Object.create()

Object.create() 方法能以指定的原型物件以及屬性來建立新的物件。

語法

Object.create(proto[, propertiesObject])

參數

proto

The object which should be the prototype of the newly-created object.

propertiesObject

Optional. If specified and not undefined, an object whose enumerable own properties (that is, those properties defined upon itself and not enumerable properties along its prototype chain) specify property descriptors to be added to the newly-created object, with the corresponding property names. These properties correspond to the second argument of Object.defineProperties().

Throws

Throws a TypeError exception if the proto parameter isn't null or an object.

範例

Classical inheritance with Object.create()

Below is an example of how to use Object.create() to achieve classical inheritance. This is for single inheritance, which is all that JavaScript supports.

// 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.'

If you wish to inherit from multiple objects, then mixins are a possibility.

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

MyClass.prototype = Object.create(SuperClass.prototype); // inherit
mixin(MyClass.prototype, OtherSuperClass.prototype); // mixin

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

The mixin function would copy the functions from the superclass prototype to the subclass prototype, the mixin function needs to be supplied by the user. An example of a mixin like function would be jQuery.extend().

Using propertiesObject argument with Object.create()

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

This polyfill covers the main use case which is creating a new object for which the prototype has been chosen but doesn't take the second argument into account.

Note that while the setting of null as [[Prototype]] is supported in the real ES5 Object.create, this polyfill cannot support it due to a limitation inherent in versions of ECMAScript lower than 5.

if (typeof Object.create != 'function') {
  Object.create = (function(undefined) {
    var Temp = function() {};
    return function (prototype, propertiesObject) {
      if(prototype !== Object(prototype) && prototype !== null) {
        throw TypeError('Argument must be an object, or null');
      }
      Temp.prototype = prototype || {};
      if (propertiesObject !== undefined) {
        Object.defineProperties(Temp.prototype, propertiesObject);
      } 
      var result = new Temp(); 
      Temp.prototype = null;
      // to imitate the case of Object.create(null)
      if(prototype === null) {
         result.__proto__ = null;
      } 
      return result;
    };
  })();
}

規範

瀏覽器相容性

參見

• Object.defineProperty()
• Object.defineProperties()
• Object.prototype.isPrototypeOf()
• John Resig's post on getPrototypeOf()