Chapter 6 - Objects

Objects

JavaScript's fundamental datatype.

6.1 Creating Objects

Literals

var obj = {};

New

var obj = new Object();

Object.create

//obj1 inherits properties x and y
var obj1 = Object.create({x:1, y:2});

//obj2 inherits no properties or methods
var obj2 = Object.create(null);

//same as new Object or {}
var obj3 = Object.create(Object.prototype);

Prototype If an Object has a prototype, the prototype is also an Object. These Objects all chain together until you reach null, which is the end of the prototype chain.

// Object is a constructor
Object.prototype; // Object {}, will be the prototype of `new Object`s
// Object.prototype is an Object
Object.getPrototypeOf(Object.prototype); // null, we are at the end of the chain

You should also note, however, that you can't keep accessing the obj.prototype property, as this is only applicable to Constructors, consider

function Foo() {
}
Foo.prototype; // Foo {}
// vs
(new Foo).prototype; // undefined

The correct way to find the prototype of an Object is by using Object.getPrototypeOf(obj),

Object.getPrototypeOf(new Foo) === Foo.prototype; // true

It may also be of note that legacy browsers may not support Object.getPrototypeOf, in which case many offer the property obj.__proto__. However, try to avoid using __proto__ outside of a shim for such browsers if you need to access the prototype chain.

Finally, using new with a Constructor isn't the only way to create this chain, you can set them up using Object.create

var a = Object.create(null),
    b = Object.create(a), // b will inherit from a
    c = Object.create(b); // c will inherit from b, hence also a
a.foo = 'foo';
b.bar = 'bar';

a instanceof Object; // false

a.bar; // undefined
c.foo + c.bar === 'foobar'; // true

Also consider

c.prototype; // undefined
// vs
Object.getPrototypeOf(c) === b; // true