JS: Prototype and Inheritance

By Xah Lee. Date: . Last updated: .
• Object.prototype
  • {…}
  • Function.prototype
    • Object
    • Function
    • Array
    • Date
    • RegExp
    • function … {…}
  • Array.prototype
    • […]
  • Date.prototype
    • new Date(…)
  • RegExp.prototype
    • /…/…
  • Math
  • JSON
JavaScript Prototype Hierarchy diagram. Image version: 510×654

Prototype of a Object

Every object obj has a Internal Slot named “prototype”, usually written as “[[Prototype]]”.

The value of the [[Prototype]] is some object P, or null. The object P is called the “prototype object” of obj. We'll call it “parent object”.

ECMAScript 2015 §Ordinary and Exotic Objects Behaviours#sec-ordinary-object-internal-methods-and-internal-slots-getprototypeof


When a property is looked up (For example, x.color), JavaScript looks at the object to see if it has that property, if not, it looks up its parent, and repeat, until a property is found or a parent is null. This is the technical meaning of inheritance in JavaScript.

Prototype Chain

A object's parent of parent of parent etc forms a chain. This is called a prototype chain. ECMAScript 2015 §ECMAScript Data Types and Values#sec-invariants-of-the-essential-internal-methods

Note: the prototype chain is never circular, and is impossible to create circular prototype chain.

Note: not all objects have a parent, but buildin objects do (except Object.prototype, which is the root, having parent of null).

You can easily creat a object without parent by Object.create(null). [see JS: Object.create]

How to Find a Object's Parent?

JS: Get/Set Prototype

How to Create Object with Specified Parent?

JS: How to Create Object with Parent X?

Dot Notation a.b.c Does Not Indicate Prototype Chain

When you see a.b.c.d in JavaScript, it has very different meaning than Java or Python, Ruby. This is a major point of confusion.

In Java, in a.b.c.p, the dot means parent/child except the last dot, is object/property. So, a is parent of b, b is parent of c, and c is the object that has member p.

In JavaScript, dot always means object/property relationship. There's no direct syntax to show parent chain.

in JavaScript, when you see a.b.c.d, the left/right thing of any dot has a object/property relationship.

It's parsed like this: (((a).b).c).d.

The value of the property, such as a.b, could be any object, having nothing to do with any of the object a or a.b or a.b.c.

For example, the expression Array.prototype.slice.call:

Prototype of Standard Objects

All function's parent is Function.prototype, by default.

console.log (Object.getPrototypeOf(Object) === Function.prototype ); // true

console.log (Object.getPrototypeOf(Function) === Function.prototype ); // true

console.log (Object.getPrototypeOf(Array) === Function.prototype ); // true

console.log (Object.getPrototypeOf(Date) === Function.prototype ); // true

console.log (Object.getPrototypeOf(RegExp) === Function.prototype ); // true

The Math and JSON objects are used as namespace for “static functions”. Math and JSON are not functions. [see JS: Math] [see JS: JSON]

console.log ( Object.getPrototypeOf(Math) === Object.prototype ); // true

console.log ( Object.getPrototypeOf(JSON) === Object.prototype ); // true

Parent of Function.prototype is Object.prototype.

    Object.getPrototypeOf(Function.prototype) === Object.prototype
); // true

Each of the {Function, Array, Date, RegExp} function object has a property key "prototype". [see JS: Property Key "prototype"]

For example, you can eval the expression Array.prototype. That means, accessing a property key "prototype" from the object Array. The value of Array.prototype, is a object. There is no special syntax to express this object other than accessing property syntax such as Array.prototype or Array["prototype"].

These objects: {Function.prototype, Array.prototype, Date.prototype, RegExp.prototype} serve as parents of user created objects of particular sub-datatype. These objects's parent is Object.prototype. [see JS: Data Types]

console.log ( Object.getPrototypeOf ( Function.prototype ) === Object.prototype ); // true

console.log ( Object.getPrototypeOf ( Array.prototype ) === Object.prototype ); // true

console.log ( Object.getPrototypeOf ( Date.prototype ) === Object.prototype ); // true

console.log ( Object.getPrototypeOf ( RegExp.prototype ) === Object.prototype ); // true

Root Prototype of All Objects is 「Object.prototype」

The root of all JavaScript standard objects is Object.prototype. Itself doesn't have any parent. Its parent is null.

Object.getPrototypeOf(Object.prototype) === null // true

Note: it is possible to have a object without parent.

// creating object without parent
const tt = Object.create(null);
console.log( Object.getPrototypeOf(tt) === null ); // true

[see JS: Object.create]

Prototype of User Created Objects

Here's the prototype of user created objects via literal expression.

// parent of user created objects via literal expression

console.log ( Object.getPrototypeOf ( {} ) === Object.prototype ); // true

console.log ( Object.getPrototypeOf ( [] ) === Array.prototype ); // true

console.log ( Object.getPrototypeOf ( function () {} ) === Function.prototype ); // true

console.log ( Object.getPrototypeOf ( /./ ) === RegExp.prototype ); // true

console.log ( Object.getPrototypeOf ( new Date() ) === Date.prototype ); // true

Parent of Object Created by Constructor Function

The parent of object created by new F(…) is:

// example of inheritance by constructor

const o1 = {};
const FF = function () { };
FF.prototype = o1;
const o2 = new FF();

console.log( Object.getPrototypeOf(o2) === o1 ); // true

Another example, using the default constructor function Date:

// example of inheritance by constructor
const dd = new Date();

console.log( Object.getPrototypeOf(dd) === Date["prototype"]); // true

[see JS: Operator “new”]

Object and Inheritance Topic

  1. JS: Object System Overview
  2. JS: What's Object?
  3. JS: Prototype and Inheritance
  4. JS: Create Object
  5. JS: Object Literal Expression
  6. JS: Get/Set Prototype
  7. JS: How to Create Object with Parent X?
  8. JS: Prevent Adding Property
  9. JS: Determine Type of Object
  10. JS: Primitive Value Object Wrapper
  11. JS: Clone, Deep Copy Object/Array
  12. JS: Test Equality of Objects

  1. JS: Object Object
  2. JS: Object.prototype
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