JavaScript override methods

Question

Let's say you have the below code:

function A() {     function modify() {        x = 300;        y = 400;     }      var c = new C(); }  function B() {     function modify(){        x = 3000;        y = 4000;     }      var c = new C(); }  C = function () {    var x = 10;    var y = 20;     function modify() {       x = 30;       y = 40;    };     modify();    alert("The sum is: " + (x+y)); } 

Now the question is, if there is any way in which I can override the method modify from C with the methods that are in A and B. In Java you would use the super-keyword, but how can you achieve something like this in JavaScript?

Answer 1

Edit: It's now six years since the original answer was written and a lot has changed!

• If you're using a newer version of JavaScript, possibly compiled with a tool like Babel, you can use real classes.
• If you're using the class-like component constructors provided by Angular or React, you'll want to look in the docs for that framework.
• If you're using ES5 and making "fake" classes by hand using prototypes, the answer below is still as right as it ever was.

Good luck!

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JavaScript inheritance looks a bit different from Java. Here is how the native JavaScript object system looks:

// Create a class function Vehicle(color){   this.color = color; }  // Add an instance method Vehicle.prototype.go = function(){   return "Underway in " + this.color; }  // Add a second class function Car(color){   this.color = color; }  // And declare it is a subclass of the first Car.prototype = new Vehicle();  // Override the instance method Car.prototype.go = function(){   return Vehicle.prototype.go.call(this) + " car" }  // Create some instances and see the overridden behavior. var v = new Vehicle("blue"); v.go() // "Underway in blue"  var c = new Car("red"); c.go() // "Underway in red car" 

Unfortunately this is a bit ugly and it does not include a very nice way to "super": you have to manually specify which parent classes' method you want to call. As a result, there are a variety of tools to make creating classes nicer. Try looking at Prototype.js, Backbone.js, or a similar library that includes a nicer syntax for doing OOP in js.