Does a new object in Javascript have a prototype property?

Question

This is a purely trivial question for academic value:

If I create a new object, either by doing:

var o = { x:5, y:6 };

or

Answer 1

There is a difference between instances and their constructors.

When creating an object like {a: 1}, you're creating an instance of the Object constructor. Object.prototype is indeed available, and all functions inside that prototype are available:

var o = {a: 1};
o.hasOwnProperty === Object.prototype.hasOwnProperty; // true

But Object.create does something different. It creates an instance (an object), but inserts an additional prototype chain:

var o = {a: 1};
var p = Object.create(o);

The chain will be:

Object.prototype  -  o  -  p

This means that:

p.hasOwnProperty === Object.prototype.hasOwnProperty; // true
p.a === o.a; // true

To get the prototype "under" an instance, you can use Object.getPrototypeOf:

var o = {a: 1};
var p = Object.create(o);

Object.getPrototypeOf(p) === o; // true
Object.getPrototypeOf(o) === Object.prototype; // true

(Previously, you could access an instance's prototype with o.__proto__, but this has been deprecated.)

Note that you could also access the prototype as follows:

o.constructor === Object; // true

So:

o.constructor.prototype === Object.prototype // true
o.constructor.prototype === Object.getPrototypeOf(o); // true

This fails for Object.create-created objects because they do not have a constructor (or rather, their constructor is Object and not what you passed to Object.create because the constructor function is absent).