Simple JavaScript Inheritance

转自: http://ejohn.org/blog/simple-javascript-inheritance/
// 略有改动，以便自己阅读

// Inspired by base2 and Prototype
( function() {
	var NAMESPACE = 'Class';

	var initializing = false;

	var fnTest = /xyz/.test( function() {
		'xyz';
	} ) ? /\b_super\b/ : /.*/;

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

	// Check whether fn is a function
	var isFunction = function(fn) {
		return (typeof fn == 'function');
	}

	// Create a new Class that inherits from this class
	Class.extend = function(obj) {
		var _super = this.prototype;

		// Instantiate a base class (but only create the instance,
		// don't run the init constructor)
		initializing = true;
		var proto = new this();
		initializing = false;

		// Copy the properties over onto the new prototype
		for (var name in obj) {
			// Check if we're overwriting an existing function
			var isOverwriting = isFunction(obj[name]) && isFunction(_super[name])
					&& fnTest.test(obj[name]);
			proto[name] = isOverwriting ? ( function(name, fn) {
				return function() {
					var tmp = this._super;

					// Add a new ._super() method that is the same method
					// but on the super-class
					this._super = _super[name];

					// The method only need to be bound temporarily, so we
					// remove it when we're done executing
					var ret = isFunction(fn) ? fn.apply(this, arguments) : void (0);

					this._super = tmp;

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

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

		// Populate our constructed prototype object
		Klass.prototype = proto;

		// Enforce the constructor to be what we expect
		Klass.constructor = Klass;

		// And make this class extendable
		Klass.extend = arguments.callee;

		return Klass;
	};

	window[NAMESPACE] = Class;
})();