Simple JavaScript Inheritance

John Resig published a great article on his blog, which brought out a smiple javascript inheritance resolution:

// Inspired by base2 and Prototype
(function(){
var initializing = false, fnTest = /xyz/.test(function(){xyz;}) ? /\b_super\b/ : /.*/;

// The base Class implementation (does nothing)
this.Class = function(){};

// Create a new Class that inherits from this class
Class.extend = function(prop) {
// "super" is a FutureReservedWord in ECMAScript v3.
var _super = this.prototype;

// Instantiate a base class (but only create the instance,
// don't run the init constructor)
// SubClass may define init() function, "new this()" should prevent init() function to be excuted.
initializing = true;
var prototype = new this();
initializing = false;

// Copy the properties over onto the new prototype
for (var name in prop) {
// Check if we're overwriting an existing function
prototype[name] = typeof prop[name] == "function" &&
typeof _super[name] == "function" && fnTest.test(prop[name]) ?
(function(name, fn){
// return a cloures, which return results of function prop[name](arguments)
return function() {
//save a reference to the old this._super (disregarding if it actually exists)
//and restore it after we're done.
var tmp = this._super;

// Add a new ._super() method that is the same method
// but on the super-class
// fn function body has statement such as "this._super()"
// this._super will be invoked by function fn
this._super = _super[name];

// The method only need to be bound temporarily, so we
// remove it when we're done executing
var ret = fn.apply(this, arguments);
this._super = tmp;

return ret;
};
})(name, prop[name]) :
prop[name];
}

// The dummy SubClass constructor
function SubClass() {
// All construction is actually done in the init method
if ( !initializing && this.init )
this.init.apply(this, arguments);
}

// Populate our constructed prototype object
SubClass.prototype = prototype;

// Enforce the constructor to be what we expect
// else SubClass.constructor is Function
SubClass.constructor = SubClass;

// And make this SubClass extendable
SubClass.extend = arguments.callee;

return SubClass;
};
})();

Example:

var A = Class.extend({
value:"A",
doStuff:function(){
return this.value;
}
});
var B = A.extend({});
var C = B.extend({
doStuff:function(){
return this._super();
}
});
var c = new C;
alert(c.doStuff());

function doc(argument) {
document.write(argument);
}

var Person = Class.extend({
init: function(isDancing){
this.dancing = isDancing;
},
dance: function(){
return this.dancing;
}
});
doc(Person.constructor);

var Ninja = Person.extend({
init: function(){
this._super( false );
},
dance: function(){
// Call the inherited version of dance()
return this._super();
},
swingSword: function(){
return true;
}
});
doc(Ninja.constructor);

var p = new Person(true);
doc(p.dance()); // => true

var n = new Ninja();
doc(n.dance()); // => false
doc(n.swingSword()); // => true

// Should all be true
doc(p instanceof Person && p instanceof Class && n instanceof Ninja && n instanceof Person && n instanceof Class);