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Object.prototype.__proto__

Warning: Changing the [[Prototype]] of an object is, by the nature of how modern JavaScript engines optimize property accesses, a very slow operation, in every browser and JavaScript engine. The effects on performance of altering inheritance are subtle and far-flung, and are not limited to simply the time spent in obj.__proto__ = ... statement, but may extend to any code that has access to any object whose [[Prototype]] has been altered. If you care about performance you should avoid setting the [[Prototype]] of an object. Instead, create a new object with the desired [[Prototype]] using Object.create().

Warning: While Object.prototype.__proto__ is supported today in most browsers, its existence and exact behavior has only been standardized in the ECMAScript 6 specification as a legacy feature to ensure compatibility for web browsers. For better support, it is recommended that only Object.getPrototypeOf() be used instead.

The __proto__ property of Object.prototype is an accessor property (a getter function and a setter function) that exposes the internal [[Prototype]] (either an object or null) of the object through which it is accessed.

The use of __proto__ is controversial, and has been discouraged. It was never originally included in the EcmaScript language spec, but modern browsers decided to implement it anyway. Only recently, the __proto__ property has been standardized in the ECMAScript 6 language specification for web browsers to ensure compatibility, so will be supported into the future. It is deprecated in favor of Object.getPrototypeOf/Reflect.getPrototypeOf and Object.setPrototypeOf/Reflect.setPrototypeOf (though still, setting the [[Prototype]] of an object is a slow operation that should be avoided if performance is a concern).

The __proto__ property can also be used in an object literal definition to set the object [[Prototype]] on creation, as an alternative to Object.create(). See: object initializer / literal syntax.

Syntax

var shape = {}, circle = new Circle();

// Set the object prototype
shape.__proto__ = circle;
// Get the object prototype
console.log(shape.__proto__ === circle); // true

Note: that is two underscores, followed by the five characters "proto", followed by two more underscores.

Description

The __proto__ getter function exposes the value of the internal [[Prototype]] of an object. For objects created using an object literal, this value is Object.prototype. For objects created using array literals, this value is Array.prototype. For functions, this value is Function.prototype. For objects created using new fun, where fun is one of the built-in constructor functions provided by JavaScript (Array, Boolean, Date, Number, Object, String, and so on — including new constructors added as JavaScript evolves), this value is always fun.prototype. For objects created using new fun, where fun is a function defined in a script, this value is the value of fun.prototype. (That is, if the constructor didn't return an other object explicitly, or the fun.prototype has been reassigned since the instance was created).

The __proto__ setter allows the [[Prototype]] of an object to be mutated. The object must be extensible according to Object.isExtensible(): if it is not, a TypeError is thrown. The value provided must be an object or null. Providing any other value will do nothing.

To understand how prototypes are used for inheritance, see guide article Inheritance and the prototype chain.

The __proto__ property is a simple accessor property on Object.prototype consisting of a getter and setter function. A property access for __proto__ that eventually consults Object.prototype will find this property, but an access that does not consult Object.prototype will not find it. If some other __proto__ property is found before Object.prototype is consulted, that property will hide the one found on Object.prototype.

var noProto = Object.create(null);

console.log(typeof noProto.__proto__); // undefined
console.log(Object.getPrototypeOf(noProto)); // null

noProto.__proto__ = 17;

console.log(noProto.__proto__); // 17
console.log(Object.getPrototypeOf(noProto)); // null

var protoHidden = {};
Object.defineProperty(protoHidden, "__proto__",
                      { value: 42, writable: true, configurable: true, enumerable: true });

console.log(protoHidden.__proto__); // 42
console.log(Object.getPrototypeOf(protoHidden) === Object.prototype); // true

Examples

In the following, a new instance of Employee is created, then tested to show that its __proto__ is the same object as its constructor's prototype.

// Declare a function to be used as a constructor
function Employee() {
  /* initialise instance */
}

// Create a new instance of Employee
var fred = new Employee();

// Test equivalence
fred.__proto__ === Employee.prototype; // true

At this point, fred inherits from Employee, however assigning a different object to fred.__proto__ can change that:

function Cow() {
  /* initialise instance */
}

// Assign a new object to __proto__
fred.__proto__ = Cow.prototype;

Now fred inherits directly from Cow.prototype instead of Employee.prototype, and loses the properties it originally inherited from Employee.prototype.

However, this only applies to extensible objects, a non–extensible object's __proto__ property cannot be changed:

var obj = {};
Object.preventExtensions(obj);

obj.__proto__ = {}; // throws a TypeError

Note that even Object.prototype's __proto__ property can be redefined as long as the chain leads to null:

var b = {};

Object.prototype.__proto__ =
  Object.create(null, // [[Prototype]]
                { hi: { value: function() { console.log('hi'); } } });

b.hi();

If Object.prototype's __proto__ had not been set to null, or had not been set to another object whose prototype chain did not eventually lead explicitly to null, a "cyclic __proto__ value" TypeError would result since the chain must eventually lead to null (as it normally does on Object.prototype).

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Object.prototype.__proto__' in that specification.
Standard Included in the (normative) annex for additional ECMAScript legacy features for Web browsers (note that the specification codifies what is already in implementations).

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes) (Yes) 11 (Yes) (Yes)
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)

Compatibility notes

While the ECMAScript 2015 (ES6) specification dictates that support for __proto__ is required only for web browsers (although being normative), other environments may support it as well for legacy usage.

See also