14. New OOP features besides classes
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14. New OOP features besides classes

Classes (which are explained in the next chapter) are the major new OOP feature in ECMAScript 6. However, it also includes new features for object literals and new utility methods in Object. This chapter describes them.



14.1 Overview

14.1.1 New object literal features

Method definitions:

const obj = {
    myMethod(x, y) {
        ···
    }
};

Property value shorthands:

const first = 'Jane';
const last = 'Doe';

const obj = { first, last };
// Same as:
const obj = { first: first, last: last };

Computed property keys:

const propKey = 'foo';
const obj = {
    [propKey]: true,
    ['b'+'ar']: 123
};

This new syntax can also be used for method definitions:

const obj = {
    ['h'+'ello']() {
        return 'hi';
    }
};
console.log(obj.hello()); // hi

The main use case for computed property keys is to make it easy to use symbols as property keys.

14.1.2 New methods in Object

The most important new method of Object is assign(). Traditionally, this functionality was called extend() in the JavaScript world. In contrast to how this classic operation works, Object.assign() only considers own (non-inherited) properties.

const obj = { foo: 123 };
Object.assign(obj, { bar: true });
console.log(JSON.stringify(obj));
    // {"foo":123,"bar":true}

14.2 New features of object literals

14.2.1 Method definitions

In ECMAScript 5, methods are properties whose values are functions:

var obj = {
    myMethod: function (x, y) {
        ···
    }
};

In ECMAScript 6, methods are still function-valued properties, but there is now a more compact way of defining them:

const obj = {
    myMethod(x, y) {
        ···
    }
};

Getters and setters continue to work as they did in ECMAScript 5 (note how syntactically similar they are to method definitions):

const obj = {
    get foo() {
        console.log('GET foo');
        return 123;
    },
    set bar(value) {
        console.log('SET bar to '+value);
        // return value is ignored
    }
};

Let’s use obj:

> obj.foo
GET foo
123
> obj.bar = true
SET bar to true
true

There is also a way to concisely define properties whose values are generator functions:

const obj = {
    * myGeneratorMethod() {
        ···
    }
};

This code is equivalent to:

const obj = {
    myGeneratorMethod: function* () {
        ···
    }
};

14.2.2 Property value shorthands

Property value shorthands let you abbreviate the definition of a property in an object literal: If the name of the variable that specifies the property value is also the property key then you can omit the key. This looks as follows.

const x = 4;
const y = 1;
const obj = { x, y };

The last line is equivalent to:

const obj = { x: x, y: y };

Property value shorthands work well together with destructuring:

const obj = { x: 4, y: 1 };
const {x,y} = obj;
console.log(x); // 4
console.log(y); // 1

One use case for property value shorthands are multiple return values (which are explained in the chapter on destructuring).

14.2.3 Computed property keys

Remember that there are two ways of specifying a key when you set a property.

  1. Via a fixed name: obj.foo = true;
  2. Via an expression: obj['b'+'ar'] = 123;

In object literals, you only have option #1 in ECMAScript 5. ECMAScript 6 additionally provides option #2:

const propKey = 'foo';
const obj = {
    [propKey]: true,
    ['b'+'ar']: 123
};

This new syntax can also be used for method definitions:

const obj = {
    ['h'+'ello']() {
        return 'hi';
    }
};
console.log(obj.hello()); // hi

The main use case for computed property keys are symbols: you can define a public symbol and use it as a special property key that is always unique. One prominent example is the symbol stored in Symbol.iterator. If an object has a method with that key, it becomes iterable: The method must return an iterator, which is used by constructs such as the for-of loop to iterate over the object. The following code demonstrates how that works.

const obj = {
    * [Symbol.iterator]() { // (A)
        yield 'hello';
        yield 'world';
    }
};
for (const x of obj) {
    console.log(x);
}
// Output:
// hello
// world

Line A starts a generator method definition with a computed key (the symbol stored in Symbol.iterator).

