Warning: 基於現代Javascript引擎最佳化物件屬性存取的方法,改變一個物件的 [[Prototype]] 在任何瀏覽器或是Javascript引擎都是非常慢的操作?。改變繼承屬性對效能的影響微妙且深遠,不僅僅只是影響執行 obj.__proto__ = ... 的時間,而是會影響到所有有存取到被改變 [[Prototype]] 的物件的程式碼的執行時間。如果你在乎效能的話就應該避免改變一個物件的 [[Prototype]] 。反之,請用 Object.create() 來產生一個擁有 [[Prototype]] 的物件。

Warning: 雖然 Object.prototype.__proto__ 在今日已經被絕大部分的瀏覽器所支援,其存在與確切的行為只有在 ECMAScript 2015 規範才被標準化成一個歷史功能來確保相容性。為了更好的支援,建議使用Object.getPrototypeOf()

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 2015 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 (en-US) and Object.setPrototypeOf (en-US)/Reflect.setPrototypeOf (en-US) (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.


var Circle = function () {};
var shape = {};
var 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); // true
var shape = function () {};
var p = {
    a: function () {
shape.prototype.__proto__ = p;

var circle = new shape();
circle.a(); // aaa
console.log(shape.prototype === circle.__proto__); // true

// or
var shape = function () {};
var p = {
    a: function () {

var circle = new shape();
circle.__proto__ = p;
circle.a(); // a
console.log(shape.prototype === circle.__proto__); // false

// or
function test() {};
test.prototype.myname = function () {

var a = new test();
console.log(a.__proto__ === test.prototype); // true
a.myname(); // myname

// or
var fn = function () {};
fn.prototype.myname = function () {

var obj = {
    __proto__: fn.prototype

obj.myname(); // myname

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


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() (en-US): if it is not, a TypeError (en-US) 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.


ECMAScript Language Specification
# sec-additional-properties-of-the-object.prototype-object

Browser compatibility

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Compatibility notes

While the ECMAScript 2015 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