^SyntaxError: Rest parameter must be last formal parameter$/

);

Enforcing a certain number of arguments via a rest parameter

You can use a rest parameter to enforce a certain number of arguments. Take, for example, the following function:

function createPoint(x, y) {

return {x, y};

// same as {x: x, y: y}

}

This is how we force callers to always provide two arguments:

function createPoint(...args) {

if (args.length !== 2) {

throw new Error('Please provide exactly 2 arguments!');

}

const [x, y] = args; // (A)

return {x, y};

}

In line A, we access the elements of args via destructuring.

Named parameters

When someone calls a function, the arguments provided by the caller are assigned to the parameters received by the callee. Two common ways of performing the mapping are:

1. Positional parameters: An argument is assigned to a parameter if they have the same position. A function call with only positional arguments looks as follows.

selectEntries(3, 20, 2)

2. Named parameters: An argument is assigned to a parameter if they have the same name. JavaScript doesn’t have named parameters, but you can simulate them. For example, this is a function call with only (simulated) named arguments:

selectEntries({start: 3, end: 20, step: 2})

Named parameters have several benefits:

- They lead to more self-explanatory code because each argument has a descriptive label. Just compare the two versions of selectEntries(): with the second one, it is much easier to see what happens.

- The order of the arguments doesn’t matter (as long as the names are correct).

- Handling more than one optional parameter is more convenient: callers can easily provide any subset of all optional parameters and don’t have to be aware of the ones they omit (with positional parameters, you have to fill in preceding optional parameters, with undefined).

Simulating named parameters

JavaScript doesn’t have real named parameters. The official way of simulating them is via object literals:

function selectEntries({start=0, end=-1, step=1}) {

return {start, end, step};

}

This function uses destructuring to access the properties of its single parameter. The pattern it uses is an abbreviation for the following pattern:

{start: start=0, end: end=-1, step: step=1}

This destructuring pattern works for empty object literals:

> selectEntries({})

{ start: 0, end: -1, step: 1 }

But it does not work if you call the function without any parameters:

> selectEntries()

TypeError: Cannot read properties of undefined (reading 'start')

You can fix this by providing a default value for the whole pattern. This default value works the same as default values for simpler parameter definitions: if the parameter is missing, the default is used.

function selectEntries({start=0, end=-1, step=1} = {}) {

return {start, end, step};

}

assert.deepEqual(

selectEntries(),

{ start: 0, end: -1, step: 1 });

Spreading (...) into function calls

If you put three dots (...) in front of the argument of a function call, then you spread it. That means that the argument must be an iterable object and the iterated values all become arguments. In other words, a single argument is expanded into multiple arguments – for example:

function func(x, y) {

console.log(x);

console.log(y);

}

const someIterable = ['a', 'b'];

func(...someIterable);

// same as func('a', 'b')

// Output:

// 'a'

// 'b'

Spreading and rest parameters use the same syntax (...), but they serve opposite purposes:

- Rest parameters are used when defining functions or methods. They collect arguments into Arrays.

- Spread arguments are used when calling functions or methods. They turn iterable objects into arguments.

Example: spreading into Math.max()

Math.max() returns the largest one of its zero or more arguments. Alas, it can’t be used for Arrays, but spreading gives us a way out:

> Math.max(-1, 5, 11, 3)

11

> Math.max(...[-1, 5, 11, 3])

11

> Math.max(-1, ...[-5, 11], 3)

11

Example: spreading into Array.prototype.push()

Similarly, the Array method .push() destructively adds its zero or more parameters to the end of its Array. JavaScript has no method for destructively appending an Array to another one. Once again, we are saved by spreading:

const arr1 = ['a', 'b'];

const arr2 = ['c', 'd'];

arr1.push(...arr2);

assert.deepEqual(arr1, ['a', 'b', 'c', 'd']);

Exercises: Parameter handling

- Positional parameters: exercises/callables/positional_parameters_test.mjs

- Named parameters: exercises/callables/named_parameters_test.mjs

Methods of functions: .call(), .apply(), .bind()

Functions are objects and have methods. In this section, we look at three of those methods: .call(), .apply(), and .bind().

The function method .call()

Each function someFunc has the following method:

someFunc.call(thisValue, arg1, arg2, arg3);

This method invocation is loosely equivalent to the following function call:

someFunc(arg1, arg2, arg3);

However, with .call(), we can also specify a value for the implicit parameter this. In other words: .call() makes the implicit parameter this explicit.

The following code demonstrates the use of .call():

function func(x, y) {

return [this, x, y];

}

assert.deepEqual(

func.call('hello', 'a', 'b'),

['hello', 'a', 'b']);

As we have seen before, if we function-call an ordinary function, its this is undefined:

assert.deepEqual(

func('a', 'b'),

[undefined, 'a', 'b']);

Therefore, the previous function call is equivalent to:

assert.deepEqual(

func.call(undefined, 'a', 'b'),

[undefined, 'a', 'b']);

In arrow functions, the value for this provided via .call() (or other means) is ignored.

The function method .apply()

Each function someFunc has the following method:

someFunc.apply(thisValue, [arg1, arg2, arg3]);

This method invocation is loosely equivalent to the following function call (which uses spreading):

someFunc(...[arg1, arg2, arg3]);

However, with .apply(), we can also specify a value for the implicit parameter this.

The following code demonstrates the use of .apply():

function func(x, y) {

return [this, x, y];

}

const args = ['a', 'b'];

assert.deepEqual(

func.apply('hello', args),

['hello', 'a', 'b']);

The function method .bind()

.bind() is another method of function objects. This method is invoked as follows:

const boundFunc = someFunc.bind(thisValue, arg1, arg2);

.bind() returns a new function boundFunc(). Calling that function invokes someFunc() with this set to thisValue and these parameters: arg1, arg2, followed by the parameters of boundFunc().

That is, the following two function calls are equivalent:

boundFunc('a', 'b')

someFunc.call(thisValue, arg1, arg2, 'a', 'b')

An alternative to .bind()

Another way of pre-filling this and parameters is via an arrow function:

const boundFunc2 = (...args) =>

someFunc.call(thisValue, arg1, arg2, ...args);

An implementation of .bind()

Considering the previous section, .bind() can be implemented as a real function as follows:

function bind(func, thisValue, ...boundArgs) {

return (...args) =>

func.call(thisValue, ...boundArgs, ...args);

}

Example: binding a real function

Using .bind() for real functions is somewhat unintuitive because we have to provide a value for this. Given that it is undefined during function calls, it is usually set to undefined or null.

In the following example, we create add8(), a function that has one parameter, by binding the first parameter of add() to 8.

function add(x, y) {

return x + y;

}

const add8 = add.bind(undefined, 8);

assert.equal(add8(1), 9);

Quiz

See quiz app.

Evaluating code dynamically: eval(), new Function() (advanced)

- 26.1 eval()

- 26.2 new Function()

- 26.3 Recommendations

In this chapter, we’ll look at two ways of evaluating code dynamically: eval() and new Function().

eval()

Given a string str with JavaScript code, eval(str) evaluates that code and returns the result:

> eval('2 ** 4')

16

There are two ways of invoking eval():

- Directly, via a function call. Then the code in its argument is evaluated inside the current scope.

- Indirectly, not via a function call. Then it evaluates its code in global scope.

“Not via a function call” means “anything that looks different than eval(···)”:

- eval.call(undefined, '···') (uses method .call() of functions)

- eval?.() () (uses optional chaining)

- (0, eval)('···') (uses the comma operator)

- globalThis.eval('···')

- const e = eval; e('···')

- Etc.

The following code illustrates the difference:

globalThis.myVariable = 'global';

function func() {

const myVariable = 'local';

// Direct eval

assert.equal(eval('myVariable'), 'local');

// Indirect eval

assert.equal(eval.call(undefined, 'myVariable'), 'global');

}

Evaluating code in global context is safer because the code has access to fewer internals.

new Function()

new Function() creates a function object and is invoked as follows:

const func = new Function('«param_1»', ···, '«param_n»', '«func_body»');

The previous statement is equivalent to the next statement. Note that «param_1», etc., are not inside string literals, anymore.

const func = function («param_1», ···, «param_n») {

«func_body»

};

In the next example, we create the same function twice, first via new Function(), then via a function expression:

const times1 = new Function('a', 'b', 'return a * b');

const times2 = function (a, b) { return a * b };

new Function() creates non-strict mode functions

By default, functions created via new Function() are sloppy. If we want the function body to be in strict mode, we have to switch it on manually.

Recommendations

Avoid dynamic evaluation of code as much as you can:

- It’s a security risk because it may enable an attacker to execute arbitrary code with the privileges of your code.

- It may be switched off – for example, in browsers, via a Content Security Policy.

Very often, JavaScript is dynamic enough so that you don’t need eval() or similar. In the following example, what we are doing with eval() (line A) can be achieved just as well without it (line B).

const obj = {a: 1, b: 2};

const propKey = 'b';

assert.equal(eval('obj.' + propKey), 2); // (A)

assert.equal(obj[propKey], 2); // (B)

If you have to dynamically evaluate code:

- Prefer new Function() over eval(): it always executes its code in global context and a function provides a clean interface to the evaluated code.

- Prefer indirect eval over direct eval: evaluating code in global context is safer.

Modules

- 27.1 Cheat sheet: modules

- 27.1.1 Exporting

- 27.1.2 Importing

- 27.2 JavaScript source code formats

- 27.2.1 Code before built-in modules was written in ECMAScript 5

- 27.3 Before we had modules, we had scripts

- 27.4 Module systems created prior to ES6

- 27.4.1 Server side: CommonJS modules

- 27.4.2 Client side: AMD (Asynchronous Module Definition) modules

- 27.4.3 Characteristics of JavaScript modules

- 27.5 ECMAScript modules

- 27.5.1 ES modules: syntax, semantics, loader API

- 27.6 Named exports and imports

- 27.6.1 Named exports

- 27.6.2 Named imports

- 27.6.3 Namespace imports

- 27.6.4 Named exporting styles: inline versus clause (advanced)

- 27.7 Default exports and imports

- 27.7.1 The two styles of default-exporting

- 27.7.2 The default export as a named export (advanced)

- 27.8 More details on exporting and importing

- 27.8.1 Imports are read-only views on exports

- 27.8.2 ESM’s transparent support for cyclic imports (advanced)

- 27.9 npm packages

- 27.9.1 Packages are installed inside a directory node_modules/

- 27.9.2 Why can npm be used to install frontend libraries?

- 27.10 Naming modules

- 27.11 Module specifiers

- 27.11.1 Categories of module specifiers

- 27.11.2 ES module specifiers in browsers

- 27.11.3 ES module specifiers on Node.js

- 27.12 import.meta – metadata for the current module [ES2020]

- 27.12.1 import.meta.url

- 27.12.2 import.meta.url and class URL

- 27.12.3 import.meta.url on Node.js

- 27.13 Loading modules dynamically via import() [ES2020] (advanced)

- 27.13.1 The limitations of static import statements

- 27.13.2 Dynamic imports via the import() operator

- 27.13.3 Use cases for import()

- 27.14 Top-level await in modules [ES2022] (advanced)

- 27.14.1 Use cases for top-level await

- 27.14.2 How does top-level await work under the hood?

- 27.14.3 The pros and cons of top-level await

- 27.15 Polyfills: emulating native web platform features (advanced)

- 27.15.1 Sources of this section

Cheat sheet: modules

Exporting

// Named exports

export function f() {}

export const one = 1;

export {foo, b as bar};

// Default exports

export default function f() {} // declaration with optional name

// Replacement for `const` (there must be exactly one value)

export default 123;

// Re-exporting from another module

export {foo, b as bar} from './some-module.mjs';

export * from './some-module.mjs';

export * as ns from './some-module.mjs'; // ES2020

Importing

// Named imports

import {foo, bar as b} from './some-module.mjs';

// Namespace import

import * as someModule from './some-module.mjs';

// Default import

import someModule from './some-module.mjs';

// Combinations:

import someModule, * as someModule from './some-module.mjs';

import someModule, {foo, bar as b} from './some-module.mjs';

// Empty import (for modules with side effects)

import './some-module.mjs';

JavaScript source code formats

The current landscape of JavaScript modules is quite diverse: ES6 brought built-in modules, but the source code formats that came before them, are still around, too. Understanding the latter helps understand the former, so let’s investigate. The next sections describe the following ways of delivering JavaScript source code:

- Scripts are code fragments that browsers run in global scope. They are precursors of modules.

- CommonJS modules are a module format that is mainly used on servers (e.g., via Node.js).

- AMD modules are a module format that is mainly used in browsers.

- ECMAScript modules are JavaScript’s built-in module format. It supersedes all previous formats.

Tbl. 18 gives an overview of these code formats. Note that for CommonJS modules and ECMAScript modules, two filename extensions are commonly used. Which one is appropriate depends on how we want to use a file. Details are given later in this chapter.

Table 18: Ways of delivering JavaScript source code.

Runs on

Loaded

Filename ext.

Script

browsers

async

.js

CommonJS module

servers

sync

.js .cjs

AMD module

browsers

async

.js

ECMAScript module

browsers and servers

async

.js .mjs

Code before built-in modules was written in ECMAScript 5

Before we get to built-in modules (which were introduced with ES6), all code that we’ll see, will be written in ES5. Among other things:

- ES5 did not have const and let, only var.

- ES5 did not have arrow functions, only function expressions.

Before we had modules, we had scripts

Initially, browsers only had scripts – pieces of code that were executed in global scope. As an example, consider an HTML file that loads script files via the following HTML:

<script src="other-module1.js"></script>

<script src="other-module2.js"></script>

<script src="my-module.js"></script>

The main file is my-module.js, where we simulate a module:

var myModule = (function () { // Open IIFE

// Imports (via global variables)

var importedFunc1 = otherModule1.importedFunc1;

var importedFunc2 = otherModule2.importedFunc2;

// Body

function internalFunc() {

// ···

}

function exportedFunc() {

importedFunc1();

importedFunc2();

internalFunc();

}

// Exports (assigned to global variable `myModule`)

return {

exportedFunc: exportedFunc,

};

})(); // Close IIFE

myModule is a global variable that is assigned the result of immediately invoking a function expression. The function expression starts in the first line. It is invoked in the last line.

This way of wrapping a code fragment is called immediately invoked function expression (IIFE, coined by Ben Alman). What do we gain from an IIFE? var is not block-scoped (like const and let), it is function-scoped: the only way to create new scopes for var-declared variables is via functions or methods (with const and let, we can use either functions, methods, or blocks {}). Therefore, the IIFE in the example hides all of the following variables from global scope and minimizes name clashes: importedFunc1, importedFunc2, internalFunc, exportedFunc.

Note that we are using an IIFE in a particular manner: at the end, we pick what we want to export and return it via an object literal. That is called the revealing module pattern (coined by Christian Heilmann).

This way of simulating modules, has several issues:

- Libraries in script files export and import functionality via global variables, which risks name clashes.

- Dependencies are not stated explicitly, and there is no built-in way for a script to load the scripts it depends on. Therefore, the web page has to load not just the scripts that are needed by the page but also the dependencies of those scripts, the dependencies’ dependencies, etc. And it has to do so in the right order!

Module systems created prior to ES6

Prior to ECMAScript 6, JavaScript did not have built-in modules. Therefore, the flexible syntax of the language was used to implement custom module systems within the language. Two popular ones are:

- CommonJS (targeting the server side)

- AMD (Asynchronous Module Definition, targeting the client side)

Server side: CommonJS modules

The original CommonJS standard for modules was created for server and desktop platforms. It was the foundation of the original Node.js module system, where it achieved enormous popularity. Contributing to that popularity were the npm package manager for Node and tools that enabled using Node modules on the client side (browserify, webpack, and others).

From now on, CommonJS module means the Node.js version of this standard (which has a few additional features). This is an example of a CommonJS module:

// Imports

var importedFunc1 = require('./other-module1.js').importedFunc1;

var importedFunc2 = require('./other-module2.js').importedFunc2;

// Body

function internalFunc() {

// ···

}

function exportedFunc() {

importedFunc1();

importedFunc2();

internalFunc();

}

// Exports

module.exports = {

exportedFunc: exportedFunc,

};

CommonJS can be characterized as follows:

- Designed for servers.

- Modules are meant to be loaded synchronously (the importer waits while the imported module is loaded and executed).

- Compact syntax.

Client side: AMD (Asynchronous Module Definition) modules

The AMD module format was created to be easier to use in browsers than the CommonJS format. Its most popular implementation is RequireJS. The following is an example of an AMD module.

define(['./other-module1.js', './other-module2.js'],

function (otherModule1, otherModule2) {

var importedFunc1 = otherModule1.importedFunc1;

var importedFunc2 = otherModule2.importedFunc2;

function internalFunc() {

// ···

}

function exportedFunc() {

importedFunc1();

importedFunc2();

internalFunc();

}

return {

exportedFunc: exportedFunc,

};

});

AMD can be characterized as follows:

- Designed for browsers.

- Modules are meant to be loaded asynchronously. That’s a crucial requirement for browsers, where code can’t wait until a module has finished downloading. It has to be notified once the module is available.

- The syntax is slightly more complicated.

On the plus side, AMD modules can be executed directly. In contrast, CommonJS modules must either be compiled before deployment or custom source code must be generated and evaluated dynamically (think eval()). That isn’t always permitted on the web.

Characteristics of JavaScript modules

Looking at CommonJS and AMD, similarities between JavaScript module systems emerge:

- There is one module per file.

- Such a file is basically a piece of code that is executed:

- Local scope: The code is executed in a local “module scope”. Therefore, by default, all of the variables, functions, and classes declared in it are internal and not global.

- Exports: If we want any declared entity to be exported, we must explicitly mark it as an export.

- Imports: Each module can import exported entities from other modules. Those other modules are identified via module specifiers (usually paths, occasionally full URLs).

- Modules are singletons: Even if a module is imported multiple times, only a single “instance” of it exists.

- No global variables are used. Instead, module specifiers serve as global IDs.

ECMAScript modules

ECMAScript modules (ES modules or ESM) were introduced with ES6. They continue the tradition of JavaScript modules and have all of their aforementioned characteristics. Additionally:

- With CommonJS, ES modules share the compact syntax and support for cyclic dependencies.

- With AMD, ES modules share being designed for asynchronous loading.

ES modules also have new benefits:

- The syntax is even more compact than CommonJS’s.

- Modules have static structures (which can’t be changed at runtime). That helps with static checking, optimized access of imports, dead code elimination, and more.

- Support for cyclic imports is completely transparent.

This is an example of ES module syntax:

import {importedFunc1} from './other-module1.mjs';

import {importedFunc2} from './other-module2.mjs';

function internalFunc() {

···

}

export function exportedFunc() {

importedFunc1();

importedFunc2();

internalFunc();

}

From now on, “module” means “ECMAScript module”.

ES modules: syntax, semantics, loader API

The full standard of ES modules comprises the following parts:

1. Syntax (how code is written): What is a module? How are imports and exports declared? Etc.

2. Semantics (how code is executed): How are variable bindings exported? How are imports connected with exports? Etc.

3. A programmatic loader API for configuring module loading.

Parts 1 and 2 were introduced with ES6. Work on part 3 is ongoing.