14.3 New methods of Object

14.3.1 Object.assign(target, source_1, source_2, ···)

This method merges the sources into the target: It modifies target, first copies all enumerable own (non-inherited) properties of source_1 into it, then all own properties of source_2, etc. At the end, it returns the target.

const obj = { foo: 123 };
Object.assign(obj, { bar: true });
console.log(JSON.stringify(obj));
    // {"foo":123,"bar":true}

Let’s look more closely at how Object.assign() works:

This is how you’d copy all properties (not just enumerable ones), while correctly transferring getters and setters, without invoking setters on the target:

function copyAllProperties(target, ...sources) {
    for (const source of sources) {
        for (const key of Reflect.ownKeys(source)) {
            const desc = Object.getOwnPropertyDescriptor(source, key);
            Object.defineProperty(target, key, desc);
        }
    }
    return target;
}
14.3.1.1 Caveat: Object.assign() doesn’t work well for moving methods

On one hand, you can’t move a method that uses super: Such a method has the internal slot [[HomeObject]] that ties it to the object it was created in. If you move it via Object.assign(), it will continue to refer to the super-properties of the original object. Details are explained in a section in the chapter on classes.

On the other hand, enumerability is wrong if you move methods created by an object literal into the prototype of a class. The former methods are all enumerable (otherwise Object.assign() wouldn’t see them, anyway), but the prototype only has non-enumerable methods by default.

14.3.1.2 Use cases for Object.assign()

Let’s look at a few use cases.

14.3.1.2.1 Adding properties to this

You can use Object.assign() to add properties to this in a constructor:

class Point {
    constructor(x, y) {
        Object.assign(this, {x, y});
    }
}
14.3.1.2.2 Providing default values for object properties

Object.assign() is also useful for filling in defaults for missing properties. In the following example, we have an object DEFAULTS with default values for properties and an object options with data.

const DEFAULTS = {
    logLevel: 0,
    outputFormat: 'html'
};
function processContent(options) {
    options = Object.assign({}, DEFAULTS, options); // (A)
    ···
}

In line A, we created a fresh object, copied the defaults into it and then copied options into it, overriding the defaults. Object.assign() returns the result of these operations, which we assign to options.

14.3.1.2.3 Adding methods to objects

Another use case is adding methods to objects:

Object.assign(SomeClass.prototype, {
    someMethod(arg1, arg2) {
        ···
    },
    anotherMethod() {
        ···
    }
});

You could also manually assign functions, but then you don’t have the nice method definition syntax and need to mention SomeClass.prototype each time:

SomeClass.prototype.someMethod = function (arg1, arg2) {
    ···
};
SomeClass.prototype.anotherMethod = function () {
    ···
};
14.3.1.2.4 Cloning objects

One last use case for Object.assign() is a quick way of cloning objects:

function clone(orig) {
    return Object.assign({}, orig);
}

This way of cloning is also somewhat dirty, because it doesn’t preserve the property attributes of orig. If that is what you need, you have to use property descriptors.

If you want the clone to have the same prototype as the original, you can use Object.getPrototypeOf() and Object.create():

function clone(orig) {
    const origProto = Object.getPrototypeOf(orig);
    return Object.assign(Object.create(origProto), orig);
}

14.3.2 Object.getOwnPropertySymbols(obj)

Object.getOwnPropertySymbols(obj) retrieves all own (non-inherited) symbol-valued property keys of obj. It complements Object.getOwnPropertyNames(), which retrieves all string-valued own property keys. Consult a later section for more details on traversing properties.

14.3.3 Object.is(value1, value2)

The strict equals operator (===) treats two values differently than one might expect.

First, NaN is not equal to itself.

> NaN === NaN
false

That is unfortunate, because it often prevents us from detecting NaN:

> [0,NaN,2].indexOf(NaN)
-1

Second, JavaScript has two zeros, but strict equals treats them as if they were the same value:

> -0 === +0
true

Doing this is normally a good thing.

Object.is() provides a way of comparing values that is a bit more precise than ===. It works as follows:

> Object.is(NaN, NaN)
true
> Object.is(-0, +0)
false

Everything else is compared as with ===.

