JavaScript for impatient programmers (ES2022 edition)
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31 Arrays (Array)

31.1 Cheat sheet: Arrays

JavaScript Arrays are a very flexible data structure and used as lists, stacks, queues, tuples (e.g. pairs), and more.

Some Array-related operations destructively change Arrays. Others non-destructively produce new Arrays with the changes applied to a copy of the original content.

31.1.1 Using Arrays

Creating an Array, reading and writing elements:

// Creating an Array
const arr = ['a', 'b', 'c']; // Array literal
  [ // Array literal
    'c', // trailing commas are ignored

// Reading elements
  arr[0], 'a' // negative indices don’t work
assert.equal(, 'c' // negative indices work

// Writing an element
arr[0] = 'x';
  arr, ['x', 'b', 'c']

The length of an Array:

const arr = ['a', 'b', 'c'];
  arr.length, 3 // number of elements
arr.length = 1; // removing elements
  arr, ['a']
arr[arr.length] = 'b'; // adding an element
  arr, ['a', 'b']

Adding elements destructively via .push():

const arr = ['a', 'b'];

arr.push('c'); // adding an element
  arr, ['a', 'b', 'c']

// Pushing Arrays (used as arguments via spreading (...)):
arr.push(...['d', 'e']);
  arr, ['a', 'b', 'c', 'd', 'e']

Adding elements non-destructively via spreading (...):

const arr1 = ['a', 'b'];
const arr2 = ['c'];
  [...arr1, ...arr2, 'd', 'e'],
  ['a', 'b', 'c', 'd', 'e']

Clearing Arrays (removing all elements):

// Destructive – affects everyone referring to the Array:
const arr1 = ['a', 'b', 'c'];
arr1.length = 0;
  arr1, []

// Non-destructive – does not affect others referring to the Array:
let arr2 = ['a', 'b', 'c'];
arr2 = [];
  arr2, []

Looping over elements:

const arr = ['a', 'b', 'c'];
for (const value of arr) {

// Output:
// 'a'
// 'b'
// 'c'

Looping over index-value pairs:

const arr = ['a', 'b', 'c'];
for (const [index, value] of arr.entries()) {
  console.log(index, value);

// Output:
// 0, 'a'
// 1, 'b'
// 2, 'c'

Creating and filling Arrays when we can’t use Array literals (e.g. because we don’t know their lengths in advance or they are too large):

const four = 4;

// Empty Array that we’ll fill later
  new Array(four),
  [ , , , ,] // four holes; last comma is ignored

// An Array filled with a primitive value
  new Array(four).fill(0),
  [0, 0, 0, 0]

// An Array filled with objects
// Why not .fill()? We’d get single object, shared multiple times.
  Array.from({length: four}, () => ({})),
  [{}, {}, {}, {}]

// A range of integers
  Array.from({length: four}, (_, i) => i),
  [0, 1, 2, 3]

31.1.2 Array methods

This section gives a brief overview of the Array API. There is a more comprehensive quick reference at the end of this chapter.

Deriving a new Array from an existing Array:

> ['■','●','▲'].slice(1, 3)
> ['■','●','■'].filter(x => x==='■') 

> ['▲','●'].map(x => x+x)
> ['▲','●'].flatMap(x => [x,x])

Removing an Array element at a given index:

// .filter(): remove non-destructively
const arr1 = ['■','●','▲'];
  arr1.filter((_, index) => index !== 1),
  arr1, ['■','●','▲'] // unchanged

// .splice(): remove destructively
const arr2 = ['■','●','▲'];
arr2.splice(1, 1); // start at 1, delete 1 element
  arr2, ['■','▲'] // changed

Computing a summary of an Array:

> ['■','●','▲'].some(x => x==='●')
> ['■','●','▲'].every(x => x==='●')

> ['■','●','▲'].join('-')

> ['■','▲'].reduce((result,x) => result+x, '●')
> ['■','▲'].reduceRight((result,x) => result+x, '●')

Reversing and filling:

// .reverse() changes and returns `arr`
const arr = ['■','●','▲'];
  arr.reverse(), arr
// `arr` was changed:
  arr, ['▲','●','■']

// .fill() works the same way:

.sort() also modifies an Array and returns it:

// By default, string representations of the Array elements
// are sorted lexicographically:
  [200, 3, 10].sort(),
  [10, 200, 3]

// Sorting can be customized via a callback:
  [200, 3, 10].sort((a,b) => a - b), // sort numerically
  [ 3, 10, 200 ]