Named exports and imports

Named exports

Each module can have zero or more named exports.

As an example, consider the following two files:

lib/my-math.mjs

main.mjs

Module my-math.mjs has two named exports: square and LIGHTSPEED.

// Not exported, private to module

function times(a, b) {

return a * b;

}

export function square(x) {

return times(x, x);

}

export const LIGHTSPEED = 299792458;

To export something, we put the keyword export in front of a declaration. Entities that are not exported are private to a module and can’t be accessed from outside.

Named imports

Module main.mjs has a single named import, square:

import {square} from './lib/my-math.mjs';

assert.equal(square(3), 9);

It can also rename its import:

import {square as sq} from './lib/my-math.mjs';

assert.equal(sq(3), 9);

Syntactic pitfall: named importing is not destructuring

Both named importing and destructuring look similar:

import {foo} from './bar.mjs'; // import

const {foo} = require('./bar.mjs'); // destructuring

But they are quite different:

- Imports remain connected with their exports.

- We can destructure again inside a destructuring pattern, but the {} in an import statement can’t be nested.

- The syntax for renaming is different:

import {foo as f} from './bar.mjs'; // importing

const {foo: f} = require('./bar.mjs'); // destructuring

Rationale: Destructuring is reminiscent of an object literal (including nesting), while importing evokes the idea of renaming.

Exercise: Named exports

exercises/modules/export_named_test.mjs

Namespace imports

Namespace imports are an alternative to named imports. If we namespace-import a module, it becomes an object whose properties are the named exports. This is what main.mjs looks like if we use a namespace import:

import * as myMath from './lib/my-math.mjs';

assert.equal(myMath.square(3), 9);

assert.deepEqual(

Object.keys(myMath), ['LIGHTSPEED', 'square']);

Named exporting styles: inline versus clause (advanced)

The named export style we have seen so far was inline: We exported entities by prefixing them with the keyword export.

But we can also use separate export clauses. For example, this is what lib/my-math.mjs looks like with an export clause:

function times(a, b) {

return a * b;

}

function square(x) {

return times(x, x);

}

const LIGHTSPEED = 299792458;

export { square, LIGHTSPEED }; // semicolon!

With an export clause, we can rename before exporting and use different names internally:

function times(a, b) {

return a * b;

}

function sq(x) {

return times(x, x);

}

const LS = 299792458;

export {

sq as square,

LS as LIGHTSPEED, // trailing comma is optional

};

Default exports and imports

Each module can have at most one default export. The idea is that the module is the default-exported value.

Avoid mixing named exports and default exports

A module can have both named exports and a default export, but it’s usually better to stick to one export style per module.

As an example for default exports, consider the following two files:

my-func.mjs

main.mjs

Module my-func.mjs has a default export:

const GREETING = 'Hello!';

export default function () {

return GREETING;

}

Module main.mjs default-imports the exported function:

import myFunc from './my-func.mjs';

assert.equal(myFunc(), 'Hello!');

Note the syntactic difference: the curly braces around named imports indicate that we are reaching into the module, while a default import is the module.

What are use cases for default exports?

The most common use case for a default export is a module that contains a single function or a single class.

The two styles of default-exporting

There are two styles of doing default exports.

First, we can label existing declarations with export default:

export default function myFunc() {} // no semicolon!

export default class MyClass {} // no semicolon!

Second, we can directly default-export values. This style of export default is much like a declaration.

export default myFunc; // defined elsewhere

export default MyClass; // defined previously

export default Math.sqrt(2); // result of invocation is default-exported

export default 'abc' + 'def';

export default { no: false, yes: true };

Why are there two default export styles?

The reason is that export default can’t be used to label const: const may define multiple values, but export default needs exactly one value. Consider the following hypothetical code:

// Not legal JavaScript!

export default const foo = 1, bar = 2, baz = 3;

With this code, we don’t know which one of the three values is the default export.

Exercise: Default exports

exercises/modules/export_default_test.mjs

The default export as a named export (advanced)

Internally, a default export is simply a named export whose name is default. As an example, consider the previous module my-func.mjs with a default export:

const GREETING = 'Hello!';

export default function () {

return GREETING;

}

The following module my-func2.mjs is equivalent to that module:

const GREETING = 'Hello!';

function greet() {

return GREETING;

}

export {

greet as default,

};

For importing, we can use a normal default import:

import myFunc from './my-func2.mjs';

assert.equal(myFunc(), 'Hello!');

Or we can use a named import:

import {default as myFunc} from './my-func2.mjs';

assert.equal(myFunc(), 'Hello!');

The default export is also available via property .default of namespace imports:

import * as mf from './my-func2.mjs';

assert.equal(mf.default(), 'Hello!');

Isn’t default illegal as a variable name?

default can’t be a variable name, but it can be an export name and it can be a property name:

const obj = {

default: 123,

};

assert.equal(obj.default, 123);

More details on exporting and importing

Imports are read-only views on exports

So far, we have used imports and exports intuitively, and everything seems to have worked as expected. But now it is time to take a closer look at how imports and exports are really related.

Consider the following two modules:

counter.mjs

main.mjs

counter.mjs exports a (mutable!) variable and a function:

export let counter = 3;

export function incCounter() {

counter++;

}

main.mjs name-imports both exports. When we use incCounter(), we discover that the connection to counter is live – we can always access the live state of that variable:

import { counter, incCounter } from './counter.mjs';

// The imported value `counter` is live

assert.equal(counter, 3);

incCounter();

assert.equal(counter, 4);

Note that while the connection is live and we can read counter, we cannot change this variable (e.g., via counter++).

There are two benefits to handling imports this way:

- It is easier to split modules because previously shared variables can become exports.

- This behavior is crucial for supporting transparent cyclic imports. Read on for more information.

ESM’s transparent support for cyclic imports (advanced)

ESM supports cyclic imports transparently. To understand how that is achieved, consider the following example: fig. 7 shows a directed graph of modules importing other modules. P importing M is the cycle in this case.

Figure 7: A directed graph of modules importing modules: M imports N and O, N imports P and Q, etc.

After parsing, these modules are set up in two phases:

- Instantiation: Every module is visited and its imports are connected to its exports. Before a parent can be instantiated, all of its children must be instantiated.

- Evaluation: The bodies of the modules are executed. Once again, children are evaluated before parents.

This approach handles cyclic imports correctly, due to two features of ES modules:

- Due to the static structure of ES modules, the exports are already known after parsing. That makes it possible to instantiate P before its child M: P can already look up M’s exports.

- When P is evaluated, M hasn’t been evaluated, yet. However, entities in P can already mention imports from M. They just can’t use them, yet, because the imported values are filled in later. For example, a function in P can access an import from M. The only limitation is that we must wait until after the evaluation of M, before calling that function.

Imports being filled in later is enabled by them being “live immutable views” on exports.

npm packages

The npm software registry is the dominant way of distributing JavaScript libraries and apps for Node.js and web browsers. It is managed via the npm package manager (short: npm). Software is distributed as so-called packages. A package is a directory containing arbitrary files and a file package.json at the top level that describes the package. For example, when npm creates an empty package inside a directory my-package/, we get this package.json:

{

"name": "my-package",

"version": "1.0.0",

"description": "",

"main": "index.js",

"scripts": {

"test": "echo \"Error: no test specified\" && exit 1"

},

"keywords": [],

"author": "",

"license": "ISC"

}

Some of these properties contain simple metadata:

- name specifies the name of this package. Once it is uploaded to the npm registry, it can be installed via npm install my-package.

- version is used for version management and follows semantic versioning, with three numbers:

- Major version: is incremented when incompatible API changes are made.

- Minor version: is incremented when functionality is added in a backward compatible manner.

- Patch version: is incremented when backward compatible changes are made.

- description, keywords, author make it easier to find packages.

- license clarifies how we can use this package.

Other properties enable advanced configuration:

- main: specifies the module that “is” the package (explained later in this chapter).

- scripts: are commands that we can execute via npm run. For example, the script test can be executed via npm run test.

For more information on package.json, consult the npm documentation.

Packages are installed inside a directory node_modules/

npm always installs packages inside a directory node_modules. There are usually many of these directories. Which one npm uses, depends on the directory where one currently is. For example, if we are inside a directory /tmp/a/b/, npm tries to find a node_modules in the current directory, its parent directory, the parent directory of the parent, etc. In other words, it searches the following chain of locations:

- /tmp/a/b/node_modules

- /tmp/a/node_modules

- /tmp/node_modules

When installing a package some-pkg, npm uses the closest node_modules. If, for example, we are inside /tmp/a/b/ and there is a node_modules in that directory, then npm puts the package inside the directory:

/tmp/a/b/node_modules/some-pkg/

When importing a module, we can use a special module specifier to tell Node.js that we want to import it from an installed package. How exactly that works, is explained later. For now, consider the following example:

// /home/jane/proj/main.mjs

import * as theModule from 'the-package/the-module.mjs';

To find the-module.mjs (Node.js prefers the filename extension .mjs for ES modules), Node.js walks up the node_module chain and searches the following locations:

- /home/jane/proj/node_modules/the-package/the-module.mjs

- /home/jane/node_modules/the-package/the-module.mjs

- /home/node_modules/the-package/the-module.mjs

Why can npm be used to install frontend libraries?

Finding installed modules in node_modules directories is only supported on Node.js. So why can we also use npm to install libraries for browsers?

That is enabled via bundling tools, such as webpack, that compile and optimize code before it is deployed online. During this compilation process, the code in npm packages is adapted so that it works in browsers.

Naming modules

There are no established best practices for naming module files and the variables they are imported into.

In this chapter, I’m using the following naming style:

- The names of module files are dash-cased and start with lowercase letters:

./my-module.mjs

./some-func.mjs

- The names of namespace imports are lowercased and camel-cased:

import * as myModule from './my-module.mjs';

- The names of default imports are lowercased and camel-cased:

import someFunc from './some-func.mjs';

What are the rationales behind this style?

- npm doesn’t allow uppercase letters in package names (source). Thus, we avoid camel case, so that “local” files have names that are consistent with those of npm packages. Using only lowercase letters also minimizes conflicts between file systems that are case-sensitive and file systems that aren’t: the former distinguish files whose names have the same letters, but with different cases; the latter don’t.

- There are clear rules for translating dash-cased file names to camel-cased JavaScript variable names. Due to how we name namespace imports, these rules work for both namespace imports and default imports.

I also like underscore-cased module file names because we can directly use these names for namespace imports (without any translation):

import * as my_module from './my_module.mjs';

But that style does not work for default imports: I like underscore-casing for namespace objects, but it is not a good choice for functions, etc.

Module specifiers

Module specifiers are the strings that identify modules. They work slightly differently in browsers and Node.js. Before we can look at the differences, we need to learn about the different categories of module specifiers.

Categories of module specifiers

In ES modules, we distinguish the following categories of specifiers. These categories originated with CommonJS modules.

- Relative path: starts with a dot. Examples:

'./some/other/module.mjs'

'../../lib/counter.mjs'

- Absolute path: starts with a slash. Example:

'/home/jane/file-tools.mjs'

- URL: includes a protocol (technically, paths are URLs, too). Examples:

'https://example.com/some-module.mjs'

'file:///home/john/tmp/main.mjs'

- Bare path: does not start with a dot, a slash or a protocol, and consists of a single filename without an extension. Examples:

'lodash'

'the-package'

- Deep import path: starts with a bare path and has at least one slash. Example:

'the-package/dist/the-module.mjs'

ES module specifiers in browsers

Browsers handle module specifiers as follows:

- Relative paths, absolute paths, and URLs work as expected. They all must point to real files (in contrast to CommonJS, which lets us omit filename extensions and more).

- The file name extensions of modules don’t matter, as long as they are served with the content type text/javascript.

- How bare paths will end up being handled is not yet clear. We will probably eventually be able to map them to other specifiers via lookup tables.

Note that bundling tools such as webpack, which combine modules into fewer files, are often less strict with specifiers than browsers. That’s because they operate at build/compile time (not at runtime) and can search for files by traversing the file system.

ES module specifiers on Node.js

Node.js handles module specifiers as follows:

- Relative paths are resolved as they are in web browsers – relative to the path of the current module.

- Absolute paths are currently not supported. As a workaround, we can use URLs that start with file:///. We can create such URLs via url.pathToFileURL().

- Only file: is supported as a protocol for URL specifiers.

- A bare path is interpreted as a package name and resolved relative to the closest node_modules directory. What module should be loaded, is determined by looking at property "main" of the package’s package.json (similarly to CommonJS).

- Deep import paths are also resolved relatively to the closest node_modules directory. They contain file names, so it is always clear which module is meant.

All specifiers, except bare paths, must refer to actual files. That is, ESM does not support the following CommonJS features:

- CommonJS automatically adds missing filename extensions.

- CommonJS can import a directory dir if there is a dir/package.json with a "main" property.

- CommonJS can import a directory dir if there is a module dir/index.js.

All built-in Node.js modules are available via bare paths and have named ESM exports – for example:

import * as assert from 'assert/strict';

import * as path from 'path';

assert.equal(

path.join('a/b/c', '../d'), 'a/b/d');

Filename extensions on Node.js

Node.js supports the following default filename extensions:

- .mjs for ES modules

- .cjs for CommonJS modules

The filename extension .js stands for either ESM or CommonJS. Which one it is is configured via the “closest” package.json (in the current directory, the parent directory, etc.). Using package.json in this manner is independent of packages.

In that package.json, there is a property "type", which has two settings:

- "commonjs" (the default): files with the extension .js or without an extension are interpreted as CommonJS modules.

- "module": files with the extension .js or without an extension are interpreted as ESM modules.

Interpreting non-file source code as either CommonJS or ESM

Not all source code executed by Node.js comes from files. We can also send it code via stdin, --eval, and --print. The command line option --input-type lets us specify how such code is interpreted:

- As CommonJS (the default): --input-type=commonjs

- As ESM: --input-type=module

import.meta – metadata for the current module [ES2020]

The object import.meta holds metadata for the current module.

import.meta.url

The most important property of import.meta is .url which contains a string with the URL of the current module’s file – for example:

'https://example.com/code/main.mjs'

import.meta.url and class URL

Class URL is available via a global variable in browsers and on Node.js. We can look up its full functionality in the Node.js documentation. When working with import.meta.url, its constructor is especially useful:

new URL(input: string, base?: string|URL)

Parameter input contains the URL to be parsed. It can be relative if the second parameter, base, is provided.

In other words, this constructor lets us resolve a relative path against a base URL:

> new URL('other.mjs', 'https://example.com/code/main.mjs').href

'https://example.com/code/other.mjs'

> new URL('../other.mjs', 'https://example.com/code/main.mjs').href

'https://example.com/other.mjs'

This is how we get a URL instance that points to a file data.txt that sits next to the current module:

const urlOfData = new URL('data.txt', import.meta.url);

import.meta.url on Node.js

On Node.js, import.meta.url is always a string with a file: URL – for example:

'file:///Users/rauschma/my-module.mjs'

Example: reading a sibling file of a module

Many Node.js file system operations accept either strings with paths or instances of URL. That enables us to read a sibling file data.txt of the current module:

import * as fs from 'fs';

function readData() {

// data.txt sits next to current module

const urlOfData = new URL('data.txt', import.meta.url);

return fs.readFileSync(urlOfData, {encoding: 'UTF-8'});

}

Module fs and URLs

For most functions of the module fs, we can refer to files via:

- Paths – in strings or instances of Buffer.

- URLs – in instances of URL (with the protocol file:)

For more information on this topic, see the Node.js API documentation.

Converting between file: URLs and paths

The Node.js module url has two functions for converting between file: URLs and paths:

- fileURLToPath(url: URL|string): string

Converts a file: URL to a path.

- pathToFileURL(path: string): URL

Converts a path to a file: URL.

If we need a path that can be used in the local file system, then property .pathname of URL instances does not always work:

assert.equal(

new URL('file:///tmp/with%20space.txt').pathname,

'/tmp/with%20space.txt');

Therefore, it is better to use fileURLToPath():

import * as url from 'url';

assert.equal(

url.fileURLToPath('file:///tmp/with%20space.txt'),

'/tmp/with space.txt'); // result on Unix

Similarly, pathToFileURL() does more than just prepend 'file://' to an absolute path.

Loading modules dynamically via import() [ES2020] (advanced)

The import() operator uses Promises

Promises are a technique for handling results that are computed asynchronously (i.e., not immediately). They are explained in [content not included]. It may make sense to postpone reading this section until you understand them.

The limitations of static import statements

So far, the only way to import a module has been via an import statement. That statement has several limitations:

- We must use it at the top level of a module. That is, we can’t, for example, import something when we are inside a function or inside an if statement.

- The module specifier is always fixed. That is, we can’t change what we import depending on a condition. And we can’t assemble a specifier dynamically.

Dynamic imports via the import() operator

The import() operator doesn’t have the limitations of import statements. It looks like this:

import(moduleSpecifierStr)

.then((namespaceObject) => {

console.log(namespaceObject.namedExport);

});

This operator is used like a function, receives a string with a module specifier and returns a Promise that resolves to a namespace object. The properties of that object are the exports of the imported module.

import() is even more convenient to use via await:

const namespaceObject = await import(moduleSpecifierStr);

console.log(namespaceObject.namedExport);

Note that await can be used at the top levels of modules (see next section).

Let’s look at an example of using import().

Example: loading a module dynamically

Consider the following files:

lib/my-math.mjs

main1.mjs

main2.mjs

We have already seen module my-math.mjs:

// Not exported, private to module

function times(a, b) {

return a * b;

}

export function square(x) {

return times(x, x);

}

export const LIGHTSPEED = 299792458;

We can use import() to load this module on demand:

// main1.mjs

const moduleSpecifier = './lib/my-math.mjs';

function mathOnDemand() {

return import(moduleSpecifier)

.then(myMath => {

const result = myMath.LIGHTSPEED;

assert.equal(result, 299792458);

return result;

});

}

mathOnDemand()

.then((result) => {

assert.equal(result, 299792458);

});

Two things in this code can’t be done with import statements:

- We are importing inside a function (not at the top level).

- The module specifier comes from a variable.

Next, we’ll implement the same functionality as in main1.mjs but via a feature called async function or async/await which provides nicer syntax for Promises.

// main2.mjs

const moduleSpecifier = './lib/my-math.mjs';

async function mathOnDemand() {

const myMath = await import(moduleSpecifier);

const result = myMath.LIGHTSPEED;

assert.equal(result, 299792458);

return result;

}

Why is import() an operator and not a function?

import() looks like a function but couldn’t be implemented as a function:

- It needs to know the URL of the current module in order to resolve relative module specifiers.

- If import() were a function, we’d have to explicitly pass this information to it (e.g. via an parameter).

- In contrast, an operator is a core language construct and has implicit access to more data, including the URL of the current module.