14.3.3.1 Using Object.is() to find Array elements

If we combine Object.is() with the new ES6 Array method findIndex(), we can find NaN in Arrays:

function myIndexOf(arr, elem) {
    return arr.findIndex(x => Object.is(x, elem));
}

myIndexOf([0,NaN,2], NaN); // 1

In contrast, indexOf() does not handle NaN well:

> [0,NaN,2].indexOf(NaN)
-1

14.3.4 Object.setPrototypeOf(obj, proto)

This method sets the prototype of obj to proto. The non-standard way of doing so in ECMAScript 5, that is supported by many engines, is via assigning to the special property __proto__. The recommended way of setting the prototype remains the same as in ECMAScript 5: during the creation of an object, via Object.create(). That will always be faster than first creating an object and then setting its prototype. Obviously, it doesn’t work if you want to change the prototype of an existing object.

14.4 Traversing properties in ES6

14.4.1 Five operations that traverse properties

In ECMAScript 6, the key of a property can be either a string or a symbol. The following are five operations that traverse the property keys of an object obj:

14.4.2 Traversal order of properties

ES6 defines two traversal orders for properties.

Own Property Keys:

Enumerable Own Names:

The order in which for-in traverses properties is not defined. Quoting Allen Wirfs-Brock:

Historically, the for-in order was not defined and there has been variation among browser implementations in the order they produce (and other specifics). ES5 added Object.keys and the requirement that it should order the keys identically to for-in. During development of both ES5 and ES6, the possibility of defining a specific for-in order was considered but not adopted because of web legacy compatibility concerns and uncertainty about the willingness of browsers to make changes in the ordering they currently produce.

14.4.2.1 Integer indices

Many engines treat integer indices specially, even though they are still strings (at least as far as the ES6 spec is concerned). Therefore, it makes sense to treat them as a separate category of keys.

Roughly, an integer index is a string that, if converted to a 53-bit non-negative integer and back is the same value. Therefore:

In ES6, instances of String and Typed Arrays have integer indices. The indices of normal Arrays are a subset of integer indices: they have a smaller range of 32 bits. For more information on Array indices, consult “Array Indices in Detail” in “Speaking JavaScript”.

14.4.2.2 Example

The following code demonstrates the traversal order “Own Property Keys”:

const obj = {
    [Symbol('first')]: true,
    '02': true,
    '10': true,
    '01': true,
    '2': true,
    [Symbol('second')]: true,
};
Reflect.ownKeys(obj);
    // [ '2', '10', '02', '01',
    //   Symbol('first'), Symbol('second') ]

Explanation:

14.4.2.3 Why does the spec standardize in which order property keys are returned?

Answer by Tab Atkins Jr.:

Because, for objects at least, all implementations used approximately the same order (matching the current spec), and lots of code was inadvertently written that depended on that ordering, and would break if you enumerated it in a different order. Since browsers have to implement this particular ordering to be web-compatible, it was specced as a requirement.

There was some discussion about breaking from this in Maps/Sets, but doing so would require us to specify an order that is impossible for code to depend on; in other words, we’d have to mandate that the ordering be random, not just unspecified. This was deemed too much effort, and creation-order is reasonable valuable (see OrderedDict in Python, for example), so it was decided to have Maps and Sets match Objects.

14.4.2.4 The order of properties in the spec

The following parts of the spec are relevant for this section:

14.5 Assigning versus defining properties

There are two similar ways of adding a property prop to an object obj:

There are three cases in which assigning does not create an own property prop, even if it doesn’t exist, yet:

  1. A read-only property prop exists in the prototype chain. Then the assignment causes a TypeError in strict mode.
  2. A setter for prop exists in the prototype chain. Then that setter is called.
  3. A getter for prop without a setter exists in the prototype chain. Then a TypeError is thrown in strict mode. This case is similar to the first one.

None of these cases prevent Object.defineProperty() from creating an own property. The next section looks at case #3 in more detail.