Finding Array elements:

> ['■','●','■'].includes('■')
> ['■','●','■'].indexOf('■')
> ['■','●','■'].lastIndexOf('■')
> ['■','●','■'].find(x => x==='■')
> ['■','●','■'].findIndex(x => x==='■')

Adding or removing an element at the start or the end:

// Adding and removing at the start
const arr1 = ['■','●'];
  arr1, ['▲','■','●']
  arr1, ['■','●']

// Adding and removing at the end
const arr2 = ['■','●'];
  arr2, ['■','●','▲']
  arr2, ['■','●']

31.2 The two ways of using Arrays in JavaScript

There are two ways of using Arrays in JavaScript:

In practice, these two ways are often mixed.

Notably, sequence Arrays are so flexible that we can use them as (traditional) arrays, stacks, and queues. We’ll see how later.

31.3 Basic Array operations

31.3.1 Creating, reading, writing Arrays

The best way to create an Array is via an Array literal:

const arr = ['a', 'b', 'c'];

The Array literal starts and ends with square brackets []. It creates an Array with three elements: 'a', 'b', and 'c'.

Trailing commas are allowed and ignored in Array literals:

const arr = [

To read an Array element, we put an index in square brackets (indices start at zero):

const arr = ['a', 'b', 'c'];
assert.equal(arr[0], 'a');

To change an Array element, we assign to an Array with an index:

const arr = ['a', 'b', 'c'];
arr[0] = 'x';
assert.deepEqual(arr, ['x', 'b', 'c']);

The range of Array indices is 32 bits (excluding the maximum length): [0, 232−1)

31.3.2 The .length of an Array

Every Array has a property .length that can be used to both read and change(!) the number of elements in an Array.

The length of an Array is always the highest index plus one:

> const arr = ['a', 'b'];
> arr.length

If we write to the Array at the index of the length, we append an element:

> arr[arr.length] = 'c';
> arr
[ 'a', 'b', 'c' ]
> arr.length

Another way of (destructively) appending an element is via the Array method .push():

> arr.push('d');
> arr
[ 'a', 'b', 'c', 'd' ]

If we set .length, we are pruning the Array by removing elements:

> arr.length = 1;
> arr
[ 'a' ]

  Exercise: Removing empty lines via .push()


31.3.3 Referring to elements via negative indices

Several Array methods support negative indices. If an index is negative, it is added to the length of an Array to produce a usable index. Therefore, the following two invocations of .slice() are equivalent: They both copy arr starting at the last element.

> const arr = ['a', 'b', 'c'];
> arr.slice(-1)
[ 'c' ]
> arr.slice(arr.length - 1)
[ 'c' ] .at(): reading single elements (supports negative indices) [ES2022]

The Array method .at() returns the element at a given index. It supports positive and negative indices (-1 refers to the last element, -2 refers to the second-last element, etc.):

> ['a', 'b', 'c'].at(0)
> ['a', 'b', 'c'].at(-1)

In contrast, the bracket operator [] does not support negative indices (and can’t be changed because that would break existing code). It interprets them as keys of non-element properties:

const arr = ['a', 'b', 'c'];

arr[-1] = 'non-element property';
// The Array elements didn’t change:
  Array.from(arr), // copy just the Array elements
  ['a', 'b', 'c']

  arr[-1], 'non-element property'

31.3.4 Clearing Arrays

To clear (empty) an Array, we can either set its .length to zero:

const arr = ['a', 'b', 'c'];
arr.length = 0;
assert.deepEqual(arr, []);

or we can assign a new empty Array to the variable storing the Array:

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

The latter approach has the advantage of not affecting other locations that point to the same Array. If, however, we do want to reset a shared Array for everyone, then we need the former approach.

31.3.5 Spreading into Array literals [ES6]

Inside an Array literal, a spread element consists of three dots (...) followed by an expression. It results in the expression being evaluated and then iterated over. Each iterated value becomes an additional Array element – for example:

> const iterable = ['b', 'c'];
> ['a', ...iterable, 'd']
[ 'a', 'b', 'c', 'd' ]

That means that we can use spreading to create a copy of an Array and to convert an iterable to an Array:

const original = ['a', 'b', 'c'];

const copy = [...original];

const iterable = original.keys();
  [...iterable], [0, 1, 2]

However, for both previous use cases, I find Array.from() more self-descriptive and prefer it:

const copy2 = Array.from(original);

  Array.from(original.keys()), [0, 1, 2]