Use cases for import()

Loading code on demand

Some functionality of web apps doesn’t have to be present when they start, it can be loaded on demand. Then import() helps because we can put such functionality into modules – for example:

button.addEventListener('click', event => {

import('./dialogBox.mjs')

.then(dialogBox => {

dialogBox.open();

})

.catch(error => {

/* Error handling */

})

});

Conditional loading of modules

We may want to load a module depending on whether a condition is true. For example, a module with a polyfill that makes a new feature available on legacy platforms:

if (isLegacyPlatform()) {

import('./my-polyfill.mjs')

.then(···);

}

Computed module specifiers

For applications such as internationalization, it helps if we can dynamically compute module specifiers:

import(`messages_${getLocale()}.mjs`)

.then(···);

Top-level await in modules [ES2022] (advanced)

await is a feature of async functions

await is explained in [content not included]. It may make sense to postpone reading this section until you understand async functions.

We can use the await operator at the top level of a module. If we do that, the module becomes asynchronous and works differently. Thankfully, we don’t usually see that as programmers because it is handled transparently by the language.

Use cases for top-level await

Why would we want to use the await operator at the top level of a module? It lets us initialize a module with asynchronously loaded data. The next three subsections show three examples of where that is useful.

Loading modules dynamically

const params = new URLSearchParams(location.search);

const language = params.get('lang');

const messages = await import(`./messages-${language}.mjs`); // (A)

console.log(messages.welcome);

In line A, we dynamically import a module. Thanks to top-level await, that is almost as convenient as using a normal, static import.

Using a fallback if module loading fails

let lodash;

try {

lodash = await import('https://primary.example.com/lodash');

} catch {

lodash = await import('https://secondary.example.com/lodash');

}

Using whichever resource loads fastest

const resource = await Promise.any([

fetch('http://example.com/first.txt')

.then(response => response.text()),

fetch('http://example.com/second.txt')

.then(response => response.text()),

]);

Due to Promise.any(), variable resource is initialized via whichever download finishes first.

How does top-level await work under the hood?

Consider the following two files.

first.mjs:

const response = await fetch('http://example.com/first.txt');

export const first = await response.text();

main.mjs:

import {first} from './first.mjs';

import {second} from './second.mjs';

assert.equal(first, 'First!');

assert.equal(second, 'Second!');

Both are roughly equivalent to the following code:

first.mjs:

export let first;

export const promise = (async () => { // (A)

const response = await fetch('http://example.com/first.txt');

first = await response.text();

})();

main.mjs:

import {promise as firstPromise, first} from './first.mjs';

import {promise as secondPromise, second} from './second.mjs';

export const promise = (async () => { // (B)

await Promise.all([firstPromise, secondPromise]); // (C)

assert.equal(first, 'First content!');

assert.equal(second, 'Second content!');

})();

A module becomes asynchronous if:

1. It directly uses top-level await (first.mjs).

2. It imports one or more asynchronous modules (main.mjs).

Each asynchronous module exports a Promise (line A and line B) that is fulfilled after its body was executed. At that point, it is safe to access the exports of that module.

In case (2), the importing module waits until the Promises of all imported asynchronous modules are fulfilled, before it enters its body (line C). Synchronous modules are handled as usually.

Awaited rejections and synchronous exceptions are managed as in async functions.

The pros and cons of top-level await

The two most important benefits of top-level await are:

- It ensures that modules don’t access asynchronous imports before they are fully initialized.

- It handles asynchronicity transparently: Importers do not need to know if an imported module is asynchronous or not.

On the downside, top-level await delays the initialization of importing modules. Therefore, it‘s best used sparingly. Asynchronous tasks that take longer are better performed later, on demand.

However, even modules without top-level await can block importers (e.g. via an infinite loop at the top level), so blocking per se is not an argument against it.

Polyfills: emulating native web platform features (advanced)

Backends have polyfills, too

This section is about frontend development and web browsers, but similar ideas apply to backend development.

Polyfills help with a conflict that we are facing when developing a web application in JavaScript:

- On one hand, we want to use modern web platform features that make the app better and/or development easier.

- On the other hand, the app should run on as many browsers as possible.

Given a web platform feature X:

- A polyfill for X is a piece of code. If it is executed on a platform that already has built-in support for X, it does nothing. Otherwise, it makes the feature available on the platform. In the latter case, the polyfilled feature is (mostly) indistinguishable from a native implementation. In order to achieve that, the polyfill usually makes global changes. For example, it may modify global data or configure a global module loader. Polyfills are often packaged as modules.

- The term polyfill was coined by Remy Sharp.

- A speculative polyfill is a polyfill for a proposed web platform feature (that is not standardized, yet).

- Alternative term: prollyfill

- A replica of X is a library that reproduces the API and functionality of X locally. Such a library exists independently of a native (and global) implementation of X.

- Replica is a new term introduced in this section. Alternative term: ponyfill

- There is also the term shim, but it doesn’t have a universally agreed upon definition. It often means roughly the same as polyfill.

Every time our web applications starts, it must first execute all polyfills for features that may not be available everywhere. Afterwards, we can be sure that those features are available natively.

Sources of this section

- “What is a Polyfill?” by Remy Sharp

- Inspiration for the term replica: The Eiffel Tower in Las Vegas

- Useful clarification of “polyfill” and related terms: “Polyfills and the evolution of the Web”. Edited by Andrew Betts.

Quiz

See quiz app.

Objects

- 28.1 Cheat sheet: objects

- 28.1.1 Single objects

- 28.1.2 Prototype chains

- 28.2 What is an object?

- 28.2.1 The two ways of using objects

- 28.3 Fixed-layout objects

- 28.3.1 Object literals: properties

- 28.3.2 Object literals: property value shorthands

- 28.3.3 Getting properties

- 28.3.4 Setting properties

- 28.3.5 Object literals: methods

- 28.3.6 Object literals: accessors

- 28.4 Spreading into object literals (...) [ES2018]

- 28.4.1 Use case for spreading: copying objects

- 28.4.2 Use case for spreading: default values for missing properties

- 28.4.3 Use case for spreading: non-destructively changing properties

- 28.4.4 “Destructive spreading”: Object.assign() [ES6]

- 28.5 Methods and the special variable this

- 28.5.1 Methods are properties whose values are functions

- 28.5.2 The special variable this

- 28.5.3 Methods and .call()

- 28.5.4 Methods and .bind()

- 28.5.5 this pitfall: extracting methods

- 28.5.6 this pitfall: accidentally shadowing this

- 28.5.7 The value of this in various contexts (advanced)

- 28.6 Optional chaining for property getting and method calls [ES2020] (advanced)

- 28.6.1 Example: optional fixed property getting

- 28.6.2 The operators in more detail (advanced)

- 28.6.3 Short-circuiting with optional property getting

- 28.6.4 Optional chaining: downsides and alternatives

- 28.6.5 Frequently asked questions

- 28.7 Dictionary objects (advanced)

- 28.7.1 Quoted keys in object literals

- 28.7.2 Computed keys in object literals

- 28.7.3 The in operator: is there a property with a given key?

- 28.7.4 Deleting properties

- 28.7.5 Enumerability

- 28.7.6 Listing property keys via Object.keys() etc.

- 28.7.7 Listing property values via Object.values()

- 28.7.8 Listing property entries via Object.entries() [ES2017]

- 28.7.9 Properties are listed deterministically

- 28.7.10 Assembling objects via Object.fromEntries() [ES2019]

- 28.7.11 The pitfalls of using an object as a dictionary

- 28.8 Property attributes and freezing objects (advanced)

- 28.8.1 Property attributes and property descriptors [ES5]

- 28.8.2 Freezing objects [ES5]

- 28.9 Prototype chains

- 28.9.1 JavaScript’s operations: all properties vs. own properties

- 28.9.2 Pitfall: only the first member of a prototype chain is mutated

- 28.9.3 Tips for working with prototypes (advanced)

- 28.9.4 Object.hasOwn(): Is a given property own (non-inherited)? [ES2022]

- 28.9.5 Sharing data via prototypes

- 28.10 FAQ: objects

- 28.10.1 Why do objects preserve the insertion order of properties?

In this book, JavaScript’s style of object-oriented programming (OOP) is introduced in four steps. This chapter covers step 1 and 2; the next chapter covers step 3 and 4. The steps are (fig. 8):

1. Single objects (this chapter): How do objects, JavaScript’s basic OOP building blocks, work in isolation?

2. Prototype chains (this chapter): Each object has a chain of zero or more prototype objects. Prototypes are JavaScript’s core inheritance mechanism.

3. Classes (next chapter): JavaScript’s classes are factories for objects. The relationship between a class and its instances is based on prototypal inheritance (step 2).

4. Subclassing (next chapter): The relationship between a subclass and its superclass is also based on prototypal inheritance.

Figure 8: This book introduces object-oriented programming in JavaScript in four steps.

Cheat sheet: objects

Single objects

Creating an object via an object literal (starts and ends with a curly brace):

const myObject = { // object literal

myProperty: 1,

myMethod() {

return 2;

}, // comma!

get myAccessor() {

return this.myProperty;

}, // comma!

set myAccessor(value) {

this.myProperty = value;

}, // last comma is optional

};

assert.equal(

myObject.myProperty, 1

);

assert.equal(

myObject.myMethod(), 2

);

assert.equal(

myObject.myAccessor, 1

);

myObject.myAccessor = 3;

assert.equal(

myObject.myProperty, 3

);

Being able to create objects directly (without classes) is one of the highlights of JavaScript.

Spreading into objects:

const original = {

a: 1,

b: {

c: 3,

},

};

// Spreading (...) copies one object “into” another one:

const modifiedCopy = {

...original, // spreading

d: 4,

};

assert.deepEqual(

modifiedCopy,

{

a: 1,

b: {

c: 3,

},

d: 4,

}

);

// Caveat: spreading copies shallowly (property values are shared)

modifiedCopy.a = 5; // does not affect `original`

modifiedCopy.b.c = 6; // affects `original`

assert.deepEqual(

original,

{

a: 1, // unchanged

b: {

c: 6, // changed

},

},

);

We can also use spreading to make an unmodified (shallow) copy of an object:

const exactCopy = {...obj};

Prototype chains

Prototypes are JavaScript’s fundamental inheritance mechanism. Even classes are based on it. Each object has null or an object as its prototype. The latter object can also have a prototype, etc. In general, we get chains of prototypes.

Prototypes are managed like this:

// `obj1` has no prototype (its prototype is `null`)

const obj1 = Object.create(null); // (A)

assert.equal(

Object.getPrototypeOf(obj1), null // (B)

);

// `obj2` has the prototype `proto`

const proto = {

protoProp: 'protoProp',

};

const obj2 = {

__proto__: proto, // (C)

objProp: 'objProp',

}

assert.equal(

Object.getPrototypeOf(obj2), proto

);

Notes:

- Setting an object’s prototype while creating the object: line A, line C

- Retrieving the prototype of an object: line B

Each object inherits all the properties of its prototype:

// `obj2` inherits .protoProp from `proto`

assert.equal(

obj2.protoProp, 'protoProp'

);

assert.deepEqual(

Reflect.ownKeys(obj2),

['objProp'] // own properties of `obj2`

);

The non-inherited properties of an object are called its own properties.

The most important use case for prototypes is that several objects can share methods by inheriting them from a common prototype.

What is an object?

Objects in JavaScript:

- An object is a set of slots (key-value entries).

- Public slots are called properties:

- A property key can only be a string or a symbol.

- Private slots can only be created via classes and are explained in §29.2.4 “Public slots (properties) vs. private slots”.

The two ways of using objects

There are two ways of using objects in JavaScript:

- Fixed-layout objects: Used this way, objects work like records in databases. They have a fixed number of properties, whose keys are known at development time. Their values generally have different types.

const fixedLayoutObject = {

product: 'carrot',

quantity: 4,

};

- Dictionary objects: Used this way, objects work like lookup tables or maps. They have a variable number of properties, whose keys are not known at development time. All of their values have the same type.

const dictionaryObject = {

['one']: 1,

['two']: 2,

};

Note that the two ways can also be mixed: Some objects are both fixed-layout objects and dictionary objects.

The ways of using objects influence how they are explained in this chapter:

- First, we’ll explore fixed-layout objects. Even though property keys are strings or symbols under the hood, they will appear as fixed identifiers to us.

- Later, we’ll explore dictionary objects. Note that Maps are usually better dictionaries than objects. However, some of the operations that we’ll encounter are also useful for fixed-layout objects.

Fixed-layout objects

Let’s first explore fixed-layout objects.

Object literals: properties

Object literals are one way of creating fixed-layout objects. They are a stand-out feature of JavaScript: we can directly create objects – no need for classes! This is an example:

const jane = {

first: 'Jane',

last: 'Doe', // optional trailing comma

};

In the example, we created an object via an object literal, which starts and ends with curly braces {}. Inside it, we defined two properties (key-value entries):

- The first property has the key first and the value 'Jane'.

- The second property has the key last and the value 'Doe'.

Since ES5, trailing commas are allowed in object literals.

We will later see other ways of specifying property keys, but with this way of specifying them, they must follow the rules of JavaScript variable names. For example, we can use first_name as a property key, but not first-name). However, reserved words are allowed:

const obj = {

if: true,

const: true,

};

In order to check the effects of various operations on objects, we’ll occasionally use Object.keys() in this part of the chapter. It lists property keys:

> Object.keys({a:1, b:2})

[ 'a', 'b' ]

Object literals: property value shorthands

Whenever the value of a property is defined via a variable that has the same name as the key, we can omit the key.

function createPoint(x, y) {

return {x, y}; // Same as: {x: x, y: y}

}

assert.deepEqual(

createPoint(9, 2),

{ x: 9, y: 2 }

);

Getting properties

This is how we get (read) a property (line A):

const jane = {

first: 'Jane',

last: 'Doe',

};

// Get property .first

assert.equal(jane.first, 'Jane'); // (A)

Getting an unknown property produces undefined:

assert.equal(jane.unknownProperty, undefined);

Setting properties

This is how we set (write to) a property (line A):

const obj = {

prop: 1,

};

assert.equal(obj.prop, 1);

obj.prop = 2; // (A)

assert.equal(obj.prop, 2);

We just changed an existing property via setting. If we set an unknown property, we create a new entry:

const obj = {}; // empty object

assert.deepEqual(

Object.keys(obj), []);

obj.unknownProperty = 'abc';

assert.deepEqual(

Object.keys(obj), ['unknownProperty']);

Object literals: methods

The following code shows how to create the method .says() via an object literal:

const jane = {

first: 'Jane', // value property

says(text) {   // method

return `${this.first} says “${text}”`; // (A)

}, // comma as separator (optional at end)

};

assert.equal(jane.says('hello'), 'Jane says “hello”');

During the method call jane.says('hello'), jane is called the receiver of the method call and assigned to the special variable this (more on this in §28.5 “Methods and the special variable this”). That enables method .says() to access the sibling property .first in line A.

Object literals: accessors

An accessor is defined via syntax inside an object literal that looks like methods: a getter and/or a setter (i.e., each accessor has one or both of them).

Invoking an accessor looks like accessing a value property:

- Reading the property invokes the getter.

- Writing to the property invokes the setter.

Getters

A getter is created by prefixing a method definition with the modifier get:

const jane = {

first: 'Jane',

last: 'Doe',

get full() {

return `${this.first} ${this.last}`;

},

};

assert.equal(jane.full, 'Jane Doe');

jane.first = 'John';

assert.equal(jane.full, 'John Doe');

Setters

A setter is created by prefixing a method definition with the modifier set:

const jane = {

first: 'Jane',

last: 'Doe',

set full(fullName) {

const parts = fullName.split(' ');

this.first = parts[0];

this.last = parts[1];

},

};

jane.full = 'Richard Roe';

assert.equal(jane.first, 'Richard');

assert.equal(jane.last, 'Roe');

Exercise: Creating an object via an object literal

exercises/objects/color_point_object_test.mjs

Spreading into object literals (...) [ES2018]

Inside an object literal, a spread property adds the properties of another object to the current one:

> const obj = {one: 1, two: 2};

> {...obj, three: 3}

{ one: 1, two: 2, three: 3 }

const obj1 = {one: 1, two: 2};

const obj2 = {three: 3};

assert.deepEqual(

{...obj1, ...obj2, four: 4},

{one: 1, two: 2, three: 3,  four: 4}

);

If property keys clash, the property that is mentioned last “wins”:

> const obj = {one: 1, two: 2, three: 3};

> {...obj, one: true}

{ one: true, two: 2, three: 3 }

> {one: true, ...obj}

{ one: 1, two: 2, three: 3 }

All values are spreadable, even undefined and null:

> {...undefined}

{}

> {...null}

{}

> {...123}

{}

> {...'abc'}

{ '0': 'a', '1': 'b', '2': 'c' }

> {...['a', 'b']}

{ '0': 'a', '1': 'b' }

Property .length of strings and Arrays is hidden from this kind of operation (it is not enumerable; see §28.8.1 “Property attributes and property descriptors [ES5]” for more information).

Spreading includes properties whose keys are symbols (which are ignored by Object.keys(), Object.values() and Object.entries()):

const symbolKey = Symbol('symbolKey');

const obj = {

stringKey: 1,

[symbolKey]: 2,

};

assert.deepEqual(

{...obj, anotherStringKey: 3},

{

stringKey: 1,

[symbolKey]: 2,

anotherStringKey: 3,

}

);

Use case for spreading: copying objects

We can use spreading to create a copy of an object original:

const copy = {...original};

Caveat – copying is shallow: copy is a fresh object with duplicates of all properties (key-value entries) of original. But if property values are objects, then those are not copied themselves; they are shared between original and copy. Let’s look at an example:

const original = { a: 1, b: {prop: true} };

const copy = {...original};

The first level of copy is really a copy: If we change any properties at that level, it does not affect the original:

copy.a = 2;

assert.deepEqual(

original, { a: 1, b: {prop: true} }); // no change

However, deeper levels are not copied. For example, the value of .b is shared between original and copy. Changing .b in the copy also changes it in the original.

copy.b.prop = false;

assert.deepEqual(

original, { a: 1, b: {prop: false} });

JavaScript doesn’t have built-in support for deep copying

Deep copies of objects (where all levels are copied) are notoriously difficult to do generically. Therefore, JavaScript does not have a built-in operation for them (for now). If we need such an operation, we have to implement it ourselves.

Use case for spreading: default values for missing properties

If one of the inputs of our code is an object with data, we can make properties optional by specifying default values that are used if those properties are missing. One technique for doing so is via an object whose properties contain the default values. In the following example, that object is DEFAULTS:

const DEFAULTS = {alpha: 'a', beta: 'b'};

const providedData = {alpha: 1};

const allData = {...DEFAULTS, ...providedData};

assert.deepEqual(allData, {alpha: 1, beta: 'b'});

The result, the object allData, is created by copying DEFAULTS and overriding its properties with those of providedData.

