Object.prototype.__proto__

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弃用
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 兼容性表格 at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.

警告： 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 simply 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]] 使用 Object.create() .

警告： 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() 代替。

__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).

__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: 对象初始化器/文字句法 .

描述

__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 , 布尔 , 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.)

__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 继承和原型链 .

__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 .

范例

Using proto

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 () {
        console.log('aaa');
    }
};
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 () {
        console.log('a');
    }
};
var circle = new shape();
circle.__proto__ = p;
circle.a(); // a
console.log(shape.prototype === circle.__proto__); // false
// or
function test() {};
test.prototype.myname = function () {
    console.log('myname');
};
var a = new test();
console.log(a.__proto__ === test.prototype); // true
a.myname(); // myname
// or
var fn = function () {};
fn.prototype.myname = function () {
    console.log('myname');
};
var obj = {
    __proto__: fn.prototype
};
obj.myname(); // myname

规范

规范
ECMAScript (ECMA-262) The definition of 'Object.prototype.__proto__' in that specification.

浏览器兼容性

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. 更新 GitHub 上的兼容性数据

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`__proto__` 弃用	Chrome 1	Edge 12	Firefox 1	IE 11	Opera 10.5	Safari 3	WebView Android 1	Chrome Android 18	Firefox Android 4	Opera Android 11	Safari iOS 1	Samsung Internet Android 1.0	nodejs Yes

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Using proto

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Using __proto__

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Using proto