Spreading is also convenient for concatenating Arrays (and other iterables) into Arrays:

const arr1 = ['a', 'b'];
const arr2 = ['c', 'd'];

const concatenated = [...arr1, ...arr2, 'e'];
  ['a', 'b', 'c', 'd', 'e']);

Due to spreading using iteration, it only works if the value is iterable:

> [...'abc'] // strings are iterable
[ 'a', 'b', 'c' ]
> [...123]
TypeError: 123 is not iterable
> [...undefined]
TypeError: undefined is not iterable

  Spreading and Array.from() produce shallow copies

Copying Arrays via spreading or via Array.from() is shallow: We get new entries in a new Array, but the values are shared with the original Array. The consequences of shallow copying are demonstrated in §28.4 “Spreading into object literals (...) [ES2018]”.

31.3.6 Arrays: listing indices and entries [ES6]

Method .keys() lists the indices of an Array:

const arr = ['a', 'b'];
  Array.from(arr.keys()), // (A)
  [0, 1]);

.keys() returns an iterable. In line A, we convert that iterable to an Array.

Listing Array indices is different from listing properties. The former produces numbers; the latter produces stringified numbers (in addition to non-index property keys):

const arr = ['a', 'b'];
arr.prop = true;

  ['0', '1', 'prop']);

Method .entries() lists the contents of an Array as [index, element] pairs:

const arr = ['a', 'b'];
  [[0, 'a'], [1, 'b']]);

31.3.7 Is a value an Array?

Following are two ways of checking if a value is an Array:

> [] instanceof Array
> Array.isArray([])

instanceof is usually fine. We need Array.isArray() if a value may come from another realm. Roughly, a realm is an instance of JavaScript’s global scope. Some realms are isolated from each other (e.g., Web Workers in browsers), but there are also realms between which we can move data – for example, same-origin iframes in browsers. x instanceof Array checks the prototype chain of x and therefore returns false if x is an Array from another realm.

typeof categorizes Arrays as objects:

> typeof []

31.4 for-of and Arrays [ES6]

We have already encountered the for-of loop earlier in this book. This section briefly recaps how to use it for Arrays.

31.4.1 for-of: iterating over elements

The following for-of loop iterates over the elements of an Array:

for (const element of ['a', 'b']) {
// Output:
// 'a'
// 'b'

31.4.2 for-of: iterating over indices

This for-of loop iterates over the indices of an Array:

for (const element of ['a', 'b'].keys()) {
// Output:
// 0
// 1

31.4.3 for-of: iterating over [index, element] pairs

The following for-of loop iterates over [index, element] pairs. Destructuring (described later), gives us convenient syntax for setting up index and element in the head of for-of.

for (const [index, element] of ['a', 'b'].entries()) {
  console.log(index, element);
// Output:
// 0, 'a'
// 1, 'b'

31.5 Array-like objects

Some operations that work with Arrays require only the bare minimum: values must only be Array-like. An Array-like value is an object with the following properties:

For example, Array.from() accepts Array-like objects and converts them to Arrays:

// If we omit .length, it is interpreted as 0

  Array.from({length:2, 0:'a', 1:'b'}),
  [ 'a', 'b' ]);

The TypeScript interface for Array-like objects is:

interface ArrayLike<T> {
  length: number;
  [n: number]: T;

  Array-like objects are relatively rare in modern JavaScript

Array-like objects used to be common before ES6; now we don’t see them very often.

31.6 Converting iterables and Array-like values to Arrays

There are two common ways of converting iterables and Array-like values to Arrays:

I prefer the latter – I find it more self-explanatory.

31.6.1 Converting iterables to Arrays via spreading (...)

Inside an Array literal, spreading via ... converts any iterable object into a series of Array elements. For example:

// Get an Array-like collection from a web browser’s DOM
const domCollection = document.querySelectorAll('a');

// Alas, the collection is missing many Array methods
assert.equal('map' in domCollection, false);

// Solution: convert it to an Array
const arr = [...domCollection];
assert.deepEqual( => x.href),
  ['', '']);

The conversion works because the DOM collection is iterable.

31.6.2 Converting iterables and Array-like objects to Arrays via Array.from()

Array.from() can be used in two modes. Mode 1 of Array.from(): converting

The first mode has the following type signature:

.from<T>(iterable: Iterable<T> | ArrayLike<T>): T[]

Interface Iterable is shown in the chapter on synchronous iteration. Interface ArrayLike appeared earlier in this chapter.