But we don’t need an object to specify the default values; we can also specify them inside the object literal, individually:

const providedData = {alpha: 1};

const allData = {alpha: 'a', beta: 'b', ...providedData};

assert.deepEqual(allData, {alpha: 1, beta: 'b'});

Use case for spreading: non-destructively changing properties

So far, we have encountered one way of changing a property .alpha of an object: We set it (line A) and mutate the object. That is, this way of changing a property is destructive.

const obj = {alpha: 'a', beta: 'b'};

obj.alpha = 1; // (A)

assert.deepEqual(obj, {alpha: 1, beta: 'b'});

With spreading, we can change .alpha non-destructively – we make a copy of obj where .alpha has a different value:

const obj = {alpha: 'a', beta: 'b'};

const updatedObj = {...obj, alpha: 1};

assert.deepEqual(updatedObj, {alpha: 1, beta: 'b'});

Exercise: Non-destructively updating a property via spreading (fixed key)

exercises/objects/update_name_test.mjs

“Destructive spreading”: Object.assign() [ES6]

Object.assign() is a tool method:

Object.assign(target, source_1, source_2, ···)

This expression assigns all properties of source_1 to target, then all properties of source_2, etc. At the end, it returns target – for example:

const target = { a: 1 };

const result = Object.assign(

target,

{b: 2},

{c: 3, b: true});

assert.deepEqual(

result, { a: 1, b: true, c: 3 });

// target was modified and returned:

assert.equal(result, target);

The use cases for Object.assign() are similar to those for spread properties. In a way, it spreads destructively.

Methods and the special variable this

Methods are properties whose values are functions

Let’s revisit the example that was used to introduce methods:

const jane = {

first: 'Jane',

says(text) {

return `${this.first} says “${text}”`;

},

};

Somewhat surprisingly, methods are functions:

assert.equal(typeof jane.says, 'function');

Why is that? We learned in the chapter on callable values that ordinary functions play several roles. Method is one of those roles. Therefore, internally, jane roughly looks as follows.

const jane = {

first: 'Jane',

says: function (text) {

return `${this.first} says “${text}”`;

},

};

The special variable this

Consider the following code:

const obj = {

someMethod(x, y) {

assert.equal(this, obj); // (A)

assert.equal(x, 'a');

assert.equal(y, 'b');

}

};

obj.someMethod('a', 'b'); // (B)

In line B, obj is the receiver of a method call. It is passed to the function stored in obj.someMethod via an implicit (hidden) parameter whose name is this (line A).

How to understand this

The best way to understand this is as an implicit parameter of ordinary functions (and therefore methods, too).

Methods and .call()

Methods are functions and functions have methods themselves. One of those methods is .call(). Let’s look at an example to understand how this method works.

In the previous section, there was this method invocation:

obj.someMethod('a', 'b')

This invocation is equivalent to:

obj.someMethod.call(obj, 'a', 'b');

Which is also equivalent to:

const func = obj.someMethod;

func.call(obj, 'a', 'b');

.call() makes the normally implicit parameter this explicit: When invoking a function via .call(), the first parameter is this, followed by the regular (explicit) function parameters.

As an aside, this means that there are actually two different dot operators:

1. One for accessing properties: obj.prop

2. Another one for calling methods: obj.prop()

They are different in that (2) is not just (1) followed by the function call operator (). Instead, (2) additionally provides a value for this.

Methods and .bind()

.bind() is another method of function objects. In the following code, we use .bind() to turn method .says() into the stand-alone function func():

const jane = {

first: 'Jane',

says(text) {

return `${this.first} says “${text}”`; // (A)

},

};

const func = jane.says.bind(jane, 'hello');

assert.equal(func(), 'Jane says “hello”');

Setting this to jane via .bind() is crucial here. Otherwise, func() wouldn’t work properly because this is used in line A. In the next section, we’ll explore why that is.

this pitfall: extracting methods

We now know quite a bit about functions and methods and are ready to take a look at the biggest pitfall involving methods and this: function-calling a method extracted from an object can fail if we are not careful.

In the following example, we fail when we extract method jane.says(), store it in the variable func, and function-call func.

const jane = {

first: 'Jane',

says(text) {

return `${this.first} says “${text}”`;

},

};

const func = jane.says; // extract the method

assert.throws(

() => func('hello'), // (A)

{

name: 'TypeError',

message: "Cannot read properties of undefined (reading 'first')",

});

In line A, we are making a normal function call. And in normal function calls, this is undefined (if strict mode is active, which it almost always is). Line A is therefore equivalent to:

assert.throws(

() => jane.says.call(undefined, 'hello'), // `this` is undefined!

{

name: 'TypeError',

message: "Cannot read properties of undefined (reading 'first')",

}

);

How do we fix this? We need to use .bind() to extract method .says():

const func2 = jane.says.bind(jane);

assert.equal(func2('hello'), 'Jane says “hello”');

The .bind() ensures that this is always jane when we call func().

We can also use arrow functions to extract methods:

const func3 = text => jane.says(text);

assert.equal(func3('hello'), 'Jane says “hello”');

Example: extracting a method

The following is a simplified version of code that we may see in actual web development:

class ClickHandler {

constructor(id, elem) {

this.id = id;

elem.addEventListener('click', this.handleClick); // (A)

}

handleClick(event) {

alert('Clicked ' + this.id);

}

}

In line A, we don’t extract the method .handleClick() properly. Instead, we should do:

const listener = this.handleClick.bind(this);

elem.addEventListener('click', listener);

// Later, possibly:

elem.removeEventListener('click', listener);

Each invocation of .bind() creates a new function. That’s why we need to store the result somewhere if we want to remove it later on.

How to avoid the pitfall of extracting methods

Alas, there is no simple way around the pitfall of extracting methods: Whenever we extract a method, we have to be careful and do it properly – for example, by binding this or by using an arrow function.

Exercise: Extracting a method

exercises/objects/method_extraction_exrc.mjs

this pitfall: accidentally shadowing this

Accidentally shadowing this is only an issue with ordinary functions

Arrow functions don’t shadow this.

Consider the following problem: when we are inside an ordinary function, we can’t access the this of the surrounding scope because the ordinary function has its own this. In other words, a variable in an inner scope hides a variable in an outer scope. That is called shadowing. The following code is an example:

const prefixer = {

prefix: '==> ',

prefixStringArray(stringArray) {

return stringArray.map(

function (x) {

return this.prefix + x; // (A)

});

},

};

assert.throws(

() => prefixer.prefixStringArray(['a', 'b']),

{

name: 'TypeError',

message: "Cannot read properties of undefined (reading 'prefix')",

}

);

In line A, we want to access the this of .prefixStringArray(). But we can’t since the surrounding ordinary function has its own this that shadows (and blocks access to) the this of the method. The value of the former this is undefined due to the callback being function-called. That explains the error message.

The simplest way to fix this problem is via an arrow function, which doesn’t have its own this and therefore doesn’t shadow anything:

const prefixer = {

prefix: '==> ',

prefixStringArray(stringArray) {

return stringArray.map(

(x) => {

return this.prefix + x;

});

},

};

assert.deepEqual(

prefixer.prefixStringArray(['a', 'b']),

['==> a', '==> b']);

We can also store this in a different variable (line A), so that it doesn’t get shadowed:

prefixStringArray(stringArray) {

const that = this; // (A)

return stringArray.map(

function (x) {

return that.prefix + x;

});

},

Another option is to specify a fixed this for the callback via .bind() (line A):

prefixStringArray(stringArray) {

return stringArray.map(

function (x) {

return this.prefix + x;

}.bind(this)); // (A)

},

Lastly, .map() lets us specify a value for this (line A) that it uses when invoking the callback:

prefixStringArray(stringArray) {

return stringArray.map(

function (x) {

return this.prefix + x;

},

this); // (A)

},

Avoiding the pitfall of accidentally shadowing this

If we follow the advice in §25.3.4 “Recommendation: prefer specialized functions over ordinary functions”, we can avoid the pitfall of accidentally shadowing this. This is a summary:

- Use arrow functions as anonymous inline functions. They don’t have this as an implicit parameter and don’t shadow it.

- For named stand-alone function declarations we can either use arrow functions or function declarations. If we do the latter, we have to make sure this isn’t mentioned in their bodies.

The value of this in various contexts (advanced)

What is the value of this in various contexts?

Inside a callable entity, the value of this depends on how the callable entity is invoked and what kind of callable entity it is:

- Function call:

- Ordinary functions: this === undefined (in strict mode)

- Arrow functions: this is same as in surrounding scope (lexical this)

- Method call: this is receiver of call

- new: this refers to the newly created instance

We can also access this in all common top-level scopes:

- <script> element: this === globalThis

- ECMAScript modules: this === undefined

- CommonJS modules: this === module.exports

Tip: pretend that this doesn’t exist in top-level scopes

I like to do that because top-level this is confusing and there are better alternatives for its (few) use cases.

Optional chaining for property getting and method calls [ES2020] (advanced)

The following kinds of optional chaining operations exist:

obj?.prop     // optional fixed property getting

obj?.[«expr»] // optional dynamic property getting

func?.(«arg0», «arg1») // optional function or method call

The rough idea is:

- If the value before the question mark is neither undefined nor null, then perform the operation after the question mark.

- Otherwise, return undefined.

Each of the three syntaxes is covered in more detail later. These are a few first examples:

> null?.prop

undefined

> {prop: 1}?.prop

1

> null?.(123)

undefined

> String?.(123)

'123'

Example: optional fixed property getting

Consider the following data:

const persons = [

{

surname: 'Zoe',

address: {

street: {

name: 'Sesame Street',

number: '123',

},

},

},

{

surname: 'Mariner',

},

{

surname: 'Carmen',

address: {

},

},

];

We can use optional chaining to safely extract street names:

const streetNames = persons.map(

p => p.address?.street?.name);

assert.deepEqual(

streetNames, ['Sesame Street', undefined, undefined]

);

Handling defaults via nullish coalescing

The nullish coalescing operator allows us to use the default value '(no name)' instead of undefined:

const streetNames = persons.map(

p => p.address?.street?.name ?? '(no name)');

assert.deepEqual(

streetNames, ['Sesame Street', '(no name)', '(no name)']

);

The operators in more detail (advanced)

Optional fixed property getting

The following two expressions are equivalent:

o?.prop

(o !== undefined && o !== null) ? o.prop : undefined

Examples:

assert.equal(undefined?.prop, undefined);

assert.equal(null?.prop,      undefined);

assert.equal({prop:1}?.prop,  1);

Optional dynamic property getting

The following two expressions are equivalent:

o?.[«expr»]

(o !== undefined && o !== null) ? o[«expr»] : undefined

Examples:

const key = 'prop';

assert.equal(undefined?.[key], undefined);

assert.equal(null?.[key], undefined);

assert.equal({prop:1}?.[key], 1);

Optional function or method call

The following two expressions are equivalent:

f?.(arg0, arg1)

(f !== undefined && f !== null) ? f(arg0, arg1) : undefined

Examples:

assert.equal(undefined?.(123), undefined);

assert.equal(null?.(123), undefined);

assert.equal(String?.(123), '123');

Note that this operator produces an error if its left-hand side is not callable:

assert.throws(

() => true?.(123),

TypeError);

Why? The idea is that the operator only tolerates deliberate omissions. An uncallable value (other than undefined and null) is probably an error and should be reported, rather than worked around.

Short-circuiting with optional property getting

In a chain of property gettings and method invocations, evaluation stops once the first optional operator encounters undefined or null at its left-hand side:

function invokeM(value) {

return value?.a.b.m(); // (A)

}

const obj = {

a: {

b: {

m() { return 'result' }

}

}

};

assert.equal(

invokeM(obj), 'result'

);

assert.equal(

invokeM(undefined), undefined // (B)

);

Consider invokeM(undefined) in line B: undefined?.a is undefined. Therefore we’d expect .b to fail in line A. But it doesn’t: The ?. operator encounters the value undefined and the evaluation of the whole expression immediately returns undefined.

This behavior differs from a normal operator where JavaScript always evaluates all operands before evaluating the operator. It is called short-circuiting. Other short-circuiting operators are:

- (a && b): b is only evaluated if a is truthy.

- (a || b): b is only evaluated if a is falsy.

- (c ? t : e): If c is truthy, t is evaluated. Otherwise, e is evaluated.

Optional chaining: downsides and alternatives

Optional chaining also has downsides:

- Deeply nested structures are more difficult to manage. For example, refactoring is harder if there are many sequences of property names: Each one enforces the structure of multiple objects.

- Being so forgiving when accessing data hides problems that will surface much later and are then harder to debug. For example, a typo early in a sequence of optional property names has more negative effects than a normal typo.

An alternative to optional chaining is to extract the information once, in a single location:

- We can either write a helper function that extracts the data.

- Or we can write a function whose input is deeply nested data and whose output is simpler, normalized data.

With either approach, it is possible to perform checks and to fail early if there are problems.

Further reading:

- “Overly defensive programming” by Carl Vitullo

- Thread on Twitter by Cory House

Frequently asked questions

What is a good mnemonic for the optional chaining operator (?.)?

Are you occasionally unsure if the optional chaining operator starts with a dot (.?) or a question mark (?.)? Then this mnemonic may help you:

- IF (?) the left-hand side is not nullish

- THEN (.) access a property.

Why are there dots in o?.[x] and f?.()?

The syntaxes of the following two optional operator are not ideal:

obj?.[«expr»]          // better: obj?[«expr»]

func?.(«arg0», «arg1») // better: func?(«arg0», «arg1»)

Alas, the less elegant syntax is necessary because distinguishing the ideal syntax (first expression) from the conditional operator (second expression) is too complicated:

obj?['a', 'b', 'c'].map(x => x+x)

obj ? ['a', 'b', 'c'].map(x => x+x) : []

Why does null?.prop evaluate to undefined and not null?

The operator ?. is mainly about its right-hand side: Does property .prop exist? If not, stop early. Therefore, keeping information about its left-hand side is rarely useful. However, only having a single “early termination” value does simplify things.

Dictionary objects (advanced)

Objects work best as fixed-layout objects. But before ES6, JavaScript did not have a data structure for dictionaries (ES6 brought Maps). Therefore, objects had to be used as dictionaries, which imposed a signficant constraint: Dictionary keys had to be strings (symbols were also introduced with ES6).

We first look at features of objects that are related to dictionaries but also useful for fixed-layout objects. This section concludes with tips for actually using objects as dictionaries. (Spoiler: If possible, it’s better to use Maps.)

Quoted keys in object literals

So far, we have always used fixed-layout objects. Property keys were fixed tokens that had to be valid identifiers and internally became strings:

const obj = {

mustBeAnIdentifier: 123,

};

// Get property

assert.equal(obj.mustBeAnIdentifier, 123);

// Set property

obj.mustBeAnIdentifier = 'abc';

assert.equal(obj.mustBeAnIdentifier, 'abc');

As a next step, we’ll go beyond this limitation for property keys: In this subsection, we’ll use arbitrary fixed strings as keys. In the next subsection, we’ll dynamically compute keys.

Two syntaxes enable us to use arbitrary strings as property keys.

First, when creating property keys via object literals, we can quote property keys (with single or double quotes):

const obj = {

'Can be any string!': 123,

};

Second, when getting or setting properties, we can use square brackets with strings inside them:

// Get property

assert.equal(obj['Can be any string!'], 123);

// Set property

obj['Can be any string!'] = 'abc';

assert.equal(obj['Can be any string!'], 'abc');

We can also use these syntaxes for methods:

const obj = {

'A nice method'() {

return 'Yes!';

},

};

assert.equal(obj['A nice method'](), 'Yes!');

Computed keys in object literals

In the previous subsection, property keys were specified via fixed strings inside object literals. In this section we learn how to dynamically compute property keys. That enables us to use either arbitrary strings or symbols.

The syntax of dynamically computed property keys in object literals is inspired by dynamically accessing properties. That is, we can use square brackets to wrap expressions:

const obj = {

['Hello world!']: true,

['p'+'r'+'o'+'p']: 123,

[Symbol.toStringTag]: 'Goodbye', // (A)

};

assert.equal(obj['Hello world!'], true);

assert.equal(obj.prop, 123);

assert.equal(obj[Symbol.toStringTag], 'Goodbye');

The main use case for computed keys is having symbols as property keys (line A).

Note that the square brackets operator for getting and setting properties works with arbitrary expressions:

assert.equal(obj['p'+'r'+'o'+'p'], 123);

assert.equal(obj['==> prop'.slice(4)], 123);

Methods can have computed property keys, too:

const methodKey = Symbol();

const obj = {

[methodKey]() {

return 'Yes!';

},

};

assert.equal(obj[methodKey](), 'Yes!');

For the remainder of this chapter, we’ll mostly use fixed property keys again (because they are syntactically more convenient). But all features are also available for arbitrary strings and symbols.

Exercise: Non-destructively updating a property via spreading (computed key)

exercises/objects/update_property_test.mjs

The in operator: is there a property with a given key?

The in operator checks if an object has a property with a given key:

const obj = {

alpha: 'abc',

beta: false,

};

assert.equal('alpha' in obj, true);

assert.equal('beta' in obj, true);

assert.equal('unknownKey' in obj, false);

Checking if a property exists via truthiness

We can also use a truthiness check to determine if a property exists:

assert.equal(

obj.alpha ? 'exists' : 'does not exist',

'exists');

assert.equal(

obj.unknownKey ? 'exists' : 'does not exist',

'does not exist');

The previous checks work because obj.alpha is truthy and because reading a missing property returns undefined (which is falsy).

There is, however, one important caveat: truthiness checks fail if the property exists, but has a falsy value (undefined, null, false, 0, "", etc.):

assert.equal(

obj.beta ? 'exists' : 'does not exist',

'does not exist'); // should be: 'exists'

Deleting properties

We can delete properties via the delete operator:

const obj = {

myProp: 123,

};

assert.deepEqual(Object.keys(obj), ['myProp']);

delete obj.myProp;

assert.deepEqual(Object.keys(obj), []);

Enumerability

Enumerability is an attribute of a property. Non-enumerable properties are ignored by some operations – for example, by Object.keys() and when spreading properties. By default, most properties are enumerable. The next example shows how to change that and how it affects spreading.

const enumerableSymbolKey = Symbol('enumerableSymbolKey');

const nonEnumSymbolKey = Symbol('nonEnumSymbolKey');

// We create enumerable properties via an object literal

const obj = {

enumerableStringKey: 1,

[enumerableSymbolKey]: 2,

}

// For non-enumerable properties, we need a more powerful tool

Object.defineProperties(obj, {

nonEnumStringKey: {

value: 3,

enumerable: false,

},

[nonEnumSymbolKey]: {

value: 4,

enumerable: false,

},

});

// Non-enumerable properties are ignored by spreading:

assert.deepEqual(

{...obj},

{

enumerableStringKey: 1,

[enumerableSymbolKey]: 2,

}

);

Object.defineProperties() is explained later in this chapter. The next subsection shows how these operations are affected by enumerability:

Listing property keys via Object.keys() etc.

Table 19: Standard library methods for listing own (non-inherited) property keys. All of them return Arrays with strings and/or symbols.

enumerable

non-e.

string

symbol

Object.keys()

✔

✔

Object.getOwnPropertyNames()

✔

✔

✔

Object.getOwnPropertySymbols()

✔

✔

✔

Reflect.ownKeys()

✔

✔

✔

✔

Each of the methods in tbl. 19 returns an Array with the own property keys of the parameter. In the names of the methods, we can see that the following distinction is made:

- A property key can be either a string or a symbol.

- A property name is a property key whose value is a string.

- A property symbol is a property key whose value is a symbol.

