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.

---

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.