17.01.whenReady.js

/*
 * Pass a function to whenReady() and it will be invoked (as a method of the
 * document) when the document is parsed and ready for manipulation. Registered
 * functions are triggered by the first DOMContentLoaded, readystatechange, or
 * load event that occurs. Once the document is ready and all functions have
 * been invoked, any functions passed to whenReady() will be invoked 
 * immediately.
 */
var whenReady = (function() { // This function returns the whenReady() function
    var funcs = [];    // The functions to run when we get an event
    var ready = false; // Switches to true when the handler is triggered

    // The event handler invoked when the document becomes ready
    function handler(e) {
        // If we've already run once, just return
        if (ready) return;

        // If this was a readystatechange event where the state changed to
        // something other than "complete", then we're not ready yet
        if (e.type === "readystatechange" && document.readyState !== "complete")
            return;
        
        // Run all registered functions.
        // Note that we look up funcs.length each time, in case calling
        // one of these functions causes more functions to be registered.
        for(var i = 0; i < funcs.length; i++) 
            funcs[i].call(document);

        // Now set the ready flag to true and forget the functions
        ready = true;
        funcs = null;
    }

    // Register the handler for any event we might receive
    if (document.addEventListener) {
        document.addEventListener("DOMContentLoaded", handler, false);
        document.addEventListener("readystatechange", handler, false);
        window.addEventListener("load", handler, false);
    }
    else if (document.attachEvent) {
        document.attachEvent("onreadystatechange", handler);
        window.attachEvent("onload", handler);
    }

    // Return the whenReady function
    return function whenReady(f) {
        if (ready) f.call(document); // If already ready, just run it
        else funcs.push(f);          // Otherwise, queue it for later.
    }
}());