With a single parameter, Array.from() converts anything iterable or Array-like to an Array:

> Array.from(new Set(['a', 'b']))
[ 'a', 'b' ]
> Array.from({length: 2, 0:'a', 1:'b'})
[ 'a', 'b' ] Mode 2 of Array.from(): converting and mapping

The second mode of Array.from() involves two parameters:

.from<T, U>(
  iterable: Iterable<T> | ArrayLike<T>,
  mapFunc: (v: T, i: number) => U,
  thisArg?: any)
  : U[]

In this mode, Array.from() does several things:

In other words: we are going from an iterable with elements of type T to an Array with elements of type U.

This is an example:

> Array.from(new Set(['a', 'b']), x => x + x)
[ 'aa', 'bb' ]

31.7 Creating and filling Arrays with arbitrary lengths

The best way of creating an Array is via an Array literal. However, we can’t always use one: The Array may be too large, we may not know its length during development, or we may want to keep its length flexible. Then I recommend the following techniques for creating, and possibly filling, Arrays.

31.7.1 Do we need to create an empty Array that we’ll fill completely later on?

> new Array(3)
[ , , ,]

Note that the result has three holes (empty slots) – the last comma in an Array literal is always ignored.

31.7.2 Do we need to create an Array filled with a primitive value?

> new Array(3).fill(0)
[0, 0, 0]

Caveat: If we use .fill() with an object, then each Array element will refer to this object (sharing it).

const arr = new Array(3).fill({});
arr[0].prop = true;
  arr, [
    {prop: true},
    {prop: true},
    {prop: true},

The next subsection explains how to fix this.

31.7.3 Do we need to create an Array filled with objects?

> new Array(3).fill(0)
[0, 0, 0]

For large sizes, the temporary Array can consume quite a bit of memory. The following approach doesn’t have this downside but is less self-descriptive:

> Array.from({length: 3}, () => ({}))
[{}, {}, {}]

Instead of a temporary Array, we are using a temporary Array-like object.

31.7.4 Do we need to create a range of integers?

function createRange(start, end) {
  return Array.from({length: end-start}, (_, i) => i+start);
  createRange(2, 5),
  [2, 3, 4]);

Here is an alternative, slightly hacky technique for creating integer ranges that start at zero:

/** Returns an iterable */
function createRange(end) {
  return new Array(end).keys();
  [0, 1, 2, 3]);

This works because .keys() treats holes like undefined elements and lists their indices.

31.7.5 Use a Typed Array if the elements are all integers or all floats

When dealing with Arrays of integers or floats, we should consider Typed Arrays, which were created for this purpose.

31.8 Multidimensional Arrays

JavaScript does not have real multidimensional Arrays; we need to resort to Arrays whose elements are Arrays:

function initMultiArray(...dimensions) {
  function initMultiArrayRec(dimIndex) {
    if (dimIndex >= dimensions.length) {
      return 0;
    } else {
      const dim = dimensions[dimIndex];
      const arr = [];
      for (let i=0; i<dim; i++) {
      return arr;
  return initMultiArrayRec(0);

const arr = initMultiArray(4, 3, 2);
arr[3][2][1] = 'X'; // last in each dimension
assert.deepEqual(arr, [
  [ [ 0, 0 ], [ 0, 0 ], [ 0, 0 ] ],
  [ [ 0, 0 ], [ 0, 0 ], [ 0, 0 ] ],
  [ [ 0, 0 ], [ 0, 0 ], [ 0, 0 ] ],
  [ [ 0, 0 ], [ 0, 0 ], [ 0, 'X' ] ],

31.9 More Array features (advanced)

In this section, we look at phenomena we don’t encounter often when working with Arrays.

31.9.1 Array indices are (slightly special) property keys

You’d think that Array elements are special because we are accessing them via numbers. But the square brackets operator [] for doing so is the same operator that is used for accessing properties. It coerces any value (that is not a symbol) to a string. Therefore, Array elements are (almost) normal properties (line A) and it doesn’t matter if we use numbers or strings as indices (lines B and C):

const arr = ['a', 'b'];
arr.prop = 123;
  ['0', '1', 'prop']); // (A)

assert.equal(arr[0], 'a');  // (B)
assert.equal(arr['0'], 'a'); // (C)

To make matters even more confusing, this is only how the language specification defines things (the theory of JavaScript, if you will). Most JavaScript engines optimize under the hood and do use actual integers to access Array elements (the practice of JavaScript, if you will).

