Object.create

Object.create() 方法会使用指定的原型对象及其属性去创建一个新的对象。

语法

Object.create(proto[, propertiesObject])

参数

proto新创建对象的原型对象。

propertiesObject可选。如果没有指定为 undefined，则是要添加到新创建对象的可枚举属性（即其自身定义的属性，而不是其原型链上的枚举属性）对象的属性描述符以及相应的属性名称。这些属性对应Object.defineProperties()的第二个参数。

返回值

在指定原型对象上添加新属性后的对象。

异常

如果proto参数不是 null或一个对象，则抛出一个TypeError异常。

例子

用 Object.create实现类式继承

下面的例子演示了如何使用Object.create()来实现类式继承。这是一个所有版本JavaScript都支持的单继承。

// Shape - superclass
function Shape() {
  this.x = 0;
  this.y = 0;
}

// superclass method
Shape.prototype.move = function(x, y) {
  this.x += x;
  this.y += y;
  console.info('Shape moved.');
};

// Rectangle - subclass
function Rectangle() {
  Shape.call(this); // call super constructor.
}

// subclass extends superclass
Rectangle.prototype = Object.create(Shape.prototype);
Rectangle.prototype.constructor = Rectangle;

var rect = new Rectangle();

console.log('Is rect an instance of Rectangle?',
  rect instanceof Rectangle); // true
console.log('Is rect an instance of Shape?',
  rect instanceof Shape); // true
rect.move(1, 1); // Outputs, 'Shape moved.'

如果你希望能继承到多个对象，则可以使用混入的方式。

function MyClass() {
  SuperClass.call(this);
  OtherSuperClass.call(this);
}

// inherit one class
MyClass.prototype = Object.create(SuperClass.prototype);
// mixin another
Object.assign(MyClass.prototype, OtherSuperClass.prototype);
// re-assign constructor
MyClass.prototype.constructor = MyClass;

MyClass.prototype.myMethod = function() {
  // do a thing
};

Object.assign 会把  OtherSuperClass原型上的函数拷贝到MyClass原型上，使 MyClass 的所有实例都可用 OtherSuperClass 的方法。Object.assign 是在 ES2015 引入的，且可用 polyfilled。要支持旧浏览器的话，可用使用 jQuery.extend() 或者 _.assign()。

使用 Object.create 的 propertyObject参数

var o;

// create an object with null as prototype
o = Object.create(null);


o = {};
// is equivalent to:
o = Object.create(Object.prototype);


// Example where we create an object with a couple of
// sample properties. (Note that the second parameter
// maps keys to *property descriptors*.)
o = Object.create(Object.prototype, {
  // foo is a regular 'value property'
  foo: {
    writable: true,
    configurable: true,
    value: 'hello'
  },
  // bar is a getter-and-setter (accessor) property
  bar: {
    configurable: false,
    get: function() { return 10; },
    set: function(value) {
      console.log('Setting `o.bar` to', value);
    }
/* with ES5 Accessors our code can look like this
    get function() { return 10; },
    set function(value) {
      console.log('Setting `o.bar` to', value);
    } */
  }
});


function Constructor() {}
o = new Constructor();
// is equivalent to:
o = Object.create(Constructor.prototype);
// Of course, if there is actual initialization code
// in the Constructor function, 
// the Object.create() cannot reflect it


// Create a new object whose prototype is a new, empty
// object and add a single property 'p', with value 42.
o = Object.create({}, { p: { value: 42 } });

// by default properties ARE NOT writable,
// enumerable or configurable:
o.p = 24;
o.p;
// 42

o.q = 12;
for (var prop in o) {
  console.log(prop);
}
// 'q'

delete o.p;
// false

// to specify an ES3 property
o2 = Object.create({}, {
  p: {
    value: 42,
    writable: true,
    enumerable: true,
    configurable: true
  }
});

Polyfill

这个 polyfill 涵盖了主要的应用场景，它创建一个已经选择了原型的新对象，但没有把第二个参数考虑在内。

请注意，尽管在 ES5 中 Object.create支持设置为[[Prototype]]为null，但因为那些ECMAScript5以前版本限制，此 polyfill 无法支持该特性。

 if (typeof Object.create !== "function") {
    Object.create = function (proto, propertiesObject) {
        if (!(proto === null || typeof proto === "object" || typeof proto === "function")) {
            throw TypeError('Argument must be an object, or null');
        }
        var temp = new Object();
        temp.__proto__ = proto;
        if(typeof propertiesObject ==="object")
            Object.defineProperties(temp,propertiesObject);
        return temp;
    };
}

规范

Specification	Status	Comment
ECMAScript 5.1 (ECMA-262)The definition of 'Object.create' in that specification.	Standard	Initial definition. Implemented in JavaScript 1.8.5.
ECMAScript 2015 (6th Edition, ECMA-262)The definition of 'Object.create' in that specification.	Standard	​
ECMAScript Latest Draft (ECMA-262)The definition of 'Object.create' in that specification.	Living Standard	​

浏览器兼容

Feature	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari
Basic Support	5	(Yes)	4	9	11.6	5

Feature	Android	Chrome for Android	Edge mobile	Firefox for Android	IE mobile	Opera Android	iOS Safari
Basic Support	(Yes)	(Yes)	(Yes)	4	(Yes)	11.5	(Yes)