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Object.create()

The Object.create() method creates a new object with the specified prototype object and properties.

Syntax

Object.create(proto[, propertiesObject])

Parameters

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.

Examples

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 depends on Object.prototype.hasOwnProperty working correctly.

if (typeof Object.create != 'function') {
  // Production steps of ECMA-262, Edition 5, 15.2.3.5
  // Reference: http://es5.github.io/#x15.2.3.5
  Object.create = (function() {
    // To save on memory, use a shared constructor
    function Temp() {}

    // make a safe reference to Object.prototype.hasOwnProperty
    var hasOwn = Object.prototype.hasOwnProperty;

    return function (O) {
      // 1. If Type(O) is not Object or Null throw a TypeError exception.
      if (typeof O != 'object') {
        throw TypeError('Object prototype may only be an Object or null');
      }

      // 2. Let obj be the result of creating a new object as if by the
      //    expression new Object() where Object is the standard built-in
      //    constructor with that name
      // 3. Set the [[Prototype]] internal property of obj to O.
      Temp.prototype = O;
      var obj = new Temp();
      Temp.prototype = null; // Let's not keep a stray reference to O...

      // 4. If the argument Properties is present and not undefined, add
      //    own properties to obj as if by calling the standard built-in
      //    function Object.defineProperties with arguments obj and
      //    Properties.
      if (arguments.length > 1) {
        // Object.defineProperties does ToObject on its first argument.
        var Properties = Object(arguments[1]);
        for (var prop in Properties) {
          if (hasOwn.call(Properties, prop)) {
            obj[prop] = Properties[prop];
          }
        }
      }

      // 5. Return obj
      return obj;
    };
  })();
}

Specifications

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  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 5 4.0 (2) 9 11.60 5
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) 4.0 (2) (Yes) 11.5 (Yes)

See also