Property keys (strings!) that are used for Array elements are called indices. A string str is an index if converting it to a 32-bit unsigned integer and back results in the original value. Written as a formula:

ToString(ToUint32(str)) === str Listing indices

When listing property keys, indices are treated specially – they always come first and are sorted like numbers ('2' comes before '10'):

const arr = [];
arr.prop = true;
arr[1] = 'b';
arr[0] = 'a';

  ['0', '1', 'prop']);

Note that .length, .entries() and .keys() treat Array indices as numbers and ignore non-index properties:

assert.equal(arr.length, 2);
  Array.from(arr.keys()), [0, 1]);
  Array.from(arr.entries()), [[0, 'a'], [1, 'b']]);

We used Array.from() to convert the iterables returned by .keys() and .entries() to Arrays.

31.9.2 Arrays are dictionaries and can have holes

We distinguish two kinds of Arrays in JavaScript:

Arrays can be sparse in JavaScript because Arrays are actually dictionaries from indices to values.

  Recommendation: avoid holes

So far, we have only seen dense Arrays and it’s indeed recommended to avoid holes: They make our code more complicated and are not handled consistently by Array methods. Additionally, JavaScript engines optimize dense Arrays, making them faster. Creating holes

We can create holes by skipping indices when assigning elements:

const arr = [];
arr[0] = 'a';
arr[2] = 'c';

assert.deepEqual(Object.keys(arr), ['0', '2']); // (A)

assert.equal(0 in arr, true); // element
assert.equal(1 in arr, false); // hole

In line A, we are using Object.keys() because arr.keys() treats holes as if they were undefined elements and does not reveal them.

Another way of creating holes is to skip elements in Array literals:

const arr = ['a', , 'c'];

assert.deepEqual(Object.keys(arr), ['0', '2']);

We can also delete Array elements:

const arr = ['a', 'b', 'c'];
assert.deepEqual(Object.keys(arr), ['0', '1', '2']);
delete arr[1];
assert.deepEqual(Object.keys(arr), ['0', '2']); How do Array operations treat holes?

Alas, there are many different ways in which Array operations treat holes.

Some Array operations remove holes:

> ['a',,'b'].filter(x => true)
[ 'a', 'b' ]

Some Array operations ignore holes:

> ['a', ,'a'].every(x => x === 'a')

Some Array operations ignore but preserve holes:

> ['a',,'b'].map(x => 'c')
[ 'c', , 'c' ]

Some Array operations treat holes as undefined elements:

> Array.from(['a',,'b'], x => x)
[ 'a', undefined, 'b' ]
> Array.from(['a',,'b'].entries())
[[0, 'a'], [1, undefined], [2, 'b']]

Object.keys() works differently than .keys() (strings vs. numbers, holes don’t have keys):

> Array.from(['a',,'b'].keys())
[ 0, 1, 2 ]
> Object.keys(['a',,'b'])
[ '0', '2' ]

There is no rule to remember here. If it ever matters how an Array operation treats holes, the best approach is to do a quick test in a console.

31.10 Adding and removing elements (destructively and non-destructively)

JavaScript’s Array is quite flexible and more like a combination of array, stack, and queue. This section explores ways of adding and removing Array elements. Most operations can be performed both destructively (modifying the Array) and non-destructively (producing a modified copy).

31.10.1 Prepending elements and Arrays

In the following code, we destructively prepend single elements to arr1 and an Array to arr2:

const arr1 = ['a', 'b'];
arr1.unshift('x', 'y'); // prepend single elements
assert.deepEqual(arr1, ['x', 'y', 'a', 'b']);

const arr2 = ['a', 'b'];
arr2.unshift(...['x', 'y']); // prepend Array
assert.deepEqual(arr2, ['x', 'y', 'a', 'b']);

Spreading lets us unshift an Array into arr2.

Non-destructive prepending is done via spread elements:

const arr1 = ['a', 'b'];
  ['x', 'y', ...arr1], // prepend single elements
  ['x', 'y', 'a', 'b']);
assert.deepEqual(arr1, ['a', 'b']); // unchanged!

const arr2 = ['a', 'b'];
  [...['x', 'y'], ...arr2], // prepend Array
  ['x', 'y', 'a', 'b']);
assert.deepEqual(arr2, ['a', 'b']); // unchanged!

31.10.2 Appending elements and Arrays

In the following code, we destructively append single elements to arr1 and an Array to arr2:

const arr1 = ['a', 'b'];
arr1.push('x', 'y'); // append single elements
assert.deepEqual(arr1, ['a', 'b', 'x', 'y']);

const arr2 = ['a', 'b'];
arr2.push(...['x', 'y']); // (A) append Array
assert.deepEqual(arr2, ['a', 'b', 'x', 'y']);

Spreading (...) lets us push an Array into arr2 (line A).