14.5.1 Overriding inherited read-only properties

If an object obj inherits a property prop that is read-only then you can’t assign to that property:

const proto = Object.defineProperty({}, 'prop', {
    writable: false,
    configurable: true,
    value: 123,
});
const obj = Object.create(proto);
obj.prop = 456;
    // TypeError: Cannot assign to read-only property

This is similar to how an inherited property works that has a getter, but no setter. It is in line with viewing assignment as changing the value of an inherited property. It does so non-destructively: the original is not modified, but overridden by a newly created own property. Therefore, an inherited read-only property and an inherited setter-less property both prevent changes via assignment. You can, however, force the creation of an own property by defining a property:

const proto = Object.defineProperty({}, 'prop', {
    writable: false,
    configurable: true,
    value: 123,
});
const obj = Object.create(proto);
Object.defineProperty(obj, 'prop', {value: 456});
console.log(obj.prop); // 456

14.6 __proto__ in ECMAScript 6

The property __proto__ (pronounced “dunder proto”) has existed for a while in most JavaScript engines. This section explains how it worked prior to ECMAScript 6 and what changes with ECMAScript 6.

For this section, it helps if you know what prototype chains are. Consult Sect. “Layer 2: The Prototype Relationship Between Objects” in “Speaking JavaScript”, if necessary.

14.6.1 __proto__ prior to ECMAScript 6

14.6.1.1 Prototypes

Each object in JavaScript starts a chain of one or more objects, a so-called prototype chain. Each object points to its successor, its prototype via the internal slot [[Prototype]] (which is null if there is no successor). That slot is called internal, because it only exists in the language specification and cannot be directly accessed from JavaScript. In ECMAScript 5, the standard way of getting the prototype p of an object obj is:

var p = Object.getPrototypeOf(obj);

There is no standard way to change the prototype of an existing object, but you can create a new object obj that has the given prototype p:

var obj = Object.create(p);
14.6.1.2 __proto__

A long time ago, Firefox got the non-standard property __proto__. Other browsers eventually copied that feature, due to its popularity.

Prior to ECMAScript 6, __proto__ worked in obscure ways:

14.6.1.3 Subclassing Array via __proto__

The main reason why __proto__ became popular was because it enabled the only way to create a subclass MyArray of Array in ES5: Array instances were exotic objects that couldn’t be created by ordinary constructors. Therefore, the following trick was used:

function MyArray() {
    var instance = new Array(); // exotic object
    instance.__proto__ = MyArray.prototype;
    return instance;
}
MyArray.prototype = Object.create(Array.prototype);
MyArray.prototype.customMethod = function (···) { ··· };

Subclassing in ES6 works differently than in ES5 and supports subclassing builtins out of the box.

14.6.1.4 Why __proto__ is problematic in ES5

The main problem is that __proto__ mixes two levels: the object level (normal properties, holding data) and the meta level.

If you accidentally use __proto__ as a normal property (object level!), to store data, you get into trouble, because the two levels clash. The situation is compounded by the fact that you have to abuse objects as maps in ES5, because it has no built-in data structure for that purpose. Maps should be able to hold arbitrary keys, but you can’t use the key '__proto__' with objects-as-maps.

In theory, one could fix the problem by using a symbol instead of the special name __proto__, but keeping meta-operations completely separate (as done via Object.getPrototypeOf()) is the best approach.

14.6.2 The two kinds of __proto__ in ECMAScript 6

Because __proto__ was so widely supported, it was decided that its behavior should be standardized for ECMAScript 6. However, due to its problematic nature, it was added as a deprecated feature. These features reside in Annex B in the ECMAScript specification, which is described as follows:

The ECMAScript language syntax and semantics defined in this annex are required when the ECMAScript host is a web browser. The content of this annex is normative but optional if the ECMAScript host is not a web browser.

JavaScript has several undesirable features that are required by a significant amount of code on the web. Therefore, web browsers must implement them, but other JavaScript engines don’t have to.

