How to inherit from a class in javascript?

Question

In PHP/Java one can do:

class Sub extends Base { } 

And automatically all public/protected methods, properties, fields, etc of the Super class become a part of the Sub class which can be overridden if necessary.

What's the equivalent for that in Javascript?

Answer 1

In JavaScript you don't have classes but you can get inheritance and behavior reuse in many ways:

Pseudo-classical inheritance (through prototyping):

function Super () {   this.member1 = 'superMember1'; } Super.prototype.member2 = 'superMember2';  function Sub() {   this.member3 = 'subMember3';   //... } Sub.prototype = new Super(); 

Should be used with the new operator:

var subInstance = new Sub(); 

Function application or "constructor chaining":

function Super () {   this.member1 = 'superMember1';   this.member2 = 'superMember2'; }   function Sub() {   Super.apply(this, arguments);   this.member3 = 'subMember3'; } 

This approach should also be used with the new operator:

var subInstance = new Sub(); 

The difference with the first example is that when we apply the Super constructor to the this object inside Sub, it adds the properties assigned to this on Super, directly on the new instance, e.g. subInstance contains the properties member1 and member2 directly (subInstance.hasOwnProperty('member1') == true;).

In the first example, those properties are reached through the prototype chain, they exist on an internal [[Prototype]] object.

Parasitic inheritance or Power Constructors:

function createSuper() {   var obj = {     member1: 'superMember1',     member2: 'superMember2'   };    return obj; }  function createSub() {   var obj = createSuper();   obj.member3 = 'subMember3';   return obj; } 

This approach is based basically on "object augmenting", you don't need to use the new operator, and as you can see, the this keyword is not involved.

var subInstance = createSub(); 

ECMAScript 5th Ed. Object.create method:

// Check if native implementation available if (typeof Object.create !== 'function') {   Object.create = function (o) {     function F() {}  // empty constructor     F.prototype = o; // set base object as prototype     return new F();  // return empty object with right [[Prototype]]   }; }  var superInstance = {   member1: 'superMember1',   member2: 'superMember2' };  var subInstance = Object.create(superInstance); subInstance.member3 = 'subMember3'; 

The above method is a prototypal inheritance technique proposed by Crockford.

Object instances inherit from other object instances, that's it.

This technique can be better than simple "object augmentation" because the inherited properties aren't copied over all the new object instances, since the base object is set as the [[Prototype]] of the extended object, in the above example subInstance contains physically only the member3 property.