To demonstrate the four operations, we revisit the example from the previous subsection:

const enumerableSymbolKey = Symbol('enumerableSymbolKey');

const nonEnumSymbolKey = Symbol('nonEnumSymbolKey');

const obj = {

enumerableStringKey: 1,

[enumerableSymbolKey]: 2,

}

Object.defineProperties(obj, {

nonEnumStringKey: {

value: 3,

enumerable: false,

},

[nonEnumSymbolKey]: {

value: 4,

enumerable: false,

},

});

assert.deepEqual(

Object.keys(obj),

['enumerableStringKey']

);

assert.deepEqual(

Object.getOwnPropertyNames(obj),

['enumerableStringKey', 'nonEnumStringKey']

);

assert.deepEqual(

Object.getOwnPropertySymbols(obj),

[enumerableSymbolKey, nonEnumSymbolKey]

);

assert.deepEqual(

Reflect.ownKeys(obj),

[

'enumerableStringKey', 'nonEnumStringKey',

enumerableSymbolKey, nonEnumSymbolKey,

]

);

Listing property values via Object.values()

Object.values() lists the values of all enumerable string-keyed properties of an object:

const firstName = Symbol('firstName');

const obj = {

[firstName]: 'Jane',

lastName: 'Doe',

};

assert.deepEqual(

Object.values(obj),

['Doe']);

Listing property entries via Object.entries() [ES2017]

Object.entries() lists all enumerable string-keyed properties as key-value pairs. Each pair is encoded as a two-element Array:

const firstName = Symbol('firstName');

const obj = {

[firstName]: 'Jane',

lastName: 'Doe',

};

assert.deepEqual(

Object.entries(obj),

[

['lastName', 'Doe'],

]);

A simple implementation of Object.entries()

The following function is a simplified version of Object.entries():

function entries(obj) {

return Object.keys(obj)

.map(key => [key, obj[key]]);

}

Exercise: Object.entries()

exercises/objects/find_key_test.mjs

Properties are listed deterministically

Own (non-inherited) properties of objects are always listed in the following order:

1. Properties with string keys that contain integer indices (that includes Array indices):

In ascending numeric order

2. Remaining properties with string keys:

In the order in which they were added

3. Properties with symbol keys:

In the order in which they were added

The following example demonstrates how property keys are sorted according to these rules:

> Object.keys({b:0,a:0, 10:0,2:0})

[ '2', '10', 'b', 'a' ]

The order of properties

The ECMAScript specification describes in more detail how properties are ordered.

Assembling objects via Object.fromEntries() [ES2019]

Given an iterable over [key, value] pairs, Object.fromEntries() creates an object:

const symbolKey = Symbol('symbolKey');

assert.deepEqual(

Object.fromEntries(

[

['stringKey', 1],

[symbolKey, 2],

]

),

{

stringKey: 1,

[symbolKey]: 2,

}

);

Object.fromEntries() does the opposite of Object.entries(). However, while Object.entries() ignores symbol-keyed properties, Object.fromEntries() doesn’t (see previous example).

To demonstrate both, we’ll use them to implement two tool functions from the library Underscore in the next subsubsections.

Example: pick()

The Underscore function pick() has the following signature:

pick(object, ...keys)

It returns a copy of object that has only those properties whose keys are mentioned in the trailing arguments:

const address = {

street: 'Evergreen Terrace',

number: '742',

city: 'Springfield',

state: 'NT',

zip: '49007',

};

assert.deepEqual(

pick(address, 'street', 'number'),

{

street: 'Evergreen Terrace',

number: '742',

}

);

We can implement pick() as follows:

function pick(object, ...keys) {

const filteredEntries = Object.entries(object)

.filter(([key, _value]) => keys.includes(key));

return Object.fromEntries(filteredEntries);

}

Example: invert()

The Underscore function invert() has the following signature:

invert(object)

It returns a copy of object where the keys and values of all properties are swapped:

assert.deepEqual(

invert({a: 1, b: 2, c: 3}),

{1: 'a', 2: 'b', 3: 'c'}

);

We can implement invert() like this:

function invert(object) {

const reversedEntries = Object.entries(object)

.map(([key, value]) => [value, key]);

return Object.fromEntries(reversedEntries);

}

A simple implementation of Object.fromEntries()

The following function is a simplified version of Object.fromEntries():

function fromEntries(iterable) {

const result = {};

for (const [key, value] of iterable) {

let coercedKey;

if (typeof key === 'string' || typeof key === 'symbol') {

coercedKey = key;

} else {

coercedKey = String(key);

}

result[coercedKey] = value;

}

return result;

}

Exercise: Using Object.entries() and Object.fromEntries()

exercises/objects/omit_properties_test.mjs

The pitfalls of using an object as a dictionary

If we use plain objects (created via object literals) as dictionaries, we have to look out for two pitfalls.

The first pitfall is that the in operator also finds inherited properties:

const dict = {};

assert.equal('toString' in dict, true);

We want dict to be treated as empty, but the in operator detects the properties it inherits from its prototype, Object.prototype.

The second pitfall is that we can’t use the property key __proto__ because it has special powers (it sets the prototype of the object):

const dict = {};

dict['__proto__'] = 123;

// No property was added to dict:

assert.deepEqual(Object.keys(dict), []);

Safely using objects as dictionaries

So how do we avoid the two pitfalls?

- If we can, we use Maps. They are the best solution for dictionaries.

- If we can’t, we use a library for objects-as-dictionaries that protects us from making mistakes.

- If that’s not possible or desired, we use an object without a prototype.

The following code demonstrates using prototype-less objects as dictionaries:

const dict = Object.create(null); // prototype is `null`

assert.equal('toString' in dict, false); // (A)

dict['__proto__'] = 123;

assert.deepEqual(Object.keys(dict), ['__proto__']);

We avoided both pitfalls:

- First, a property without a prototype does not inherit any properties (line A).

- Second, in modern JavaScript, __proto__ is implemented via Object.prototype. That means that it is switched off if Object.prototype is not in the prototype chain.

Exercise: Using an object as a dictionary

exercises/objects/simple_dict_test.mjs

Property attributes and freezing objects (advanced)

Property attributes and property descriptors [ES5]

Just as objects are composed of properties, properties are composed of attributes. The value of a property is only one of several attributes. Others include:

- writable: Is it possible to change the value of the property?

- enumerable: Is the property considered by Object.keys(), spreading, etc.?

When we are using one of the operations for handling property attributes, attributes are specified via property descriptors: objects where each property represents one attribute. For example, this is how we read the attributes of a property obj.myProp:

const obj = { myProp: 123 };

assert.deepEqual(

Object.getOwnPropertyDescriptor(obj, 'myProp'),

{

value: 123,

writable: true,

enumerable: true,

configurable: true,

});

And this is how we change the attributes of obj.myProp:

assert.deepEqual(Object.keys(obj), ['myProp']);

// Hide property `myProp` from Object.keys()

// by making it non-enumerable

Object.defineProperty(obj, 'myProp', {

enumerable: false,

});

assert.deepEqual(Object.keys(obj), []);

Further reading:

- Enumerability is covered in greater detail earlier in this chapter.

- For more information on property attributes and property descriptors, see Deep JavaScript.

Freezing objects [ES5]

Object.freeze(obj) makes obj completely immutable: We can’t change properties, add properties, or change its prototype – for example:

const frozen = Object.freeze({ x: 2, y: 5 });

assert.throws(

() => { frozen.x = 7 },

{

name: 'TypeError',

message: /^Cannot assign to read only property 'x'/,

});

Under the hood, Object.freeze() changes the attributes of properties (e.g., it makes them non-writable) and objects (e.g., it makes them non-extensible, meaning that no properties can be added anymore).

There is one caveat: Object.freeze(obj) freezes shallowly. That is, only the properties of obj are frozen but not objects stored in properties.

More information

For more information on freezing and other ways of locking down objects, see Deep JavaScript.

Prototype chains

Prototypes are JavaScript’s only inheritance mechanism: Each object has a prototype that is either null or an object. In the latter case, the object inherits all of the prototype’s properties.

In an object literal, we can set the prototype via the special property __proto__:

const proto = {

protoProp: 'a',

};

const obj = {

__proto__: proto,

objProp: 'b',

};

// obj inherits .protoProp:

assert.equal(obj.protoProp, 'a');

assert.equal('protoProp' in obj, true);

Given that a prototype object can have a prototype itself, we get a chain of objects – the so-called prototype chain. Inheritance gives us the impression that we are dealing with single objects, but we are actually dealing with chains of objects.

Fig. 9 shows what the prototype chain of obj looks like.

Figure 9: obj starts a chain of objects that continues with proto and other objects.

Non-inherited properties are called own properties. obj has one own property, .objProp.

JavaScript’s operations: all properties vs. own properties

Some operations consider all properties (own and inherited) – for example, getting properties:

> const obj = { one: 1 };

> typeof obj.one // own

'number'

> typeof obj.toString // inherited

'function'

Other operations only consider own properties – for example, Object.keys():

> Object.keys(obj)

[ 'one' ]

Read on for another operation that also only considers own properties: setting properties.

Pitfall: only the first member of a prototype chain is mutated

Given an object obj with a chain of prototype objects, it makes sense that setting an own property of obj only changes obj. However, setting an inherited property via obj also only changes obj. It creates a new own property in obj that overrides the inherited property. Let’s explore how that works with the following object:

const proto = {

protoProp: 'a',

};

const obj = {

__proto__: proto,

objProp: 'b',

};

In the next code snippet, we set the inherited property obj.protoProp (line A). That “changes” it by creating an own property: When reading obj.protoProp, the own property is found first and its value overrides the value of the inherited property.

// In the beginning, obj has one own property

assert.deepEqual(Object.keys(obj), ['objProp']);

obj.protoProp = 'x'; // (A)

// We created a new own property:

assert.deepEqual(Object.keys(obj), ['objProp', 'protoProp']);

// The inherited property itself is unchanged:

assert.equal(proto.protoProp, 'a');

// The own property overrides the inherited property:

assert.equal(obj.protoProp, 'x');

The prototype chain of obj is depicted in fig. 10.

Figure 10: The own property .protoProp of obj overrides the property inherited from proto.

Tips for working with prototypes (advanced)

Getting and setting prototypes

Recommendations for __proto__:

- Don’t use __proto__ as a pseudo-property (a setter of all instances of Object):

- It can’t be used with all objects (e.g. objects that are not instances of Object).

- The language specification has deprecated it.

For more information on this feature see §29.8.7 “Object.prototype.__proto__ (accessor)”.

- Using __proto__ in object literals to set prototypes is different: It’s a feature of object literals that has no pitfalls.

The recommended ways of getting and setting prototypes are:

- Getting the prototype of an object:

Object.getPrototypeOf(obj: Object) : Object

- The best time to set the prototype of an object is when we are creating it. We can do so via __proto__ in an object literal or via:

Object.create(proto: Object) : Object

If we have to, we can use Object.setPrototypeOf() to change the prototype of an existing object. But that may affect performance negatively.

This is how these features are used:

const proto1 = {};

const proto2a = {};

const proto2b = {};

const obj1 = {

__proto__: proto1,

a: 1,

b: 2,

};

assert.equal(Object.getPrototypeOf(obj1), proto1);

const obj2 = Object.create(

proto2a,

{

a: {

value: 1,

writable: true,

enumerable: true,

configurable: true,

},

b: {

value: 2,

writable: true,

enumerable: true,

configurable: true,

},

}

);

assert.equal(Object.getPrototypeOf(obj2), proto2a);

Object.setPrototypeOf(obj2, proto2b);

assert.equal(Object.getPrototypeOf(obj2), proto2b);

Checking if an object is in the prototype chain of another object

So far, “proto is a prototype of obj” always meant “proto is a direct prototype of obj”. But it can also be used more loosely and mean that proto is in the prototype chain of obj. That looser relationship can be checked via .isPrototypeOf():

For example:

const a = {};

const b = {__proto__: a};

const c = {__proto__: b};

assert.equal(a.isPrototypeOf(b), true);

assert.equal(a.isPrototypeOf(c), true);

assert.equal(c.isPrototypeOf(a), false);

assert.equal(a.isPrototypeOf(a), false);

For more information on this method see §29.8.5 “Object.prototype.isPrototypeOf()”.

Object.hasOwn(): Is a given property own (non-inherited)? [ES2022]

The in operator (line A) checks if an object has a given property. In contrast, Object.hasOwn() (lines B and C) checks if a property is own.

const proto = {

protoProp: 'protoProp',

};

const obj = {

__proto__: proto,

objProp: 'objProp',

}

assert.equal('protoProp' in obj, true); // (A)

assert.equal(Object.hasOwn(obj, 'protoProp'), false); // (B)

assert.equal(Object.hasOwn(proto, 'protoProp'), true); // (C)

Alternative before ES2022: .hasOwnProperty()

Before ES2022, we can use another feature: §29.8.8 “Object.prototype.hasOwnProperty()”. This feature has pitfalls, but the referenced section explains how to work around them.

Sharing data via prototypes

Consider the following code:

const jane = {

firstName: 'Jane',

describe() {

return 'Person named '+this.firstName;

},

};

const tarzan = {

firstName: 'Tarzan',

describe() {

return 'Person named '+this.firstName;

},

};

assert.equal(jane.describe(), 'Person named Jane');

assert.equal(tarzan.describe(), 'Person named Tarzan');

We have two objects that are very similar. Both have two properties whose names are .firstName and .describe. Additionally, method .describe() is the same. How can we avoid duplicating that method?

We can move it to an object PersonProto and make that object a prototype of both jane and tarzan:

const PersonProto = {

describe() {

return 'Person named ' + this.firstName;

},

};

const jane = {

__proto__: PersonProto,

firstName: 'Jane',

};

const tarzan = {

__proto__: PersonProto,

firstName: 'Tarzan',

};

The name of the prototype reflects that both jane and tarzan are persons.

Figure 11: Objects jane and tarzan share method .describe(), via their common prototype PersonProto.

Fig. 11 illustrates how the three objects are connected: The objects at the bottom now contain the properties that are specific to jane and tarzan. The object at the top contains the properties that are shared between them.

When we make the method call jane.describe(), this points to the receiver of that method call, jane (in the bottom-left corner of the diagram). That’s why the method still works. tarzan.describe() works similarly.

assert.equal(jane.describe(), 'Person named Jane');

assert.equal(tarzan.describe(), 'Person named Tarzan');

Looking ahead to the next chapter on classes – this is how classes are organized internally:

- All instances share a common prototype with methods.

- Instance-specific data is stored in own properties in each instance.

§29.3 “The internals of classes” explains this in more detail.

FAQ: objects

Why do objects preserve the insertion order of properties?

In principle, objects are unordered. The main reason for ordering properties is so that operations that list entries, keys, or values are deterministic. That helps, e.g., with testing.

Quiz

See quiz app.

Classes [ES6]

- 29.1 Cheat sheet: classes

- 29.2 The essentials of classes

- 29.2.1 A class for persons

- 29.2.2 Class expressions

- 29.2.3 The instanceof operator

- 29.2.4 Public slots (properties) vs. private slots

- 29.2.5 Private slots in more detail [ES2022] (advanced)

- 29.2.6 The pros and cons of classes in JavaScript

- 29.2.7 Tips for using classes

- 29.3 The internals of classes

- 29.3.1 A class is actually two connected objects

- 29.3.2 Classes set up the prototype chains of their instances

- 29.3.3 .__proto__ vs. .prototype

- 29.3.4 Person.prototype.constructor (advanced)

- 29.3.5 Dispatched vs. direct method calls (advanced)

- 29.3.6 Classes evolved from ordinary functions (advanced)

- 29.4 Prototype members of classes

- 29.4.1 Public prototype methods and accessors

- 29.4.2 Private methods and accessors [ES2022]

- 29.5 Instance members of classes [ES2022]

- 29.5.1 Instance public fields

- 29.5.2 Instance private fields

- 29.5.3 Private instance data before ES2022 (advanced)

- 29.5.4 Simulating protected visibility and friend visibility via WeakMaps (advanced)

- 29.6 Static members of classes

- 29.6.1 Static public methods and accessors

- 29.6.2 Static public fields [ES2022]

- 29.6.3 Static private methods, accessors, and fields [ES2022]

- 29.6.4 Static initialization blocks in classes [ES2022]

- 29.6.5 Pitfall: Using this to access static private fields

- 29.6.6 All members (static, prototype, instance) can access all private members

- 29.6.7 Static private methods and data before ES2022

- 29.6.8 Static factory methods

- 29.7 Subclassing

- 29.7.1 The internals of subclassing (advanced)

- 29.7.2 instanceof and subclassing (advanced)

- 29.7.3 Not all objects are instances of Object (advanced)

- 29.7.4 Prototype chains of built-in objects (advanced)

- 29.7.5 Mixin classes (advanced)

- 29.8 The methods and accessors of Object.prototype (advanced)

- 29.8.1 Using Object.prototype methods safely

- 29.8.2 Object.prototype.toString()

- 29.8.3 Object.prototype.toLocaleString()

- 29.8.4 Object.prototype.valueOf()

- 29.8.5 Object.prototype.isPrototypeOf()

- 29.8.6 Object.prototype.propertyIsEnumerable()

- 29.8.7 Object.prototype.__proto__ (accessor)

- 29.8.8 Object.prototype.hasOwnProperty()

- 29.9 FAQ: classes

- 29.9.1 Why are they called “instance private fields” in this book and not “private instance fields”?

- 29.9.2 Why the identifier prefix #? Why not declare private fields via private?

In this book, JavaScript’s style of object-oriented programming (OOP) is introduced in four steps. This chapter covers step 3 and 4, the previous chapter covers step 1 and 2. The steps are (fig. 12):

1. Single objects (previous chapter): How do objects, JavaScript’s basic OOP building blocks, work in isolation?

2. Prototype chains (previous chapter): Each object has a chain of zero or more prototype objects. Prototypes are JavaScript’s core inheritance mechanism.

3. Classes (this chapter): JavaScript’s classes are factories for objects. The relationship between a class and its instances is based on prototypal inheritance (step 2).

4. Subclassing (this chapter): The relationship between a subclass and its superclass is also based on prototypal inheritance.

Figure 12: This book introduces object-oriented programming in JavaScript in four steps.

Cheat sheet: classes

Superclass:

class Person {

#firstName; // (A)

constructor(firstName) {

this.#firstName = firstName; // (B)

}

describe() {

return `Person named ${this.#firstName}`;

}

static extractNames(persons) {

return persons.map(person => person.#firstName);

}

}

const tarzan = new Person('Tarzan');

assert.equal(

tarzan.describe(),

'Person named Tarzan'

);

assert.deepEqual(

Person.extractNames([tarzan, new Person('Cheeta')]),

['Tarzan', 'Cheeta']

);

Subclass:

class Employee extends Person {

constructor(firstName, title) {

super(firstName);

this.title = title; // (C)

}

describe() {

return super.describe() +

` (${this.title})`;

}

}

const jane = new Employee('Jane', 'CTO');

assert.equal(

jane.title,

'CTO'

);

assert.equal(

jane.describe(),

'Person named Jane (CTO)'

);

Notes:

- .#firstName is a private field and must be declared (line A) before it can be initialized (line B).

- A private field can only be accessed inside its surrounding class. It can’t even be accessed by subclasses.