In order to explain the magic behind __proto__, two mechanisms were introduced in ES6:

14.6.2.1 Object.prototype.__proto__

ECMAScript 6 enables getting and setting the property __proto__ via a getter and a setter stored in Object.prototype. If you were to implement them manually, this is roughly what it would look like:

Object.defineProperty(Object.prototype, '__proto__', {
    get() {
        const _thisObj = Object(this);
        return Object.getPrototypeOf(_thisObj);
    },
    set(proto) {
        if (this === undefined || this === null) {
            throw new TypeError();
        }
        if (!isObject(this)) {
            return undefined;
        }
        if (!isObject(proto)) {
            return undefined;
        }
        const status = Reflect.setPrototypeOf(this, proto);
        if (! status) {
            throw new TypeError();
        }
    },
});
function isObject(value) {
    return Object(value) === value;
}

The getter and the setter for __proto__ in the ES6 spec:

14.6.2.2 The property key __proto__ as an operator in an object literal

If __proto__ appears as an unquoted or quoted property key in an object literal, the prototype of the object created by that literal is set to the property value:

> Object.getPrototypeOf({ __proto__: null })
null
> Object.getPrototypeOf({ '__proto__': null })
null

Using the string value '__proto__' as a computed property key does not change the prototype, it creates an own property:

> const obj = { ['__proto__']: null };
> Object.getPrototypeOf(obj) === Object.prototype
true
> Object.keys(obj)
[ '__proto__' ]

The special property key '__proto__' in the ES6 spec:

14.6.3 Avoiding the magic of __proto__

14.6.3.1 Defining (not assigning) __proto__

In ECMAScript 6, if you define the own property __proto__, no special functionality is triggered and the getter/setter Object.prototype.__proto__ is overridden:

const obj = {};
Object.defineProperty(obj, '__proto__', { value: 123 })

Object.keys(obj); // [ '__proto__' ]
console.log(obj.__proto__); // 123
14.6.3.2 Objects that don’t have Object.prototype as a prototype

The __proto__ getter/setter is provided via Object.prototype. Therefore, an object without Object.prototype in its prototype chain doesn’t have the getter/setter, either. In the following code, dict is an example of such an object – it does not have a prototype. As a result, __proto__ now works like any other property:

> const dict = Object.create(null);
> '__proto__' in dict
false
> dict.__proto__ = 'abc';
> dict.__proto__
'abc'
14.6.3.3 __proto__ and dict objects

If you want to use an object as a dictionary then it is best if it doesn’t have a prototype. That’s why prototype-less objects are also called dict objects. In ES6, you don’t even have to escape the property key '__proto__' for dict objects, because it doesn’t trigger any special functionality.

__proto__ as an operator in an object literal lets you create dict objects more concisely:

const dictObj = {
    __proto__: null,
    yes: true,
    no: false,
};

Note that in ES6, you should normally prefer the built-in data structure Map to dict objects, especially if keys are not fixed.

14.6.3.4 __proto__ and JSON

Prior to ES6, the following could happen in a JavaScript engine:

> JSON.parse('{"__proto__": []}') instanceof Array
true

With __proto__ being a getter/setter in ES6, JSON.parse() works fine, because it defines properties, it doesn’t assign them (if implemented properly, an older version of V8 did assign).

JSON.stringify() isn’t affected by __proto__, either, because it only considers own properties. Objects that have an own property whose name is __proto__ work fine:

> JSON.stringify({['__proto__']: true})
'{"__proto__":true}'

14.6.4 Detecting support for ES6-style __proto__

Support for ES6-style __proto__ varies from engine to engine. Consult kangax’ ECMAScript 6 compatibility table for information on the status quo:

The following two sections describe how you can programmatically detect whether an engine supports either of the two kinds of __proto__.

14.6.4.1 Feature: __proto__ as getter/setter

A simple check for the getter/setter:

var supported = {}.hasOwnProperty.call(Object.prototype, '__proto__');

A more sophisticated check:

var desc = Object.getOwnPropertyDescriptor(Object.prototype, '__proto__');
var supported = (
    typeof desc.get === 'function' && typeof desc.set === 'function'
);
14.6.4.2 Feature: __proto__ as an operator in an object literal

You can use the following check:

var supported = Object.getPrototypeOf({__proto__: null}) === null;

14.6.5 __proto__ is pronounced “dunder proto”

Bracketing names with double underscores is a common practice in Python to avoid name clashes between meta-data (such as __proto__) and data (user-defined properties). That practice will never become common in JavaScript, because it now has symbols for this purpose. However, we can look to the Python community for ideas on how to pronounce double underscores.

