Javascript inheritance and method overriding

Question

Assume I have a class like this:

function Widget() {
    this.id = new Date().getTime();
    // other fields
}
Widget.prototype = {
    load: function(args) {
        // do something
    }
}

From this class I created some other classes which inherit the same prototype but have some added methods. What I want to do is being able to define a load() method in the sub-classes which first calls the parent method and then execute some code. Something like:

SpecialWidget.prototype = {
    load: function(args) {
        super.load(args);
        // specific code here
    }
}

I know there's no super keyword in Javascript but there must be a way to do this.

Answer 1

You can simulate it like this:

SpecialWidget.prototype = {     load: function(args) {         Widget.prototype.load.call(this, args);         // specific code here     } } 

Or you can create your own super property like this:

SpecialWidget.prototype.parent = Widget.prototype;  SpecialWidget.prototype = {     load: function(args) {         this.parent.load.call(this,args);         // specific code here     } } 

Answer 2

so first, you set up your 'subclass' like so

function SubClass(name) {     Super.call(this);      // stuff here }  SubClass.prototype = new SuperClass(null); SubClass.prototype.constructor = SubClass; 

and then you can do

SuperClass.prototype.theMethod.apply(this); 

from within a subclass implementation to specifically invoke the super's implementation.

Answer 3

I don't know if this is the best solution, but you could do something like this:

function Widget() {
    this.id = new Date().getTime();
}
Widget.prototype.load = function(args) {
   alert( 'parent load' );
};

SpecialWidget = function(){};

   // Make the prototype of SpecialWidget an instance of Widget
var proto = SpecialWidget.prototype = new Widget;

   // Give the prototype a function that references the "load" from Widget
proto.parent_load = proto.load;

   // Give SpecialWidget its own "load" that first calls the parent_load
proto.load = function( args ) {
    this.parent_load( args );
    alert( 'special load' );
};

var inst = new SpecialWidget;

inst.load();

This makes the prototype of SpecialWidget an instance of Widget so that it inherits all that Widget has.

Then it makes a reference to the load() of Widget called parent_load(), and creates its own load() that calls the parent_load() when invoked.