Non-destructive appending is done via spread elements:

const arr1 = ['a', 'b'];
  [...arr1, 'x', 'y'], // append single elements
  ['a', 'b', 'x', 'y']);
assert.deepEqual(arr1, ['a', 'b']); // unchanged!

const arr2 = ['a', 'b'];
  [...arr2, ...['x', 'y']], // append Array
  ['a', 'b', 'x', 'y']);
assert.deepEqual(arr2, ['a', 'b']); // unchanged!

31.10.3 Removing elements

These are three destructive ways of removing Array elements:

// Destructively remove first element:
const arr1 = ['a', 'b', 'c'];
assert.equal(arr1.shift(), 'a');
assert.deepEqual(arr1, ['b', 'c']);

// Destructively remove last element:
const arr2 = ['a', 'b', 'c'];
assert.equal(arr2.pop(), 'c');
assert.deepEqual(arr2, ['a', 'b']);

// Remove one or more elements anywhere:
const arr3 = ['a', 'b', 'c', 'd'];
assert.deepEqual(arr3.splice(1, 2), ['b', 'c']);
assert.deepEqual(arr3, ['a', 'd']);

.splice() is covered in more detail in the quick reference at the end of this chapter.

Destructuring via a rest element lets us non-destructively remove elements from the beginning of an Array (destructuring is covered later).

const arr1 = ['a', 'b', 'c'];
// Ignore first element, extract remaining elements
const [, ...arr2] = arr1;

assert.deepEqual(arr2, ['b', 'c']);
assert.deepEqual(arr1, ['a', 'b', 'c']); // unchanged!

Alas, a rest element must come last in an Array. Therefore, we can only use it to extract suffixes.

  Exercise: Implementing a queue via an Array


31.11 Methods: iteration and transformation (.find(), .map(), .filter(), etc.)

In this section, we take a look at Array methods for iterating over Arrays and for transforming Arrays.

31.11.1 Callbacks for iteration and transformation methods

All iteration and transformation methods use callbacks. The former feed all iterated values to their callbacks; the latter ask their callbacks how to transform Arrays.

These callbacks have type signatures that look as follows:

callback: (value: T, index: number, array: Array<T>) => boolean

That is, the callback gets three parameters (it is free to ignore any of them):

What the callback is expected to return depends on the method it is passed to. Possibilities include:

Both of these methods are described in more detail later.

31.11.2 Searching elements: .find(), .findIndex()

.find() returns the first element for which its callback returns a truthy value (and undefined if it can’t find anything):

> [6, -5, 8].find(x => x < 0)
> [6, 5, 8].find(x => x < 0)

.findIndex() returns the index of the first element for which its callback returns a truthy value (and -1 if it can’t find anything):

> [6, -5, 8].findIndex(x => x < 0)
> [6, 5, 8].findIndex(x => x < 0)

.findIndex() can be implemented as follows:

function findIndex(arr, callback) {
  for (const [i, x] of arr.entries()) {
    if (callback(x, i, arr)) {
      return i;
  return -1;

31.11.3 .map(): copy while giving elements new values

.map() returns a modified copy of the receiver. The elements of the copy are the results of applying map’s callback to the elements of the receiver.

All of this is easier to understand via examples:

> [1, 2, 3].map(x => x * 3)
[ 3, 6, 9 ]
> ['how', 'are', 'you'].map(str => str.toUpperCase())
[ 'HOW', 'ARE', 'YOU' ]
> [true, true, true].map((_x, index) => index)
[ 0, 1, 2 ]

.map() can be implemented as follows:

function map(arr, mapFunc) {
  const result = [];
  for (const [i, x] of arr.entries()) {
    result.push(mapFunc(x, i, arr));
  return result;

  Exercise: Numbering lines via .map()


31.11.4 .flatMap(): mapping to zero or more values

The type signature of Array<T>.prototype.flatMap() is:

  callback: (value: T, index: number, array: T[]) => U|Array<U>,
  thisValue?: any
): U[]

Both .map() and .flatMap() take a function callback as a parameter that controls how an input Array is translated to an output Array:

This is .flatMap() in action:

> ['a', 'b', 'c'].flatMap(x => [x,x])
[ 'a', 'a', 'b', 'b', 'c', 'c' ]
> ['a', 'b', 'c'].flatMap(x => [x])
[ 'a', 'b', 'c' ]
> ['a', 'b', 'c'].flatMap(x => [])

We’ll consider use cases next, before exploring how this method could be implemented. Use case: filtering and mapping at the same time

The result of the Array method .map() always has the same length as the Array it is invoked on. That is, its callback can’t skip Array elements it isn’t interested in. The ability of .flatMap() to do so is useful in the next example.