The following pronunciation has been suggested by Ned Batchelder:

An awkward thing about programming in Python: there are lots of double underscores. For example, the standard method names beneath the syntactic sugar have names like __getattr__, constructors are __init__, built-in operators can be overloaded with __add__, and so on. […]

My problem with the double underscore is that it’s hard to say. How do you pronounce __init__? “underscore underscore init underscore underscore”? “under under init under under”? Just plain “init” seems to leave out something important.

I have a solution: double underscore should be pronounced “dunder”. So __init__ is “dunder init dunder”, or just “dunder init”.

Thus, __proto__ is pronounced “dunder proto”. The chances for this pronunciation catching on are good, JavaScript creator Brendan Eich uses it.

14.6.6 Recommendations for __proto__

It is nice how well ES6 turns __proto__ from something obscure into something that is easy to understand.

However, I still recommend not to use it. It is effectively a deprecated feature and not part of the core standard. You can’t rely on it being there for code that must run on all engines.

More recommendations:

14.7 Enumerability in ECMAScript 6

Enumerability is an attribute of object properties. This section explains how it works in ECMAScript 6. Let’s first explore what attributes are.

14.7.1 Property attributes

Each object has zero or more properties. Each property has a key and three or more attributes, named slots that store the data of the property (in other words, a property is itself much like a JavaScript object or like a record with fields in a database).

ECMAScript 6 supports the following attributes (as does ES5):

You can retrieve the attributes of a property via Object.getOwnPropertyDescriptor(), which returns the attributes as a JavaScript object:

> const obj = { foo: 123 };
> Object.getOwnPropertyDescriptor(obj, 'foo')
{ value: 123,
  writable: true,
  enumerable: true,
  configurable: true }

This section explains how the attribute enumerable works in ES6. All other attributes and how to change attributes is explained in Sect. “Property Attributes and Property Descriptors” in “Speaking JavaScript”.

14.7.2 Constructs affected by enumerability

ECMAScript 5:

ECMAScript 6:

for-in is the only built-in operations where enumerability matters for inherited properties. All other operations only work with own properties.

14.7.3 Use cases for enumerability

Unfortunately, enumerability is quite an idiosyncratic feature. This section presents several use cases for it and argues that, apart from protecting legacy code from breaking, its usefulness is limited.

14.7.3.1 Use case: Hiding properties from the for-in loop

The for-in loop traverses all enumerable properties of an object, own and inherited ones. Therefore, the attribute enumerable is used to hide properties that should not be traversed. That was the reason for introducing enumerability in ECMAScript 1.

14.7.3.1.1 Non-enumerability in the language

Non-enumerable properties occur in the following locations in the language:

The main reason for making all of these properties non-enumerable is to hide them (especially the inherited ones) from legacy code that uses the for-in loop or $.extend() (and similar operations that copy both inherited and own properties; see next section). Both operations should be avoided in ES6. Hiding them ensures that the legacy code doesn’t break.

14.7.3.2 Use case: Marking properties as not to be copied
14.7.3.2.1 Historical precedents

When it comes to copying properties, there are two important historical precedents that take enumerability into consideration:

Problems with this way of copying properties:

The only instance property that is non-enumerable in the standard library is property length of Arrays. However, that property only needs to be hidden due to it magically updating itself via other properties. You can’t create that kind of magic property for your own objects (short of using a Proxy).

14.7.3.2.2 ES6: Object.assign()

In ES6, Object.assign(target, source_1, source_2, ···) can be used to merge the sources into the target. All own enumerable properties of the sources are considered (that is, keys can be either strings or symbols). Object.assign() uses a “get” operation to read a value from a source and a “set” operation to write a value to the target.

With regard to enumerability, Object.assign() continues the tradition of Object.extend() and $.extend(). Quoting Yehuda Katz:

Object.assign would pave the cowpath of all of the extend() APIs already in circulation. We thought the precedent of not copying enumerable methods in those cases was enough reason for Object.assign to have this behavior.

In other words: Object.assign() was created with an upgrade path from $.extend() (and similar) in mind. Its approach is cleaner than $.extend’s, because it ignores inherited properties.

