Question

I want to write a small game using JavaScript and <canvas> but first I want to nail the “correct” or at least common approach to working with Objects.

One topic I am having trouble understanding in particular is how I could implement overriding of method.

When I create an Object, I may have this:

function MyObject()
{
    var base = {};

    base.i = 0;
    base.update = function()
    {
        base.i ++;
    }

   return base;
}

Then when I create another Object that should start with the same members, I use this:

function AnotherObject()
{
    var base = new MyObject();

    base.j = 0;

    return base;
}

I want to add more content to AnotherObject.update() while still running the logic I have in MyObject.update(), but when I do this within AnotherObject():

base.update = function()
{
    j ++;
}

Then I of course lose the logic I added in MyObject.update().

How can I write AnotherObject.update() so that it also calls the original update() method defined by MyObject?

Answer 1

First, I’d suggest you read this excellent excellent MDN article. It will enlighten you.

You can achieve subclassing this way:

function MyObject() {
  this.i = 0;
}

MyObject.prototype.update = function() {
  this.i++;
}

function AnotherObject() {
  MyObject.call(this);
  this.j = 0;
}

AnotherObject.prototype = new MyObject;

AnotherObject.prototype.constructor = AnotherObject;

AnotherObject.prototype.update = function() {
  MyObject.prototype.update.call(this);
  this.j++;
}

obj = new AnotherObject();
console.log(obj.i); //0
console.log(obj.j); //0

obj.update();
console.log(obj.i); //1
console.log(obj.j); //1

console.log(obj instanceof MyObject) //true
console.log(obj instanceof AnotherObject) //true