We will use the following function processArray() to create an Array that we’ll then filter and map via .flatMap():

function processArray(arr, callback) {
  return => {
    try {
      return { value: callback(x) };
    } catch (e) {
      return { error: e };

Next, we create an Array results via processArray():

const results = processArray([1, -5, 6], throwIfNegative);
assert.deepEqual(results, [
  { value: 1 },
  { error: new Error('Illegal value: -5') },
  { value: 6 },

function throwIfNegative(value) {
  if (value < 0) {
    throw new Error('Illegal value: '+value);
  return value;

We can now use .flatMap() to extract just the values or just the errors from results:

const values = results.flatMap(
  result => result.value ? [result.value] : []);
assert.deepEqual(values, [1, 6]);
const errors = results.flatMap(
  result => result.error ? [result.error] : []);
assert.deepEqual(errors, [new Error('Illegal value: -5')]); Use case: mapping single input values to multiple output values

The Array method .map() maps each input Array element to one output element. But what if we want to map it to multiple output elements?

That becomes necessary in the following example:

> stringsToCodePoints(['many', 'a', 'moon'])
['m', 'a', 'n', 'y', 'a', 'm', 'o', 'o', 'n']

We want to convert an Array of strings to an Array of Unicode characters (code points). The following function achieves that via .flatMap():

function stringsToCodePoints(strs) {
  return strs.flatMap(str => Array.from(str));
} A simple implementation

We can implement .flatMap() as follows. Note: This implementation is simpler than the built-in version, which, for example, performs more checks.

function flatMap(arr, mapFunc) {
  const result = [];
  for (const [index, elem] of arr.entries()) {
    const x = mapFunc(elem, index, arr);
    // We allow mapFunc() to return non-Arrays
    if (Array.isArray(x)) {
    } else {
  return result;

  Exercises: .flatMap()

31.11.5 .filter(): only keep some of the elements

The Array method .filter() returns an Array collecting all elements for which the callback returns a truthy value.

For example:

> [-1, 2, 5, -7, 6].filter(x => x >= 0)
[ 2, 5, 6 ]
> ['a', 'b', 'c', 'd'].filter((_x,i) => (i%2)===0)
[ 'a', 'c' ]

.filter() can be implemented as follows:

function filter(arr, filterFunc) {
  const result = [];
  for (const [i, x] of arr.entries()) {
    if (filterFunc(x, i, arr)) {
  return result;

  Exercise: Removing empty lines via .filter()


31.11.6 .reduce(): deriving a value from an Array (advanced)

Method .reduce() is a powerful tool for computing a “summary” of an Array arr. A summary can be any kind of value:

reduce is also known as foldl (“fold left”) in functional programming and popular there. One caveat is that it can make code difficult to understand.

.reduce() has the following type signature (inside an Array<T>):

  callback: (accumulator: U, element: T, index: number, array: T[]) => U,
  init?: U)
  : U

T is the type of the Array elements, U is the type of the summary. The two may or may not be different. accumulator is just another name for “summary”.

To compute the summary of an Array arr, .reduce() feeds all Array elements to its callback one at a time:

const accumulator_0 = callback(init, arr[0]);
const accumulator_1 = callback(accumulator_0, arr[1]);
const accumulator_2 = callback(accumulator_1, arr[2]);
// Etc.

callback combines the previously computed summary (stored in its parameter accumulator) with the current Array element and returns the next accumulator. The result of .reduce() is the final accumulator – the last result of callback after it has visited all elements.

In other words: callback does most of the work; .reduce() just invokes it in a useful manner.

We could say that the callback folds Array elements into the accumulator. That’s why this operation is called “fold” in functional programming. A first example

Let’s look at an example of .reduce() in action: function addAll() computes the sum of all numbers in an Array arr.

function addAll(arr) {
  const startSum = 0;
  const callback = (sum, element) => sum + element;
  return arr.reduce(callback, startSum);
assert.equal(addAll([1,  2, 3]), 6); // (A)
assert.equal(addAll([7, -4, 2]), 5);

In this case, the accumulator holds the sum of all Array elements that callback has already visited.

