How soon will jQuery(document).ready be called?

Question

If a third party javascript file hangs and takes a while to load, will

jQuery(document).ready(function() {}) 

have to wait for that to

Answer 1

Yes, it has to wait. In particular, you cannot rely on jQuery(document).ready() to fire before other scripts get a chance to execute. ready binds to DOMContentReady, readystatechanged, or onload, whichever is available.

The documentation states that "in most cases, the script can be run as soon as the DOM hierarchy has been fully constructed". Note that the only guarantee is that the DOM is ready when this event fires. It does not guarantee you anything else - because it just cannot.

This, for example will not work in IE, Firefox or Chromium, brilliant.js is always called before the ready() handler has a chance to execute no matter how you shuffle the script tags:

FYI, here is the relevant code from jquery-1.4.2:

bindReady: function() {
    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();
        }
    }
},