Object.prototype.__proto__
Deprecated: This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.
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 the performance of altering inheritance are subtle and far-flung, and are not limited to the time spent in obj.__proto__ = ...
statements, 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 2015 specification as a legacy feature to ensure compatibility for web browsers. For better support, use Object.getPrototypeOf()
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 discouraged. It was never originally included in the ECMAScript language spec, but modern browsers implemented it anyway. Only recently was the __proto__
property standardized by the ECMAScript 2015 specification for compatibility with web browsers, so it 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.
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. If some other __proto__
property is found before Object.prototype
is consulted, that property will hide the one found on Object.prototype
.
Examples
Using __proto__
function Circle() {}
const shape = {};
const circle = new Circle();
// Set the object prototype.
// DEPRECATED. This is for example purposes only. DO NOT DO THIS in real code.
shape.__proto__ = circle;
// Get the object prototype
console.log(shape.__proto__ === Circle); // false
const ShapeA = function () {};
const ShapeB = {
a() {
console.log('aaa');
}
};
console.log(ShapeA.prototype.__proto__ = ShapeB);
const shapea = new ShapeA();
shapea.a(); // aaa
console.log(ShapeA.prototype === shapea.__proto__); // true
// or
const ShapeC = function () {};
const ShapeD = {
a() {
console.log('a');
}
};
const shapeC = new ShapeC();
shapeC.__proto__ = ShapeD;
shapeC.a(); // a
console.log(ShapeC.prototype === shapeC.__proto__); // false
// or
function Test() {}
Test.prototype.myname = function () {
console.log('myname');
};
const a = new Test();
console.log(a.__proto__ === Test.prototype); // true
a.myname(); // myname
// or
const fn = function () {};
fn.prototype.myname = function () {
console.log('myname');
};
var obj = {
__proto__: fn.prototype
};
obj.myname(); // myname
Specifications
Specification |
---|
ECMAScript Language Specification # sec-additional-properties-of-the-object.prototype-object |
Browser compatibility
BCD tables only load in the browser