- .title is a property and can be initialized without a prior declaration (line C). JavaScript relatively often makes instance data public (in contrast to, e.g., Java that prefers to hide it).

The essentials of classes

Classes are basically a compact syntax for setting up prototype chains (which are explained in the previous chapter). Under the hood, JavaScript’s classes are unconventional. But that is something we rarely see when working with them. They should normally feel familiar to people who have used other object-oriented programming languages.

Note that we don’t need classes to create objects. We can also do so via object literals. That’s why the singleton pattern isn’t needed in JavaScript and classes are used less than in many other languages that have them.

A class for persons

We have previously worked with jane and tarzan, single objects representing persons. Let’s use a class declaration to implement a factory for such objects:

class Person {

#firstName; // (A)

constructor(firstName) {

this.#firstName = firstName; // (B)

}

describe() {

return `Person named ${this.#firstName}`;

}

static extractNames(persons) {

return persons.map(person => person.#firstName);

}

}

jane and tarzan can now be created via new Person():

const jane = new Person('Jane');

const tarzan = new Person('Tarzan');

Let’s examine what’s inside the body of class Person.

- .constructor() is a special method that is called after the creation of a new instance. Inside it, this refers to that instance.

- [ES2022] .#firstName is an instance private field: Such fields are stored in instances. They are accessed similarly to properties, but their names are separate – they always start with hash symbols (#). And they are invisible to the world outside the class:

assert.deepEqual(

Reflect.ownKeys(jane),

[]

);

Before we can initialize .#firstName in the constructor (line B), we need to declare it by mentioning it in the class body (line A).

- .describe() is a method. If we invoke it via obj.describe() then this refers to obj inside the body of .describe().

assert.equal(

jane.describe(), 'Person named Jane'

);

assert.equal(

tarzan.describe(), 'Person named Tarzan'

);

- .extractName() is a static method. “Static” means that it belongs to the class, not to instances:

assert.deepEqual(

Person.extractNames([jane, tarzan]),

['Jane', 'Tarzan']

);

We can also create instance properties (public fields) in constructors:

class Container {

constructor(value) {

this.value = value;

}

}

const abcContainer = new Container('abc');

assert.equal(

abcContainer.value, 'abc'

);

In contrast to instance private fields, instance properties don’t have to be declared in class bodies.

Class expressions

There are two kinds of class definitions (ways of defining classes):

- Class declarations, which we have seen in the previous section.

- Class expressions, which we’ll see next.

Class expressions can be anonymous and named:

// Anonymous class expression

const Person = class { ··· };

// Named class expression

const Person = class MyClass { ··· };

The name of a named class expression works similarly to the name of a named function expression: It can only be accessed inside the body of a class and stays the same, regardless of what the class is assigned to.

The instanceof operator

The instanceof operator tells us if a value is an instance of a given class:

> new Person('Jane') instanceof Person

true

> {} instanceof Person

false

> {} instanceof Object

true

> [] instanceof Array

true

We’ll explore the instanceof operator in more detail later, after we have looked at subclassing.

Public slots (properties) vs. private slots

In the JavaScript language, objects can have two kinds of “slots”.

- Public slots (which are are also called properties). For example, methods are public slots.

- Private slots [ES2022]. For example, private fields are private slots.

These are the most important rules we need to know about properties and private slots:

- In classes, we can use public and private versions of fields, methods, getters and setters. All of them are slots in objects. Which objects they are placed in depends on whether the keyword static is used and other factors.

- A getter and a setter that have the same key create a single accessor slot. An Accessor can also have only a getter or only a setter.

- Properties and private slots are very different – for example:

- They are stored separately.

- Their keys are different. The keys of private slots can’t even be accessed directly (see §29.2.5.2 “Each private slot has a unique key (a private name)” later in this chapter).

- Properties are inherited from prototypes, private slots aren’t.

- Private slots can only be created via classes.

More information on properties and private slots

This chapter doesn’t cover all details of properties and private slots (just the essentials). If you want to dig deeper, you can do so here:

- §28.8.1 “Property attributes and property descriptors [ES5]”

- Section “Object Internal Methods and Internal Slots” in the ECMAScript language specification explains how private slots work. Search for “[[PrivateElements]]”.

The following class demonstrates the two kinds of slots. Each of its instances has one private field and one property:

class MyClass {

#instancePrivateField = 1;

instanceProperty = 2;

getInstanceValues() {

return [

this.#instancePrivateField,

this.instanceProperty,

];

}

}

const inst = new MyClass();

assert.deepEqual(

inst.getInstanceValues(), [1, 2]

);

As expected, outside MyClass, we can only see the property:

assert.deepEqual(

Reflect.ownKeys(inst),

['instanceProperty']

);

Next, we’ll look at some of the details of private slots.

Private slots in more detail [ES2022] (advanced)

Private slots can’t be accessed in subclasses

A private slot really can only be accessed inside the body of its class. We can’t even access it from a subclass:

class SuperClass {

#superProp = 'superProp';

}

class SubClass extends SuperClass {

getSuperProp() {

return this.#superProp;

}

}

// SyntaxError: Private field '#superProp'

// must be declared in an enclosing class

Subclassing via extends is explained later in this chapter. How to work around this limitation is explained in §29.5.4 “Simulating protected visibility and friend visibility via WeakMaps”.

Each private slot has a unique key (a private name)

Private slots have unique keys that are similar to symbols. Consider the following class from earlier:

class MyClass {

#instancePrivateField = 1;

instanceProperty = 2;

getInstanceValues() {

return [

this.#instancePrivateField,

this.instanceProperty,

];

}

}

Internally, the private field of MyClass is handled roughly like this:

let MyClass;

{ // Scope of the body of the class

const instancePrivateFieldKey = Symbol();

MyClass = class {

// Very loose approximation of how this

// works in the language specification

__PrivateElements__ = new Map([

[instancePrivateFieldKey, 1],

]);

instanceProperty = 2;

getInstanceValues() {

return [

this.__PrivateElements__.get(instancePrivateFieldKey),

this.instanceProperty,

];

}

}

}

The value of instancePrivateFieldKey is called a private name. We can’t use private names directly in JavaScript, we can only use them indirectly, via the fixed identifiers of private fields, private methods, and private accessors. Where the fixed identifiers of public slots (such as getInstanceValues) are interpreted as string keys, the fixed identifiers of private slots (such as #instancePrivateField) refer to private names (similarly to how variable names refer to values).

The same private identifier refers to different private names in different classes

Because the identifiers of private slots aren’t used as keys, using the same identifier in different classes produces different slots (line A and line C):

class Color {

#name; // (A)

constructor(name) {

this.#name = name; // (B)

}

static getName(obj) {

return obj.#name;

}

}

class Person {

#name; // (C)

constructor(name) {

this.#name = name;

}

}

assert.equal(

Color.getName(new Color('green')), 'green'

);

// We can’t access the private slot #name of a Person in line B:

assert.throws(

() => Color.getName(new Person('Jane')),

{

name: 'TypeError',

message: 'Cannot read private member #name from'

+ ' an object whose class did not declare it',

}

);

The names of private fields never clash

Even if a subclass uses the same name for a private field, the two names never clash because they refer to private names (which are always unique). In the following example, .#privateField in SuperClass does not clash with .#privateField in SubClass, even though both slots are stored directly in inst:

class SuperClass {

#privateField = 'super';

getSuperPrivateField() {

return this.#privateField;

}

}

class SubClass extends SuperClass {

#privateField = 'sub';

getSubPrivateField() {

return this.#privateField;

}

}

const inst = new SubClass();

assert.equal(

inst.getSuperPrivateField(), 'super'

);

assert.equal(

inst.getSubPrivateField(), 'sub'

);

Subclassing via extends is explained later in this chapter.

Using in to check if an object has a given private slot

The in operator can be used to check if a private slot exists (line A):

class Color {

#name;

constructor(name) {

this.#name = name;

}

static check(obj) {

return #name in obj; // (A)

}

}

Let’s look at more examples of in applied to private slots.

Private methods. The following code shows that private methods create private slots in instances:

class C1 {

#priv() {}

static check(obj) {

return #priv in obj;

}

}

assert.equal(C1.check(new C1()), true);

Static private fields. We can also use in for a static private field:

class C2 {

static #priv = 1;

static check(obj) {

return #priv in obj;

}

}

assert.equal(C2.check(C2), true);

assert.equal(C2.check(new C2()), false);

Static private methods. And we can check for the slot of a static private method:

class C3 {

static #priv() {}

static check(obj) {

return #priv in obj;

}

}

assert.equal(C3.check(C3), true);

Using the same private identifier in different classes. In the next example, the two classes Color and Person both have a slot whose identifier is #name. The in operator distinguishes them correctly:

class Color {

#name;

constructor(name) {

this.#name = name;

}

static check(obj) {

return #name in obj;

}

}

class Person {

#name;

constructor(name) {

this.#name = name;

}

static check(obj) {

return #name in obj;

}

}

// Detecting Color’s #name

assert.equal(

Color.check(new Color()), true

);

assert.equal(

Color.check(new Person()), false

);

// Detecting Person’s #name

assert.equal(

Person.check(new Person()), true

);

assert.equal(

Person.check(new Color()), false

);

The pros and cons of classes in JavaScript

I recommend using classes for the following reasons:

- Classes are a common standard for object creation and inheritance that is now widely supported across libraries and frameworks. This is an improvement compared to how things were before, when almost every framework had its own inheritance library.

- They help tools such as IDEs and type checkers with their work and enable new features there.

- If you come from another language to JavaScript and are used to classes, then you can get started more quickly.

- JavaScript engines optimize them. That is, code that uses classes is almost always faster than code that uses a custom inheritance library.

- We can subclass built-in constructor functions such as Error.

That doesn’t mean that classes are perfect:

- There is a risk of overdoing inheritance.

- There is a risk of putting too much functionality in classes (when some of it is often better put in functions).

- Classes look familiar to programmers coming from other languages, but they work differently and are used differently (see next subsection). Therefore, there is a risk of those programmers writing code that doesn’t feel like JavaScript.

- How classes seem to work superficially is quite different from how they actually work. In other words, there is a disconnect between syntax and semantics. Two examples are:

- A method definition inside a class C creates a method in the object C.prototype.

- Classes are functions.

The motivation for the disconnect is backward compatibility. Thankfully, the disconnect causes few problems in practice; we are usually OK if we go along with what classes pretend to be.

This was a first look at classes. We’ll explore more features soon.

Exercise: Writing a class

exercises/classes/point_class_test.mjs

Tips for using classes

- Use inheritance sparingly – it tends to make code more complicated and spread out related functionality across multiple locations.

- Instead of static members, it is often better to use external functions and variables. We can even make those private to a module, simply by not exporting them. Two important exceptions to this rule are:

- Operations that need access to private slots

- Static factory methods

- Only put core functionality in prototype methods. Other functionality is better implemented via functions – especially algorithms that involve instances of multiple classes.

The internals of classes

A class is actually two connected objects

Under the hood, a class becomes two connected objects. Let’s revisit class Person to see how that works:

class Person {

#firstName;

constructor(firstName) {

this.#firstName = firstName;

}

describe() {

return `Person named ${this.#firstName}`;

}

static extractNames(persons) {

return persons.map(person => person.#firstName);

}

}

The first object created by the class is stored in Person. It has four properties:

assert.deepEqual(

Reflect.ownKeys(Person),

['length', 'name', 'prototype', 'extractNames']

);

// The number of parameters of the constructor

assert.equal(

Person.length, 1

);

// The name of the class

assert.equal(

Person.name, 'Person'

);

The two remaining properties are:

- Person.extractNames is the static method that we have already seen in action.

- Person.prototype points to the second object that is created by a class definition.

These are the contents of Person.prototype:

assert.deepEqual(

Reflect.ownKeys(Person.prototype),

['constructor', 'describe']

);

There are two properties:

- Person.prototype.constructor points to the constructor.

- Person.prototype.describe is the method that we have already used.

Classes set up the prototype chains of their instances

The object Person.prototype is the prototype of all instances:

const jane = new Person('Jane');

assert.equal(

Object.getPrototypeOf(jane), Person.prototype

);

const tarzan = new Person('Tarzan');

assert.equal(

Object.getPrototypeOf(tarzan), Person.prototype

);

That explains how the instances get their methods: They inherit them from the object Person.prototype.

Fig. 13 visualizes how everything is connected.

Figure 13: The class Person has the property .prototype that points to an object that is the prototype of all instances of Person. The objects jane and tarzan are two such instances.

.__proto__ vs. .prototype

It is easy to confuse .__proto__ and .prototype. Hopefully, fig. 13 makes it clear how they differ:

- .__proto__ is an accessor of class Object that lets us get and set the prototypes of its instances.

- .prototype is a normal property like any other. It is only special because the new operator uses its value as the prototype of instances. Its name is not ideal. A different name such as .instancePrototype would be more fitting.

Person.prototype.constructor (advanced)

There is one detail in fig. 13 that we haven’t looked at, yet: Person.prototype.constructor points back to Person:

> Person.prototype.constructor === Person

true

This setup exists due to backward compatibility. But it has two additional benefits.

First, each instance of a class inherits property .constructor. Therefore, given an instance, we can make “similar” objects via it:

const jane = new Person('Jane');

const cheeta = new jane.constructor('Cheeta');

// cheeta is also an instance of Person

assert.equal(cheeta instanceof Person, true);

Second, we can get the name of the class that created a given instance:

const tarzan = new Person('Tarzan');

assert.equal(tarzan.constructor.name, 'Person');

Dispatched vs. direct method calls (advanced)

In this subsection, we learn about two different ways of invoking methods:

- Dispatched method calls

- Direct method calls

Understanding both of them will give us important insights into how methods work.

We’ll also need the second way later in this chapter: It will allow us to borrow useful methods from Object.prototype.

Dispatched method calls

Let’s examine how method calls work with classes. We are revisiting jane from earlier:

class Person {

#firstName;

constructor(firstName) {

this.#firstName = firstName;

}

describe() {

return 'Person named '+this.#firstName;

}

}

const jane = new Person('Jane');

Fig. 14 has a diagram with jane’s prototype chain.

Figure 14: The prototype chain of jane starts with jane and continues with Person.prototype.

Normal method calls are dispatched – the method call

jane.describe()

happens in two steps:

- Dispatch: JavaScript traverses the prototype chain starting with jane to find the first object that has an own property with the key 'describe': It first looks at jane and doesn’t find an own property .describe. It continues with jane’s prototype, Person.prototype and finds an own property describe whose value it returns.

const func = jane.describe;

- Invocation: Method-invoking a value is different from function-invoking a value in that it not only calls what comes before the parentheses with the arguments inside the parentheses but also sets this to the receiver of the method call (in this case, jane):

func.call(jane);

This way of dynamically looking for a method and invoking it is called dynamic dispatch.

Direct method calls

We can also make method calls directly, without dispatching:

Person.prototype.describe.call(jane)

This time, we directly point to the method via Person.prototype.describe and don’t search for it in the prototype chain. We also specify this differently – via .call().

this always points to the instance

No matter where in the prototype chain of an instance a method is located, this always points to the instance (the beginning of the prototype chain). That enables .describe() to access .#firstName in the example.

When are direct method calls useful? Whenever we want to borrow a method from elsewhere that a given object doesn’t have – for example:

const obj = Object.create(null);

// `obj` is not an instance of Object and doesn’t inherit

// its prototype method .toString()

assert.throws(

() => obj.toString(),

/^TypeError: obj.toString is not a function$/

);

assert.equal(

Object.prototype.toString.call(obj),

'[object Object]'

);

Classes evolved from ordinary functions (advanced)

Before ECMAScript 6, JavaScript didn’t have classes. Instead, ordinary functions were used as constructor functions:

function StringBuilderConstr(initialString) {

this.string = initialString;

}

StringBuilderConstr.prototype.add = function (str) {

this.string += str;

return this;

};

const sb = new StringBuilderConstr('¡');

sb.add('Hola').add('!');

assert.equal(

sb.string, '¡Hola!'

);

Classes provide better syntax for this approach:

class StringBuilderClass {

constructor(initialString) {

this.string = initialString;

}

add(str) {

this.string += str;

return this;

}

}

const sb = new StringBuilderClass('¡');

sb.add('Hola').add('!');

assert.equal(

sb.string, '¡Hola!'

);

Subclassing is especially tricky with constructor functions. Classes also offer benefits that go beyond more convenient syntax:

- Built-in constructor functions such as Error can be subclassed.

- We can access overridden properties via super.

- Classes can’t be function-called.

- Methods can’t be new-called and don’t have the property .prototype.

- Support for private instance data.

- And more.

Classes are so compatible with constructor functions that they can even extend them:

function SuperConstructor() {}

class SubClass extends SuperConstructor {}

assert.equal(

new SubClass() instanceof SuperConstructor, true

);

extends and subclassing are explained later in this chapter.

A class is the constructor

This brings us to an interesting insight. On one hand, StringBuilderClass refers to its constructor via StringBuilderClass.prototype.constructor.

On the other hand, the class is the constructor (a function):

> StringBuilderClass.prototype.constructor === StringBuilderClass

true

> typeof StringBuilderClass

'function'

Constructor (functions) vs. classes

Due to how similar they are, I use the terms constructor (function) and class interchangeably.

Prototype members of classes

Public prototype methods and accessors

All members in the body of the following class declaration create properties of PublicProtoClass.prototype.

class PublicProtoClass {

constructor(args) {

// (Do something with `args` here.)

}

publicProtoMethod() {

return 'publicProtoMethod';

}

get publicProtoAccessor() {

return 'publicProtoGetter';

}

set publicProtoAccessor(value) {

assert.equal(value, 'publicProtoSetter');

}

}

assert.deepEqual(

Reflect.ownKeys(PublicProtoClass.prototype),

['constructor', 'publicProtoMethod', 'publicProtoAccessor']

);

const inst = new PublicProtoClass('arg1', 'arg2');

assert.equal(

inst.publicProtoMethod(), 'publicProtoMethod'

);

assert.equal(

inst.publicProtoAccessor, 'publicProtoGetter'

);

inst.publicProtoAccessor = 'publicProtoSetter';

All kinds of public prototype methods and accessors (advanced)

const accessorKey = Symbol('accessorKey');

const syncMethodKey = Symbol('syncMethodKey');

const syncGenMethodKey = Symbol('syncGenMethodKey');

const asyncMethodKey = Symbol('asyncMethodKey');

const asyncGenMethodKey = Symbol('asyncGenMethodKey');

class PublicProtoClass2 {

// Identifier keys

get accessor() {}

set accessor(value) {}

syncMethod() {}

* syncGeneratorMethod() {}

async asyncMethod() {}

async * asyncGeneratorMethod() {}

// Quoted keys

get 'an accessor'() {}

set 'an accessor'(value) {}

'sync method'() {}

* 'sync generator method'() {}

async 'async method'() {}

async * 'async generator method'() {}

// Computed keys

get [accessorKey]() {}

set [accessorKey](value) {}

[syncMethodKey]() {}

* [syncGenMethodKey]() {}

async [asyncMethodKey]() {}

async * [asyncGenMethodKey]() {}

}

// Quoted and computed keys are accessed via square brackets:

const inst = new PublicProtoClass2();

inst['sync method']();

inst[syncMethodKey]();

Quoted and computed keys can also be used in object literals:

- §28.7.1 “Quoted keys in object literals”

- §28.7.2 “Computed keys in object literals”

More information on accessors (defined via getters and/or setters), generators, async methods, and async generator methods:

- §28.3.6 “Object literals: accessors”

- [Content not included]

- [Content not included]

- [Content not included]

Private methods and accessors [ES2022]

Private methods (and accessors) are an interesting mix of prototype members and instance members.

