An object 'a' having a prototype of 'b' means that if a property is not found on 'a', it will then look it up on 'b'. It's just chaining. To set an object's pro

An object 'a' having a prototype of 'b' means that if a property is not found on 'a', it will then look it up on 'b'. It's just chaining. To set an object's prototype, you set the special __proto__ property on it. var a = { foo: 5 }; a.__proto__ = { foo: 10, bar: 2 }; console.log(a.foo, a.bar); // 5, 2 You can also use Object.create() to initialize an object with a given prototype. So the above is equivalent to: var a = Object.create({ foo: 10, bar: 2 }); a.foo = 5; console.log(a.foo, a.bar); // 5, 2 The real confusing part is the property called "prototype". This doesn't exist on random objects, it's only available on functions, and it sets the __proto__ of the object created with that function when used as a constructor (that is, when using the "new" operator). function Foo() {} Foo.prototype = { foo: 10, bar: 2 }; var a = new Foo(); a.foo = 5; console.log(a.foo, a.bar); // 5, 2 The "new" operator also sets a.constructor = Foo, and calls the constructor method. You can think of the new operator as syntax sugar performing the below: function Foo() {} Foo.prototype = { foo: 10, bar: 2 }; // The four lines below are equivalent to `var a = new Foo();` var a = {}; a.constructor = Foo; a.__proto__ = Foo.prototype; Foo.call(a); a.foo = 5; console.log(a.foo, a.bar); // 5, 2 The reason for all of these different mechanisms is history. The original mechanism was the constructor `prototype`, then in ES5, Object.create() was added. Then, much later on, `__proto__` was standardized (it was a non-standard extension before).