14.7.3.3 Marking properties as private

If you make a property non-enumerable, it can’t by seen by Object.keys() and the for-in loop, anymore. With regard to those mechanisms, the property is private.

However, there are several problems with this approach:

14.7.3.4 Hiding own properties from JSON.stringify()

JSON.stringify() does not include properties in its output that are non-enumerable. You can therefore use enumerability to determine which own properties should be exported to JSON. This use case is similar to marking properties as private, the previous use case. But it is also different, because this is more about exporting and slightly different considerations apply. For example: Can an object be completely reconstructed from JSON?

An alternative for specifying how an object should be converted to JSON is to use toJSON():

const obj = {
    foo: 123,
    toJSON() {
        return { bar: 456 };
    },
};
JSON.stringify(obj); // '{"bar":456}'

I find toJSON() cleaner than enumerability for the current use case. It also gives you more control, because you can export properties that don’t exist on the object.

14.7.4 Naming inconsistencies

In general, a shorter name means that only enumerable properties are considered:

However, Reflect.ownKeys() deviates from that rule, it ignores enumerability and returns the keys of all properties. Additionally, starting with ES6, the following distinction is made:

Therefore, a better name for Object.keys() would now be Object.names().

14.7.5 Looking ahead

It seems to me that enumerability is only suited for hiding properties from the for-in loop and $.extend() (and similar operations). Both are legacy features, you should avoid them in new code. As for the other use cases:

I’m not sure what the best strategy is for enumerability going forward. If, with ES6, we had started to pretend that it didn’t exist (except for making prototype properties non-enumerable so that old code doesn’t break), we might eventually have been able to deprecate enumerability. However, Object.assign() considering enumerability runs counter that strategy (but it does so for a valid reason, backward compatibility).

In my own ES6 code, I’m not using enumerability, except (implicitly) for classes whose prototype methods are non-enumerable.

Lastly, when using an interactive command line, I occasionally miss an operation that returns all property keys of an object, not just the own ones (Reflect.ownKeys). Such an operation would provide a nice overview of the contents of an object.

14.8 Customizing basic language operations via well-known symbols

This section explains how you can customize basic language operations by using the following well-known symbols as property keys:

14.8.1 Property key Symbol.hasInstance (method)

An object C can customize the behavior of the instanceof operator via a method with the key Symbol.hasInstance that has the following signature:

[Symbol.hasInstance](potentialInstance : any)

x instanceof C works as follows in ES6:

14.8.1.1 Uses in the standard library

The only method in the standard library that has this key is:

This is the implementation of instanceof that all functions (including classes) use by default. Quoting the spec:

This property is non-writable and non-configurable to prevent tampering that could be used to globally expose the target function of a bound function.

The tampering is possible because the traditional instanceof algorithm, OrdinaryHasInstance(), applies instanceof to the target function if it encounters a bound function.

Given that this property is read-only, you can’t use assignment to override it, as mentioned earlier.

14.8.1.2 Example: checking whether a value is an object

As an example, let’s implement an object ReferenceType whose “instances” are all objects, not just objects that are instances of Object (and therefore have Object.prototype in their prototype chains).

const ReferenceType = {
    [Symbol.hasInstance](value) {
        return (value !== null
            && (typeof value === 'object'
                || typeof value === 'function'));
    }
};
const obj1 = {};
console.log(obj1 instanceof Object); // true
console.log(obj1 instanceof ReferenceType); // true

const obj2 = Object.create(null);
console.log(obj2 instanceof Object); // false
console.log(obj2 instanceof ReferenceType); // true

14.8.2 Property key Symbol.toPrimitive (method)

Symbol.toPrimitive lets an object customize how it is coerced (converted automatically) to a primitive value.

Many JavaScript operations coerce values to the types that they need.