On one hand, private methods are stored in slots in instances (line A):

class MyClass {

#privateMethod() {}

static check() {

const inst = new MyClass();

assert.equal(

#privateMethod in inst, true // (A)

);

assert.equal(

#privateMethod in MyClass.prototype, false

);

assert.equal(

#privateMethod in MyClass, false

);

}

}

MyClass.check();

Why are they not stored in .prototype objects? Private slots are not inherited, only properties are.

On the other hand, private methods are shared between instances – like prototype public methods:

class MyClass {

#privateMethod() {}

static check() {

const inst1 = new MyClass();

const inst2 = new MyClass();

assert.equal(

inst1.#privateMethod,

inst2.#privateMethod

);

}

}

Due to that and due to their syntax being similar to prototype public methods, they are covered here.

The following code demonstrates how private methods and accessors work:

class PrivateMethodClass {

#privateMethod() {

return 'privateMethod';

}

get #privateAccessor() {

return 'privateGetter';

}

set #privateAccessor(value) {

assert.equal(value, 'privateSetter');

}

callPrivateMembers() {

assert.equal(this.#privateMethod(), 'privateMethod');

assert.equal(this.#privateAccessor, 'privateGetter');

this.#privateAccessor = 'privateSetter';

}

}

assert.deepEqual(

Reflect.ownKeys(new PrivateMethodClass()), []

);

All kinds of private methods and accessors (advanced)

With private slots, the keys are always identifiers:

class PrivateMethodClass2 {

get #accessor() {}

set #accessor(value) {}

#syncMethod() {}

* #syncGeneratorMethod() {}

async #asyncMethod() {}

async * #asyncGeneratorMethod() {}

}

More information on accessors (defined via getters and/or setters), generators, async methods, and async generator methods:

- §28.3.6 “Object literals: accessors”

- [Content not included]

- [Content not included]

- [Content not included]

Instance members of classes [ES2022]

Instance public fields

Instances of the following class have two instance properties (created in line A and line B):

class InstPublicClass {

// Instance public field

instancePublicField = 0; // (A)

constructor(value) {

// We don’t need to mention .property elsewhere!

this.property = value; // (B)

}

}

const inst = new InstPublicClass('constrArg');

assert.deepEqual(

Reflect.ownKeys(inst),

['instancePublicField', 'property']

);

assert.equal(

inst.instancePublicField, 0

);

assert.equal(

inst.property, 'constrArg'

);

If we create an instance property inside the constructor (line B), we don’t need to “declare” it elsewhere. As we have already seen, that is different for instance private fields.

Note that instance properties are relatively common in JavaScript; much more so than in, e.g., Java, where most instance state is private.

Instance public fields with quoted and computed keys (advanced)

const computedFieldKey = Symbol('computedFieldKey');

class InstPublicClass2 {

'quoted field key' = 1;

[computedFieldKey] = 2;

}

const inst = new InstPublicClass2();

assert.equal(inst['quoted field key'], 1);

assert.equal(inst[computedFieldKey], 2);

What is the value of this in instance public fields? (advanced)

In the initializer of a instance public field, this refers to the newly created instance:

class MyClass {

instancePublicField = this;

}

const inst = new MyClass();

assert.equal(

inst.instancePublicField, inst

);

When are instance public fields executed? (advanced)

The execution of instance public fields roughly follows these two rules:

- In base classes (classes without superclasses), instance public fields are executed immediately before the constructor.

- In derived classes (classes with superclasses):

- The superclass sets up its instance slots when super() is called.

- Instance public fields are executed immediately after super().

The following example demonstrates these rules:

class SuperClass {

superProp = console.log('superProp');

constructor() {

console.log('super-constructor');

}

}

class SubClass extends SuperClass {

subProp = console.log('subProp');

constructor() {

console.log('BEFORE super()');

super();

console.log('AFTER super()');

}

}

new SubClass();

// Output:

// 'BEFORE super()'

// 'superProp'

// 'super-constructor'

// 'subProp'

// 'AFTER super()'

extends and subclassing are explained later in this chapter.

Instance private fields

The following class contains two instance private fields (line A and line B):

class InstPrivateClass {

#privateField1 = 'private field 1'; // (A)

#privateField2; // (B) required!

constructor(value) {

this.#privateField2 = value; // (C)

}

/**

* Private fields are not accessible outside the class body.

*/

checkPrivateValues() {

assert.equal(

this.#privateField1, 'private field 1'

);

assert.equal(

this.#privateField2, 'constructor argument'

);

}

}

const inst = new InstPrivateClass('constructor argument');

inst.checkPrivateValues();

// No instance properties were created

assert.deepEqual(

Reflect.ownKeys(inst),

[]

);

Note that we can only use .#privateField2 in line C if we declare it in the class body.

Private instance data before ES2022 (advanced)

In this section, we look at two techniques for keeping instance data private. Because they don’t rely on classes, we can also use them for objects that were created in other ways – e.g., via object literals.

Before ES6: private members via naming conventions

The first technique makes a property private by prefixing its name with an underscore. This doesn’t protect the property in any way; it merely signals to the outside: “You don’t need to know about this property.”

In the following code, the properties ._counter and ._action are private.

class Countdown {

constructor(counter, action) {

this._counter = counter;

this._action = action;

}

dec() {

this._counter--;

if (this._counter === 0) {

this._action();

}

}

}

// The two properties aren’t really private:

assert.deepEqual(

Object.keys(new Countdown()),

['_counter', '_action']);

With this technique, we don’t get any protection and private names can clash. On the plus side, it is easy to use.

Private methods work similarly: They are normal methods whose names start with underscores.

ES6 and later: private instance data via WeakMaps

We can also manage private instance data via WeakMaps:

const _counter = new WeakMap();

const _action = new WeakMap();

class Countdown {

constructor(counter, action) {

_counter.set(this, counter);

_action.set(this, action);

}

dec() {

let counter = _counter.get(this);

counter--;

_counter.set(this, counter);

if (counter === 0) {

_action.get(this)();

}

}

}

// The two pseudo-properties are truly private:

assert.deepEqual(

Object.keys(new Countdown()),

[]);

How exactly that works is explained in the chapter on WeakMaps.

This technique offers us considerable protection from outside access and there can’t be any name clashes. But it is also more complicated to use.

We control the visibility of the pseudo-property _superProp by controlling who has access to it – for example: If the variable exists inside a module and isn’t exported, everyone inside the module and no one outside the module can access it. In other words: The scope of privacy isn’t the class in this case, it’s the module. We could narrow the scope, though:

let Countdown;

{ // class scope

const _counter = new WeakMap();

const _action = new WeakMap();

Countdown = class {

// ···

}

}

This technique doesn’t really support private methods. But module-local functions that have access to _superProp are the next best thing:

const _counter = new WeakMap();

const _action = new WeakMap();

class Countdown {

constructor(counter, action) {

_counter.set(this, counter);

_action.set(this, action);

}

dec() {

privateDec(this);

}

}

function privateDec(_this) { // (A)

let counter = _counter.get(_this);

counter--;

_counter.set(_this, counter);

if (counter === 0) {

_action.get(_this)();

}

}

Note that this becomes the explicit function parameter _this (line A).

Simulating protected visibility and friend visibility via WeakMaps (advanced)

As previously discussed, instance private fields are only visible inside their classes and not even in subclasses. Thus, there is no built-in way to get:

- Protected visibility: A class and all of its subclasses can access a piece instance data.

- Friend visibility: A class and its “friends” (designated functions, objects, or classes) can access a piece of instance data.

In the previous subsection, we simulated “module visibility” (everyone inside a module has access to a piece of instance data) via WeakMaps. Therefore:

- If we put a class and its subclasses into the same module, we get protected visibility.

- If we put a class and its friends into the same module, we get friend visibility.

The next example demonstrates protected visibility:

const _superProp = new WeakMap();

class SuperClass {

constructor() {

_superProp.set(this, 'superProp');

}

}

class SubClass extends SuperClass {

getSuperProp() {

return _superProp.get(this);

}

}

assert.equal(

new SubClass().getSuperProp(),

'superProp'

);

Subclassing via extends is explained later in this chapter.

Static members of classes

Static public methods and accessors

All members in the body of the following class declaration create so-called static properties – properties of StaticClass itself.

class StaticPublicMethodsClass {

static staticMethod() {

return 'staticMethod';

}

static get staticAccessor() {

return 'staticGetter';

}

static set staticAccessor(value) {

assert.equal(value, 'staticSetter');

}

}

assert.equal(

StaticPublicMethodsClass.staticMethod(), 'staticMethod'

);

assert.equal(

StaticPublicMethodsClass.staticAccessor, 'staticGetter'

);

StaticPublicMethodsClass.staticAccessor = 'staticSetter';

All kinds of static public methods and accessors (advanced)

const accessorKey = Symbol('accessorKey');

const syncMethodKey = Symbol('syncMethodKey');

const syncGenMethodKey = Symbol('syncGenMethodKey');

const asyncMethodKey = Symbol('asyncMethodKey');

const asyncGenMethodKey = Symbol('asyncGenMethodKey');

class StaticPublicMethodsClass2 {

// Identifier keys

static get accessor() {}

static set accessor(value) {}

static syncMethod() {}

static * syncGeneratorMethod() {}

static async asyncMethod() {}

static async * asyncGeneratorMethod() {}

// Quoted keys

static get 'an accessor'() {}

static set 'an accessor'(value) {}

static 'sync method'() {}

static * 'sync generator method'() {}

static async 'async method'() {}

static async * 'async generator method'() {}

// Computed keys

static get [accessorKey]() {}

static set [accessorKey](value) {}

static [syncMethodKey]() {}

static * [syncGenMethodKey]() {}

static async [asyncMethodKey]() {}

static async * [asyncGenMethodKey]() {}

}

// Quoted and computed keys are accessed via square brackets:

StaticPublicMethodsClass2['sync method']();

StaticPublicMethodsClass2[syncMethodKey]();

Quoted and computed keys can also be used in object literals:

- §28.7.1 “Quoted keys in object literals”

- §28.7.2 “Computed keys in object literals”

More information on accessors (defined via getters and/or setters), generators, async methods, and async generator methods:

- §28.3.6 “Object literals: accessors”

- [Content not included]

- [Content not included]

- [Content not included]

Static public fields [ES2022]

The following code demonstrates static public fields. StaticPublicFieldClass has three of them:

const computedFieldKey = Symbol('computedFieldKey');

class StaticPublicFieldClass {

static identifierFieldKey = 1;

static 'quoted field key' = 2;

static [computedFieldKey] = 3;

}

assert.deepEqual(

Reflect.ownKeys(StaticPublicFieldClass),

[

'length', // number of constructor parameters

'name', // 'StaticPublicFieldClass'

'prototype',

'identifierFieldKey',

'quoted field key',

computedFieldKey,

],

);

assert.equal(StaticPublicFieldClass.identifierFieldKey, 1);

assert.equal(StaticPublicFieldClass['quoted field key'], 2);

assert.equal(StaticPublicFieldClass[computedFieldKey], 3);

Static private methods, accessors, and fields [ES2022]

The following class has two static private slots (line A and line B):

class StaticPrivateClass {

// Declare and initialize

static #staticPrivateField = 'hello'; // (A)

static #twice() { // (B)

const str = StaticPrivateClass.#staticPrivateField;

return str + ' ' + str;

}

static getResultOfTwice() {

return StaticPrivateClass.#twice();

}

}

assert.deepEqual(

Reflect.ownKeys(StaticPrivateClass),

[

'length', // number of constructor parameters

'name', // 'StaticPublicFieldClass'

'prototype',

'getResultOfTwice',

],

);

assert.equal(

StaticPrivateClass.getResultOfTwice(),

'hello hello'

);

This is a complete list of all kinds of static private slots:

class MyClass {

static #staticPrivateMethod() {}

static * #staticPrivateGeneratorMethod() {}

static async #staticPrivateAsyncMethod() {}

static async * #staticPrivateAsyncGeneratorMethod() {}

static get #staticPrivateAccessor() {}

static set #staticPrivateAccessor(value) {}

}

Static initialization blocks in classes [ES2022]

To set up instance data via classes, we have two constructs:

- Fields, to create and optionally initialize instance data

- Constructors, blocks of code that are executed every time a new instance is created

For static data, we have:

- Static fields

- Static blocks that are executed when a class is created

The following code demonstrates static blocks (line A):

class Translator {

static translations = {

yes: 'ja',

no: 'nein',

maybe: 'vielleicht',

};

static englishWords = [];

static germanWords = [];

static { // (A)

for (const [english, german] of Object.entries(this.translations)) {

this.englishWords.push(english);

this.germanWords.push(german);

}

}

}

We could also execute the code inside the static block after the class (at the top level). However, using a static block has two benefits:

- All class-related code is inside the class.

- The code in a static block has access to private slots.

Rules for static initialization blocks

The rules for how static initialization blocks work, are relatively simple:

- There can be more than one static block per class.

- The execution of static blocks is interleaved with the execution of static field initializers.

- The static members of a superclass are executed before the static members of a subclass.

The following code demonstrates these rules:

class SuperClass {

static superField1 = console.log('superField1');

static {

assert.equal(this, SuperClass);

console.log('static block 1 SuperClass');

}

static superField2 = console.log('superField2');

static {

console.log('static block 2 SuperClass');

}

}

class SubClass extends SuperClass {

static subField1 = console.log('subField1');

static {

assert.equal(this, SubClass);

console.log('static block 1 SubClass');

}

static subField2 = console.log('subField2');

static {

console.log('static block 2 SubClass');

}

}

// Output:

// 'superField1'

// 'static block 1 SuperClass'

// 'superField2'

// 'static block 2 SuperClass'

// 'subField1'

// 'static block 1 SubClass'

// 'subField2'

// 'static block 2 SubClass'

Subclassing via extends is explained later in this chapter.

Pitfall: Using this to access static private fields

In static public members, we can access static public slots via this. Alas, we should not use it to access static private slots.

this and static public fields

Consider the following code:

class SuperClass {

static publicData = 1;

static getPublicViaThis() {

return this.publicData;

}

}

class SubClass extends SuperClass {

}

Subclassing via extends is explained later in this chapter.

Static public fields are properties. If we make the method call

assert.equal(SuperClass.getPublicViaThis(), 1);

then this points to SuperClass and everything works as expected. We can also invoke .getPublicViaThis() via the subclass:

assert.equal(SubClass.getPublicViaThis(), 1);

SubClass inherits .getPublicViaThis() from its prototype SuperClass. this points to SubClass and things continue to work, because SubClass also inherits the property .publicData.

As an aside, if we assigned to this.publicData in getPublicViaThis() and invoked it via SubClass.getPublicViaThis(), then we would create a new own poperty of SubClass that (non-destructively) overrides the property inherited from SuperClass.

this and static private fields

Consider the following code:

class SuperClass {

static #privateData = 2;

static getPrivateDataViaThis() {

return this.#privateData;

}

static getPrivateDataViaClassName() {

return SuperClass.#privateData;

}

}

class SubClass extends SuperClass {

}

Invoking .getPrivateDataViaThis() via SuperClass works, because this points to SuperClass:

assert.equal(SuperClass.getPrivateDataViaThis(), 2);

However, invoking .getPrivateDataViaThis() via SubClass does not work, because this now points to SubClass and SubClass has no static private field .#privateData (private slots in prototype chains are not inherited):

assert.throws(

() => SubClass.getPrivateDataViaThis(),

{

name: 'TypeError',

message: 'Cannot read private member #privateData from'

+ ' an object whose class did not declare it',

}

);

The workaround is to accesss .#privateData directly, via SuperClass:

assert.equal(SubClass.getPrivateDataViaClassName(), 2);

With static private methods, we are facing the same issue.

All members (static, prototype, instance) can access all private members

Every member inside a class can access all other members inside that class – both public and private ones:

class DemoClass {

static #staticPrivateField = 1;

#instPrivField = 2;

static staticMethod(inst) {

// A static method can access static private fields

// and instance private fields

assert.equal(DemoClass.#staticPrivateField, 1);

assert.equal(inst.#instPrivField, 2);

}

protoMethod() {

// A prototype method can access instance private fields

// and static private fields

assert.equal(this.#instPrivField, 2);

assert.equal(DemoClass.#staticPrivateField, 1);

}

}

In contrast, no one outside can access the private members:

// Accessing private fields outside their classes triggers

// syntax errors (before the code is even executed).

assert.throws(

() => eval('DemoClass.#staticPrivateField'),

{

name: 'SyntaxError',

message: "Private field '#staticPrivateField' must"

+ " be declared in an enclosing class",

}

);

// Accessing private fields outside their classes triggers

// syntax errors (before the code is even executed).

assert.throws(

() => eval('new DemoClass().#instPrivField'),

{

name: 'SyntaxError',

message: "Private field '#instPrivField' must"

+ " be declared in an enclosing class",

}

);

Static private methods and data before ES2022

The following code only works in ES2022 – due to every line that has a hash symbol (#) in it:

class StaticClass {

static #secret = 'Rumpelstiltskin';

static #getSecretInParens() {

return `(${StaticClass.#secret})`;

}

static callStaticPrivateMethod() {

return StaticClass.#getSecretInParens();

}

}

Since private slots only exist once per class, we can move #secret and #getSecretInParens to the scope surrounding the class and use a module to hide them from the world outside the module.

const secret = 'Rumpelstiltskin';

function getSecretInParens() {

return `(${secret})`;

}

// Only the class is accessible outside the module

export class StaticClass {

static callStaticPrivateMethod() {

return getSecretInParens();

}

}

Static factory methods

Sometimes there are multiple ways in which a class can be instantiated. Then we can implement static factory methods such as Point.fromPolar():

class Point {

static fromPolar(radius, angle) {

const x = radius * Math.cos(angle);

const y = radius * Math.sin(angle);

return new Point(x, y);

}

constructor(x=0, y=0) {

this.x = x;

this.y = y;

}

}

assert.deepEqual(

Point.fromPolar(13, 0.39479111969976155),

new Point(12, 5)

);

I like how descriptive static factory methods are: fromPolar describes how an instance is created. JavaScript’s standard library also has such factory methods – for example:

- Array.from()

- Object.create()

I prefer to either have no static factory methods or only static factory methods. Things to consider in the latter case:

- One factory method will probably directly call the constructor (but have a descriptive name).

- We need to find a way to prevent the constructor being called from outside.

In the following code, we use a secret token (line A) to prevent the constructor being called from outside the current module.

