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().

Return value

A new object with the specified prototype object and properties.

Exceptions

A TypeError exception if the propertiesObject 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);
}

// 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 copies properties from the OtherSuperClass prototype to the MyClass prototype, making them available to all instances of MyClass. Object.assign was introduced with ES2015 and can be polyfilled. If support for older browsers is necessary, jQuery.extend() or _.assign() can be used.

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 (proto, propertiesObject) {
        if (typeof proto !== 'object' && typeof proto !== 'function') {
            throw new TypeError('Object prototype may only be an Object: ' + proto);
        } else if (proto === null) {
            throw new Error("This browser's implementation of Object.create is a shim and doesn't support 'null' as the first argument.");
        }

        if (typeof propertiesObject != 'undefined') {
            throw new Error("This browser's implementation of Object.create is a shim and doesn't support a second argument.");
        }
        
        function F() {}
        F.prototype = proto;
        
        return new F();
    };
}

Specifications

Browser compatibility

See also

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