The following are the most common types that values are coerced to:

Thus, for numbers and strings, the first step is to ensure that a value is any kind of primitive. That is handled by the spec-internal operation ToPrimitive(), which has three modes:

The default mode is only used by:

If the value is a primitive then ToPrimitive() is already done. Otherwise, the value is an object obj, which is converted to a primitive as follows:

This normal algorithm can be overridden by giving an object a method with the following signature:

[Symbol.toPrimitive](hint : 'default' | 'string' | 'number')

In the standard library, there are two such methods:

14.8.2.1 Example

The following code demonstrates how coercion affects the object obj.

const obj = {
    [Symbol.toPrimitive](hint) {
        switch (hint) {
            case 'number':
                return 123;
            case 'string':
                return 'str';
            case 'default':
                return 'default';
            default:
                throw new Error();
        }
    }
};
console.log(2 * obj); // 246
console.log(3 + obj); // '3default'
console.log(obj == 'default'); // true
console.log(String(obj)); // 'str'

14.8.3 Property key Symbol.toStringTag (string)

In ES5 and earlier, each object had the internal own property [[Class]] whose value hinted at its type. You could not access it directly, but its value was part of the string returned by Object.prototype.toString(), which is why that method was used for type checks, as an alternative to typeof.

In ES6, there is no internal slot [[Class]], anymore, and using Object.prototype.toString() for type checks is discouraged. In order to ensure the backwards-compatibility of that method, the public property with the key Symbol.toStringTag was introduced. You could say that it replaces [[Class]].

Object.prototype.toString() now works as follows:

14.8.3.1 Default toString tags

The default values for various kinds of objects are shown in the following table.

Value toString tag
undefined 'Undefined'
null 'Null'
An Array object 'Array'
A string object 'String'
arguments 'Arguments'
Something callable 'Function'
An error object 'Error'
A boolean object 'Boolean'
A number object 'Number'
A date object 'Date'
A regular expression object 'RegExp'
(Otherwise) 'Object'

Most of the checks in the left column are performed by looking at internal slots. For example, if an object has the internal slot [[Call]], it is callable.

The following interaction demonstrates the default toString tags.

> Object.prototype.toString.call(null)
'[object Null]'
> Object.prototype.toString.call([])
'[object Array]'
> Object.prototype.toString.call({})
'[object Object]'
> Object.prototype.toString.call(Object.create(null))
'[object Object]'
14.8.3.2 Overriding the default toString tag

If an object has an (own or inherited) property whose key is Symbol.toStringTag then its value overrides the default toString tag. For example:

> ({}.toString())
'[object Object]'
> ({[Symbol.toStringTag]: 'Foo'}.toString())
'[object Foo]'

Instances of user-defined classes get the default toString tag (of objects):

class Foo { }
console.log(new Foo().toString()); // [object Object]

One option for overriding the default is via a getter:

class Bar {
    get [Symbol.toStringTag]() {
      return 'Bar';
    }
}
console.log(new Bar().toString()); // [object Bar]

In the JavaScript standard library, there are the following custom toString tags. Objects that have no global names are quoted with percent symbols (for example: %TypedArray%).

All of the built-in properties whose keys are Symbol.toStringTag have the following property descriptor:

{
    writable: false,
    enumerable: false,
    configurable: true,
}

As mentioned earlier, you can’t use assignment to override those properties, because they are read-only.

14.8.4 Property key Symbol.unscopables (Object)

Symbol.unscopables lets an object hide some properties from the with statement.

The reason for doing so is that it allows TC39 to add new methods to Array.prototype without breaking old code. Note that current code rarely uses with, which is forbidden in strict mode and therefore ES6 modules (which are implicitly in strict mode).

Why would adding methods to Array.prototype break code that uses with (such as the widely deployed Ext JS 4.2.1)? Take a look at the following code. The existence of a property Array.prototype.values breaks foo(), if you call it with an Array:

function foo(values) {
    with (values) {
        console.log(values.length); // abc (*)
    }
}
Array.prototype.values = { length: 'abc' };
foo([]);

Inside the with statement, all properties of values become local variables, shadowing even values itself. Therefore, if values has a property values then the statement in line * logs values.values.length and not values.length.

Symbol.unscopables is used only once in the standard library:

14.9 FAQ: object literals

14.9.1 Can I use super in object literals?

Yes you can! Details are explained in the chapter on classes.

Next: 15. Classes