Simple DOM ready() detection without jQuery

dom-ready.js

var DOM = new function(){
	var IS_READY = false;
	var CALLBACKS = [];
	var SELF = this;
	
	SELF.ready = function( callback ){
		//check to see if we're already finished
		if( IS_READY === true && typeof callback === 'function' ){
			callback();
			return;
		}
	
		//else, add this callback to the queue
		CALLBACKS.push( callback );
	};
	var addEvent = function( event, obj, func ){
		if( window.addEventListener ){
			obj.addEventListener( event, func, false );
		}
		else if( document.attachEvent ){
			obj.attachEvent( 'on' + event, func );
		}
	};
	var doScrollCheck = function(){
		//check to see if the callbacks have been fired already
		if( IS_READY === true ){
			return;
		}
		
		//now try the scrolling check
		try{
			document.documentElement.doScroll( 'left' );
		}
		catch( error ){
			setTimeout( doScrollCheck, 1 );
			return;
		}
		
		//there were no errors with the scroll check and the callbacks have not yet fired, so fire them now
		fireCallbacks();
	};
	var fireCallbacks = function(){
		//check to make sure these fallbacks have not been fired already
		if( IS_READY === true ){
			return;
		}
		
		//loop through the callbacks and fire each one
		var callback = false;
		for( var i = 0, len = CALLBACKS.length; i < len; i++ ){
			callback = CALLBACKS[ i ];
			if( typeof callback === 'function' ){
				callback();
			}
		}
		
		//now set a flag to indicate that callbacks have already been fired
		IS_READY = true;
	};
	var listenForDocumentReady = function(){
		//check the document readystate
		if( document.readyState === 'complete' ){
			return fireCallbacks();
		}
		
		//begin binding events based on the current browser
		if( document.addEventListener ){
			addEvent( 'DOMContentLoaded', document, fireCallbacks );
			addEvent( 'load', window, fireCallbacks );
		}
		else if( document.attachEvent ){
			addEvent( 'load', window, fireCallbacks );
			addEvent( 'readystatechange', document, fireCallbacks );
			
			//check for the scroll stuff
			if( document.documentElement.doScroll && window.frameset === null ){
				doScrollCheck();
			}
		}
	};
	
	//since we have the function declared, start listening
	listenForDocumentReady();
};

//simple use case
DOM.ready( function(){
	alert( 'Ready!' );
} );