How was the result 6 derived from the Array in line A? Via the following invocations of callback:

callback(0, 1) --> 1
callback(1, 2) --> 3
callback(3, 3) --> 6


Alternatively, we could have implemented addAll() via a for-of loop:

function addAll(arr) {
  let sum = 0;
  for (const element of arr) {
    sum = sum + element;
  return sum;

It’s hard to say which of the two implementations is “better”: the one based on .reduce() is a little more concise, while the one based on for-of may be a little easier to understand – especially if someone is not familiar with functional programming. Example: finding indices via .reduce()

The following function is an implementation of the Array method .indexOf(). It returns the first index at which the given searchValue appears inside the Array arr:

const NOT_FOUND = -1;
function indexOf(arr, searchValue) {
  return arr.reduce(
    (result, elem, index) => {
      if (result !== NOT_FOUND) {
        // We have already found something: don’t change anything
        return result;
      } else if (elem === searchValue) {
        return index;
      } else {
        return NOT_FOUND;
assert.equal(indexOf(['a', 'b', 'c'], 'b'), 1);
assert.equal(indexOf(['a', 'b', 'c'], 'x'), -1);

One limitation of .reduce() is that we can’t finish early (in a for-of loop, we can break). Here, we always immediately return the result once we have found it. Example: doubling Array elements

Function double(arr) returns a copy of inArr whose elements are all multiplied by 2:

function double(inArr) {
  return inArr.reduce(
    (outArr, element) => {
      outArr.push(element * 2);
      return outArr;
  double([1, 2, 3]),
  [2, 4, 6]);

We modify the initial value [] by pushing into it. A non-destructive, more functional version of double() looks as follows:

function double(inArr) {
  return inArr.reduce(
    // Don’t change `outArr`, return a fresh Array
    (outArr, element) => [...outArr, element * 2],
  double([1, 2, 3]),
  [2, 4, 6]);

This version is more elegant but also slower and uses more memory.

  Exercises: .reduce()

31.12 .sort(): sorting Arrays

.sort() has the following type definition:

sort(compareFunc?: (a: T, b: T) => number): this

By default, .sort() sorts string representations of the elements. These representations are compared via <. This operator compares lexicographically (the first characters are most significant). We can see that when sorting numbers:

> [200, 3, 10].sort()
[ 10, 200, 3 ]

When sorting human-language strings, we need to be aware that they are compared according to their code unit values (char codes):

> ['pie', 'cookie', 'éclair', 'Pie', 'Cookie', 'Éclair'].sort()
[ 'Cookie', 'Pie', 'cookie', 'pie', 'Éclair', 'éclair' ]

All unaccented uppercase letters come before all unaccented lowercase letters, which come before all accented letters. We can use Intl, the JavaScript internationalization API if we want proper sorting for human languages.

.sort() sorts in place; it changes and returns its receiver:

> const arr = ['a', 'c', 'b'];
> arr.sort() === arr
> arr
[ 'a', 'b', 'c' ]

31.12.1 Customizing the sort order

We can customize the sort order via the parameter compareFunc, which must return a number that is:

  Tip for remembering these rules

A negative number is less than zero (etc.).

31.12.2 Sorting numbers

We can use this helper function to sort numbers:

function compareNumbers(a, b) {
  if (a < b) {
    return -1;
  } else if (a === b) {
    return 0;
  } else {
    return 1;
  [200, 3, 10].sort(compareNumbers),
  [3, 10, 200]);

The following is a quick and dirty alternative.

> [200, 3, 10].sort((a,b) => a - b)
[ 3, 10, 200 ]

The downsides of this approach are:

31.12.3 Sorting objects

We also need to use a compare function if we want to sort objects. As an example, the following code shows how to sort objects by age.

const arr = [ {age: 200}, {age: 3}, {age: 10} ];
  arr.sort((obj1, obj2) => obj1.age - obj2.age),
  [{ age: 3 }, { age: 10 }, { age: 200 }] );

  Exercise: Sorting objects by name


31.13 Quick reference: Array


31.13.1 new Array()

new Array(n) creates an Array of length n that contains n holes:

// Trailing commas are always ignored.
// Therefore: number of commas = number of holes
assert.deepEqual(new Array(3), [,,,]);

new Array() creates an empty Array. However, I recommend to always use [] instead.

31.13.2 Static methods of Array

31.13.3 Methods of Array.prototype

31.13.4 Sources


See quiz app.