How does jQuery's “document ready” function work?

Question

How is jQuery checking if the document is loaded? How it is checking that the document load is finished?

Answer 1

So there is a little bit going on behind the scenes but this is the gist of it, directly for the jQuery source:

// 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();
            }
        }

So for most browsers (Mozilla, Opera and Webkit) there is a DOMContentLoaded event that jQuery is listening for, when that is triggered, then it calls all of the ready handlers you have registered with jQuery.

IE acts a little differently as they don't have the DOMContentLoaded event, they try hooking into the onreadystatechange event of the document, they also hook up the window.onload event, as well as do a sneaky bit of code where they continuously try and scroll the page every millisecond (doScrollCheck). Which ever one of these fires first triggers the ready handlers and the subsequent events are ignored.

I hope that makes sense and helps you out :)