How to inherit from a class in javascript?

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.

I have changed how I do this now, I try to avoid using constructor functions and their prototype property, but my old answer from 2010 is still at the bottom. I now prefer Object.create(). Object.create is available in all modern browsers.

I should note that Object.create is usually much slower than using new with a function constructor.

//The prototype is just an object when you use `Object.create()`
var Base = {};

//This is how you create an instance:
var baseInstance = Object.create(Base);

//If you want to inherit from "Base":
var subInstance = Object.create(Object.create(Base));

//Detect if subInstance is an instance of Base:
console.log(Base.isPrototypeOf(subInstance)); //True

jsfiddle

One of the big benefits of using Object.create is being able to pass in a defineProperties argument, which gives you significant control over how properties on the class can be accessed and enumerated over, and I also use functions to create instances, these serve as constructors in a way, as you can do initialization at the end instead of just returning the instance.

var Base = {};

function createBase() {
  return Object.create(Base, {
    doSomething: {
       value: function () {
         console.log("Doing something");
       },
    },
  });
}

var Sub = createBase();

function createSub() {
  return Object.create(Sub, {
    doSomethingElse: {
      value: function () {
        console.log("Doing something else");
      },
    },
  }); 
}

var subInstance = createSub();
subInstance.doSomething(); //Logs "Doing something"
subInstance.doSomethingElse(); //Logs "Doing something else"
console.log(Base.isPrototypeOf(subInstance)); //Logs "true"
console.log(Sub.isPrototypeOf(subInstance)); //Logs "true

jsfiddle

This is my original answer from 2010:

function Base ( ) {
  this.color = "blue";
}

function Sub ( ) {

}
Sub.prototype = new Base( );
Sub.prototype.showColor = function ( ) {
 console.log( this.color );
}

var instance = new Sub ( );
instance.showColor( ); //"blue"

Well, in JavaScript there is no "class inheritance", there is just "prototype inheritance". So you don't make a class "truck" and then mark it as a subclass of "automobile". Instead, you make an object "Jack" and say that it uses "John" as a prototype. If John knows, how much "4+4" is, then Jack knows it, too.

I suggest you read Douglas Crockford's article about prototypal inheritance here: http://javascript.crockford.com/prototypal.html He also shows how you can make JavaScript have "look-alike" inheritance as in other OO languages and then explains that this actually means breaking javaScript in a way it was not meant to be used.

you can't (in the classical sense). Javascript is a prototypical language. You will observe that you never declare a "class" in Javascript; you merely define the state and methods of an object. To produce inheritance, you take some object and prototype it. The prototype is extended with new functionality.

ES6 classes:

Javascript does not have classes. Classes in javascript are is just syntactical sugar build on top of the prototypal inheritance pattern that javascript. You can use the JS class to enforce prototypal inheritance but it is important to realize that you are actually still using constructor functions under the hood.

These concepts also apply when you are extending from a es6 'class' using the extends keyword. This just creates an additional link in the prototype chain. The __proto__

Example:

class Animal {
  makeSound () {
    console.log('animalSound');
  }
}

class Dog extends Animal {
   makeSound () {
    console.log('Woof');
  }
}


console.log(typeof Dog)  // classes in JS are just constructor functions under the hood

const dog = new Dog();

console.log(dog.__proto__ === Dog.prototype);   
// First link in the prototype chain is Dog.prototype

console.log(dog.__proto__.__proto__ === Animal.prototype);  
// Second link in the prototype chain is Animal.prototype
// The extends keyword places Animal in the prototype chain
// Now Dog 'inherits' the makeSound property from Animal

Object.create()

Object.create() is also a way to create inheritance in JS in javascript. Object.create() is a function which creates a new object, an takes an existing object as an argument. It will assign the object which was received as an argument to the __proto__ property of the newly created object. Again it is important to realize we are bound to the prototypal inheritance paradigm which JS embodies.

Example:

const Dog = {
  fluffy: true,
  bark: () => {
      console.log('woof im a relatively cute dog or something else??');
  }
};

const dog = Object.create(Dog);

dog.bark();

You can't inherit from a class in JavaScript, because there are no classes in JavaScript.