$(fn)
In jQuery, $(fn) is a short cut to $.fn.ready(fn). First you call the $.fn.ready(fn), in the function it call jQuery.bindReady() first, then add the fn to a readyList. In the jQuery.bindReady() function, it try to hookup DOMContentLoaded handler, in IE the event is called onreadystatechange. In the DOMContentLoaded handler, it call the jQuery.ready() function, in side of this function, the function in the readyList is invoked
if ( jQuery.isFunction( selector ) ) { rootjQuery.ready( selector ); } ready: function( fn ) { // before hookup event, bind the real event first // Attach the listeners jQuery.bindReady(); // If the DOM is already ready if ( jQuery.isReady ) { // Execute the function immediately fn.call( document, jQuery ); // Otherwise, remember the function for later } else if ( readyList ) { // Add the function to the wait list readyList.push( fn ); } return this; }, bindReady: function() { //ensure this method is only run once if ( readyBound ) { return; } readyBound = true; // Catch cases where $(document).ready() is called after the // browser event has already occurred. if ( document.readyState === "complete" ) { return jQuery.ready(); } // Mozilla, Opera and webkit nightlies currently support this event if ( document.addEventListener ) { // Use the handy event callback document.addEventListener( "DOMContentLoaded", DOMContentLoaded, false ); // A fallback to window.onload, that will always work window.addEventListener( "load", jQuery.ready, false ); // If IE event model is used } else if ( document.attachEvent ) { // ensure firing before onload, // maybe late but safe also for iframes document.attachEvent("onreadystatechange", DOMContentLoaded); // A fallback to window.onload, that will always work window.attachEvent( "onload", jQuery.ready ); // If IE and not a frame // continually check to see if the document is ready var toplevel = false; try { toplevel = window.frameElement == null; } catch(e) {} if ( document.documentElement.doScroll && toplevel ) { doScrollCheck(); } } }, if ( document.addEventListener ) { DOMContentLoaded = function() { document.removeEventListener( "DOMContentLoaded", DOMContentLoaded, false ); jQuery.ready(); }; } else if ( document.attachEvent ) { DOMContentLoaded = function() { // Make sure body exists, at least, in case IE gets a little overzealous (ticket #5443). if ( document.readyState === "complete" ) { document.detachEvent( "onreadystatechange", DOMContentLoaded ); jQuery.ready(); } }; }