// Only accessible inside the current module

const secretToken = Symbol('secretToken'); // (A)

export class Point {

static create(x=0, y=0) {

return new Point(secretToken, x, y);

}

static fromPolar(radius, angle) {

const x = radius * Math.cos(angle);

const y = radius * Math.sin(angle);

return new Point(secretToken, x, y);

}

constructor(token, x, y) {

if (token !== secretToken) {

throw new TypeError('Must use static factory method');

}

this.x = x;

this.y = y;

}

}

Point.create(3, 4); // OK

assert.throws(

() => new Point(3, 4),

TypeError

);

Subclassing

Classes can also extend existing classes. For example, the following class Employee extends Person:

class Person {

#firstName;

constructor(firstName) {

this.#firstName = firstName;

}

describe() {

return `Person named ${this.#firstName}`;

}

static extractNames(persons) {

return persons.map(person => person.#firstName);

}

}

class Employee extends Person {

constructor(firstName, title) {

super(firstName);

this.title = title;

}

describe() {

return super.describe() +

` (${this.title})`;

}

}

const jane = new Employee('Jane', 'CTO');

assert.equal(

jane.title,

'CTO'

);

assert.equal(

jane.describe(),

'Person named Jane (CTO)'

);

Terminology related to extending:

- Another word for extending is subclassing.

- Person is the superclass of Employee.

- Employee is the subclass of Person.

- A base class is a class that has no superclasses.

- A derived class is a class that has a superclass.

Inside the .constructor() of a derived class, we must call the super-constructor via super() before we can access this. Why is that?

Let’s consider a chain of classes:

- Base class A

- Class B extends A.

- Class C extends B.

If we invoke new C(), C’s constructor super-calls B’s constructor which super-calls A’s constructor. Instances are always created in base classes, before the constructors of subclasses add their slots. Therefore, the instance doesn’t exist before we call super() and we can’t access it via this, yet.

Note that static public slots are inherited. For example, Employee inherits the static method .extractNames():

> 'extractNames' in Employee

true

Exercise: Subclassing

exercises/classes/color_point_class_test.mjs

The internals of subclassing (advanced)

Figure 15: These are the objects that make up class Person and its subclass, Employee. The left column is about classes. The right column is about the Employee instance jane and its prototype chain.

The classes Person and Employee from the previous section are made up of several objects (fig. 15). One key insight for understanding how these objects are related is that there are two prototype chains:

- The instance prototype chain, on the right.

- The class prototype chain, on the left.

The instance prototype chain (right column)

The instance prototype chain starts with jane and continues with Employee.prototype and Person.prototype. In principle, the prototype chain ends at this point, but we get one more object: Object.prototype. This prototype provides services to virtually all objects, which is why it is included here, too:

> Object.getPrototypeOf(Person.prototype) === Object.prototype

true

The class prototype chain (left column)

In the class prototype chain, Employee comes first, Person next. Afterward, the chain continues with Function.prototype, which is only there because Person is a function and functions need the services of Function.prototype.

> Object.getPrototypeOf(Person) === Function.prototype

true

instanceof and subclassing (advanced)

We have not yet learned how instanceof really works. How does instanceof determine if a value x is an instance of a class C (it can be a direct instance of C or a direct instance of a subclass of C)? It checks if C.prototype is in the prototype chain of x. That is, the following two expressions are equivalent:

x instanceof C

C.prototype.isPrototypeOf(x)

If we go back to fig. 15, we can confirm that the prototype chain does lead us to the following correct answers:

> jane instanceof Employee

true

> jane instanceof Person

true

> jane instanceof Object

true

Note that instanceof always returns false if its self-hand side is a primitive value:

> 'abc' instanceof String

false

> 123 instanceof Number

false

Not all objects are instances of Object (advanced)

An object (a non-primitive value) is only an instance of Object if Object.prototype is in its prototype chain (see previous subsection). Virtually all objects are instances of Object – for example:

assert.equal(

{a: 1} instanceof Object, true

);

assert.equal(

['a'] instanceof Object, true

);

assert.equal(

/abc/g instanceof Object, true

);

assert.equal(

new Map() instanceof Object, true

);

class C {}

assert.equal(

new C() instanceof Object, true

);

In the next example, obj1 and obj2 are both objects (line A and line C), but they are not instances of Object (line B and line D): Object.prototype is not in their prototype chains because they don’t have any prototypes.

const obj1 = {__proto__: null};

assert.equal(

typeof obj1, 'object' // (A)

);

assert.equal(

obj1 instanceof Object, false // (B)

);

const obj2 = Object.create(null);

assert.equal(

typeof obj2, 'object' // (C)

);

assert.equal(

obj2 instanceof Object, false // (D)

);

Object.prototype is the object that ends most prototype chains. Its prototype is null, which means it isn’t an instance of Object either:

> typeof Object.prototype

'object'

> Object.getPrototypeOf(Object.prototype)

null

> Object.prototype instanceof Object

false

Prototype chains of built-in objects (advanced)

Next, we’ll use our knowledge of subclassing to understand the prototype chains of a few built-in objects. The following tool function p() helps us with our explorations.

const p = Object.getPrototypeOf.bind(Object);

We extracted method .getPrototypeOf() of Object and assigned it to p.

The prototype chain of {}

Let’s start by examining plain objects:

> p({}) === Object.prototype

true

> p(p({})) === null

true

Figure 16: The prototype chain of an object created via an object literal starts with that object, continues with Object.prototype, and ends with null.

Fig. 16 shows a diagram for this prototype chain. We can see that {} really is an instance of Object – Object.prototype is in its prototype chain.

The prototype chain of []

What does the prototype chain of an Array look like?

> p([]) === Array.prototype

true

> p(p([])) === Object.prototype

true

> p(p(p([]))) === null

true

Figure 17: The prototype chain of an Array has these members: the Array instance, Array.prototype, Object.prototype, null.

This prototype chain (visualized in fig. 17) tells us that an Array object is an instance of Array and of Object.

The prototype chain of function () {}

Lastly, the prototype chain of an ordinary function tells us that all functions are objects:

> p(function () {}) === Function.prototype

true

> p(p(function () {})) === Object.prototype

true

The prototype chains of built-in classes

The prototype of a base class is Function.prototype which means that it is a function (an instance of Function):

class A {}

assert.equal(

Object.getPrototypeOf(A),

Function.prototype

);

assert.equal(

Object.getPrototypeOf(class {}),

Function.prototype

);

The prototype of a derived class is its superclass:

class B extends A {}

assert.equal(

Object.getPrototypeOf(B),

A

);

assert.equal(

Object.getPrototypeOf(class extends Object {}),

Object

);

Interestingly, Object, Array, and Function are all base classes:

> Object.getPrototypeOf(Object) === Function.prototype

true

> Object.getPrototypeOf(Array) === Function.prototype

true

> Object.getPrototypeOf(Function) === Function.prototype

true

However, as we have seen, even the instances of base classes have Object.prototype in their prototype chains because it provides services that all objects need.

Why are Array and Function base classes?

Base classes are where instances are actually created. Both Array and Function need to create their own instances because they have so-called “internal slots” which can’t be added later to instances created by Object.

Mixin classes (advanced)

JavaScript’s class system only supports single inheritance. That is, each class can have at most one superclass. One way around this limitation is via a technique called mixin classes (short: mixins).

The idea is as follows: Let’s say we want a class C to inherit from two superclasses S1 and S2. That would be multiple inheritance, which JavaScript doesn’t support.

Our workaround is to turn S1 and S2 into mixins, factories for subclasses:

const S1 = (Sup) => class extends Sup { /*···*/ };

const S2 = (Sup) => class extends Sup { /*···*/ };

Each of these two functions returns a class that extends a given superclass Sup. We create class C as follows:

class C extends S2(S1(Object)) {

/*···*/

}

We now have a class C that extends the class returned by S2() which extends the class returned by S1() which extends Object.

Example: a mixin for brand management

We implement a mixin Branded that has helper methods for setting and getting the brand of an object:

const Named = (Sup) => class extends Sup {

name = '(Unnamed)';

toString() {

const className = this.constructor.name;

return `${className} named ${this.name}`;

}

};

We use this mixin to implement a class City that has a name:

class City extends Named(Object) {

constructor(name) {

super();

this.name = name;

}

}

The following code confirms that the mixin works:

const paris = new City('Paris');

assert.equal(

paris.name, 'Paris'

);

assert.equal(

paris.toString(), 'City named Paris'

);

The benefits of mixins

Mixins free us from the constraints of single inheritance:

- The same class can extend a single superclass and zero or more mixins.

- The same mixin can be used by multiple classes.

The methods and accessors of Object.prototype (advanced)

As we have seen in §29.7.3 “Not all objects are instances of Object”, almost all objects are instances of Object. This class provides several useful methods and an accessor to its instances:

- Configuring how objects are converted to primitive values (e.g. by the + operator): The following methods have default implementations but are often overridden in subclasses or instances.

- .toString(): Configures how an object is converted to a string.

- .toLocaleString(): A version of .toString() that can be configured in various ways via arguments (language, region, etc.).

- .valueOf(): Configures how an object is converted to a non-string primitive value (often a number).

- Useful methods (with pitfalls – see next subsection):

- .isPrototypeOf(): Is the receiver in the prototype chain of a given object?

- .propertyIsEnumerable(): Does the receiver have an enumerable own property with the given key?

- Avoid these features (there are better alternatives):

- .__proto__: Get and set the prototype of the receiver.

- Using this accessor is not recommended. Alternatives:

- Object.getPrototypeOf()

- Object.setPrototypeOf()

- .hasOwnProperty(): Does the receiver have an own property with a given key?

- Using this method is not recommended. Alternative in ES2022 and later: Object.hasOwn().

Before we take a closer look at each of these features, we’ll learn about an important pitfall (and how to work around it): We can’t use the features of Object.prototype with all objects.

Using Object.prototype methods safely

Invoking one of the methods of Object.prototype on an arbitrary object doesn’t always work. To illustrate why, we use method Object.prototype.hasOwnProperty, which returns true if an object has an own property with a given key:

> {ownProp: true}.hasOwnProperty('ownProp')

true

> {ownProp: true}.hasOwnProperty('abc')

false

Invoking .hasOwnProperty() on an arbitrary object can fail in two ways. On one hand, this method isn’t available if an object is not an instance of Object (see §29.7.3 “Not all objects are instances of Object”):

const obj = Object.create(null);

assert.equal(obj instanceof Object, false);

assert.throws(

() => obj.hasOwnProperty('prop'),

{

name: 'TypeError',

message: 'obj.hasOwnProperty is not a function',

}

);

On the other hand, we can’t use .hasOwnProperty() if an object overrides it with an own property (line A):

const obj = {

hasOwnProperty: 'yes' // (A)

};

assert.throws(

() => obj.hasOwnProperty('prop'),

{

name: 'TypeError',

message: 'obj.hasOwnProperty is not a function',

}

);

There is, however, a safe way to use .hasOwnProperty():

function hasOwnProp(obj, propName) {

return Object.prototype.hasOwnProperty.call(obj, propName); // (A)

}

assert.equal(

hasOwnProp(Object.create(null), 'prop'), false

);

assert.equal(

hasOwnProp({hasOwnProperty: 'yes'}, 'prop'), false

);

assert.equal(

hasOwnProp({hasOwnProperty: 'yes'}, 'hasOwnProperty'), true

);

The method invocation in line A is explained in §29.3.5 “Dispatched vs. direct method calls”.

We can also use .bind() to implement hasOwnProp():

const hasOwnProp = Object.prototype.hasOwnProperty.call

.bind(Object.prototype.hasOwnProperty);

How does this work? When we invoke .call() like in line A in the previous example, it does exactly what hasOwnProp() should do, including avoiding the pitfalls. However, if we want to function-call it, we can’t simply extract it, we must also ensure that its this always has the right value. That’s what .bind() does.

Is it never OK to use Object.prototype methods via dynamic dispatch?

In some cases we can be lazy and call Object.prototype methods like normal methods (without .call() or .bind()): If we know the receivers and they are fixed-layout objects.

If, on the other hand, we don’t know their receivers and/or they are dictionary objects, then we need to take precautions.

Object.prototype.toString()

By overriding .toString() (in a subclass or an instance), we can configure how objects are converted to strings:

> String({toString() { return 'Hello!' }})

'Hello!'

> String({})

'[object Object]'

For converting objects to strings it’s better to use String() because that also works with undefined and null:

> undefined.toString()

TypeError: Cannot read properties of undefined (reading 'toString')

> null.toString()

TypeError: Cannot read properties of null (reading 'toString')

> String(undefined)

'undefined'

> String(null)

'null'

Object.prototype.toLocaleString()

.toLocaleString() is a version of .toString() that can be configured via a locale and often additional options. Any class or instance can implement this method. In the standard library, the following classes do:

- Array.prototype.toLocaleString()

- Number.prototype.toLocaleString()

- Date.prototype.toLocaleString()

- TypedArray.prototype.toLocaleString()

- BigInt.prototype.toLocaleString()

As an example, this is how numbers with decimal fractions are converted to string differently, depending on locale ('fr' is French, 'en' is English):

> 123.45.toLocaleString('fr')

'123,45'

> 123.45.toLocaleString('en')

'123.45'

Object.prototype.valueOf()

By overriding .valueOf() (in a subclass or an instance), we can configure how objects are converted to non-string values (often numbers):

> Number({valueOf() { return 123 }})

123

> Number({})

NaN

Object.prototype.isPrototypeOf()

proto.isPrototypeOf(obj) returns true if proto is in the prototype chain of obj and false otherwise.

const a = {};

const b = {__proto__: a};

const c = {__proto__: b};

assert.equal(a.isPrototypeOf(b), true);

assert.equal(a.isPrototypeOf(c), true);

assert.equal(a.isPrototypeOf(a), false);

assert.equal(c.isPrototypeOf(a), false);

This is how to use this method safely (for details see §29.8.1 “Using Object.prototype methods safely”):

const obj = {

// Overrides Object.prototype.isPrototypeOf

isPrototypeOf: true,

};

// Doesn’t work in this case:

assert.throws(

() => obj.isPrototypeOf(Object.prototype),

{

name: 'TypeError',

message: 'obj.isPrototypeOf is not a function',

}

);

// Safe way of using .isPrototypeOf():

assert.equal(

Object.prototype.isPrototypeOf.call(obj, Object.prototype), false

);

Object.prototype.propertyIsEnumerable()

obj.propertyIsEnumerable(propKey) returns true if obj has an own enumerable property whose key is propKey and false otherwise.

const proto = {

enumerableProtoProp: true,

};

const obj = {

__proto__: proto,

enumerableObjProp: true,

nonEnumObjProp: true,

};

Object.defineProperty(

obj, 'nonEnumObjProp',

{

enumerable: false,

}

);

assert.equal(

obj.propertyIsEnumerable('enumerableProtoProp'),

false // not an own property

);

assert.equal(

obj.propertyIsEnumerable('enumerableObjProp'),

true

);

assert.equal(

obj.propertyIsEnumerable('nonEnumObjProp'),

false // not enumerable

);

assert.equal(

obj.propertyIsEnumerable('unknownProp'),

false // not a property

);

This is how to use this method safely (for details see §29.8.1 “Using Object.prototype methods safely”):

const obj = {

// Overrides Object.prototype.propertyIsEnumerable

propertyIsEnumerable: true,

enumerableProp: 'yes',

};

// Doesn’t work in this case:

assert.throws(

() => obj.propertyIsEnumerable('enumerableProp'),

{

name: 'TypeError',

message: 'obj.propertyIsEnumerable is not a function',

}

);

// Safe way of using .propertyIsEnumerable():

assert.equal(

Object.prototype.propertyIsEnumerable.call(obj, 'enumerableProp'),

true

);

Another safe alternative is to use property descriptors:

assert.deepEqual(

Object.getOwnPropertyDescriptor(obj, 'enumerableProp'),

{

value: 'yes',

writable: true,

enumerable: true,

configurable: true,

}

);

Object.prototype.__proto__ (accessor)

Property __proto__ exists in two versions:

- An accessor that all instances of Object have.

- A property of object literals that sets the prototypes of the objects created by them.

I recommend to avoid the former feature:

- As explained in §29.8.1 “Using Object.prototype methods safely”, it doesn’t work with all objects.

- The ECMAScript specification has deprecated it and calls it “optional” and “legacy”.

In contrast, __proto__ in object literals always works and is not deprecated.

Read on if you are interested in how the accessor __proto__ works.

__proto__ is an accessor of Object.prototype that is inherited by all instances of Object. Implementing it via a class would look like this:

class Object {

get __proto__() {

return Object.getPrototypeOf(this);

}

set __proto__(other) {

Object.setPrototypeOf(this, other);

}

// ···

}

Since __proto__ is inherited from Object.prototype, we can remove this feature by creating an object that doesn’t have Object.prototype in its prototype chain (see §29.7.3 “Not all objects are instances of Object”):

> '__proto__' in {}

true

> '__proto__' in Object.create(null)

false

Object.prototype.hasOwnProperty()

Better alternative to .hasOwnProperty(): Object.hasOwn() [ES2022]

See §28.9.4 “Object.hasOwn(): Is a given property own (non-inherited)? [ES2022]”.

obj.hasOwnProperty(propKey) returns true if obj has an own (non-inherited) property whose key is propKey and false otherwise.

const obj = { ownProp: true };

assert.equal(

obj.hasOwnProperty('ownProp'), true // own

);

assert.equal(

'toString' in obj, true // inherited

);

assert.equal(

obj.hasOwnProperty('toString'), false

);

This is how to use this method safely (for details see §29.8.1 “Using Object.prototype methods safely”):

const obj = {

// Overrides Object.prototype.hasOwnProperty

hasOwnProperty: true,

};

// Doesn’t work in this case:

assert.throws(

() => obj.hasOwnProperty('anyPropKey'),

{

name: 'TypeError',

message: 'obj.hasOwnProperty is not a function',

}

);

// Safe way of using .hasOwnProperty():

assert.equal(

Object.prototype.hasOwnProperty.call(obj, 'anyPropKey'), false

);

FAQ: classes

Why are they called “instance private fields” in this book and not “private instance fields”?

That is done to highlight how different properties (public slots) and private slots are: By changing the order of the adjectives, the words “public” and “field” and the words “private” and “field” are always mentioned together.

Why the identifier prefix #? Why not declare private fields via private?

Could private fields be declared via private and use normal identifiers? Let’s examine what would happen if that were possible:

class MyClass {

private value; // (A)

compare(other) {

return this.value === other.value;

}

}

Whenever an expression such as other.value appears in the body of MyClass, JavaScript has to decide:

- Is .value a property?

- Is .value a private field?

At compile time, JavaScript doesn’t know if the declaration in line A applies to other (due to it being an instance of MyClass) or not. That leaves two options for making the decision:

1. .value is always interpreted as a private field.

2. JavaScript decides at runtime:

- If other is an instance of MyClass, then .value is interpreted as a private field.

- Otherwise .value is interpreted as a property.

Both options have downsides:

- With option (1), we can’t use .value as a property, anymore – for any object.

- With option (2), performance is affected negatively.

That’s why the name prefix # was introduced. The decision is now easy: If we use #, we want to access a private field. If we don’t, we want to access a property.

private works for statically typed languages (such as TypeScript) because they know at compile time if other is an instance of MyClass and can then treat .value as private or public.

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