Question

This question relates to a previous question here:

Reducing number of calls to the methods of a JavaScript object

When profiling these two code snippets with Firebug:

function ie6PNGFixLoader(scriptURL) {
    if(arguments.length > 0) {
        for (var i = 0; i < arguments.length; i++) {
            $.ajax({// load PNG fix scripts
                url: arguments[i],
                cache: true,
                dataType: 'script'
            });
        }
    } else {
        return false;
    }               
}

var pngFix = "/Global/ICIS/Scripts/DD_belatedPNG_0.0.8a-min.js";    
var pngList = "/Global/ICIS/Scripts/DD_PNG_listing.js"; 
ie6PNGFixLoader(pngFix, pngList);

and

function InjectScriptsAndExecute(url) {
    this.url = url;
}

InjectScriptsAndExecute.prototype.InjectMethod = function() {
    var inject = $.ajax({
                        url: this.url,
                        cache: true,
                        dataType: 'script',
                        async: false, // Otherwise you cannot depend on the parse order
                        }); 
    return inject;  
}
var pngFix = new InjectScriptsAndExecute("/Global/ICIS/Scripts/DD_belatedPNG_0.0.8a-min.js");
var pngList = new InjectScriptsAndExecute("/Global/ICIS/Scripts/DD_PNG_listing.js");
pngFix.InjectMethod();
pngList.InjectMethod();

It appers that the latter’s calls to the InjectScriptsAndExecute method are much faster than the former’s calls to its function. A colleague has asked me why when i mentioned the performance improvement but i cannot explain it myself.

Any advice for better understanding would be greatfully received.

Answer 1

Arguments is not Array it’s an Object that somewhat behaves like an array.

if(arguments.length > 0) { // Slow AND superfluous
    for (var i = 0; i < arguments.length; i++) { // Even SLOWER
        arguments[i]; // Holy...

Cache the length, accessing the property is slow, IE6 won’t have no optimization at all for .length and I even suspect it to be really slow when using the arguments[i] since it is not a real Array and might therefore do an unoptimized property lookup.

If you want to get the best of both worlds, pass a normal Array, use a plain for loop, and cache the length.

function ie6PNGFixLoader(scripts) {
    for (var i = 0, l = scripts.length; i < l; i++) {
        $.ajax({// load PNG fix scripts
            url: scripts[i],
            cache: true,
            dataType: 'script'
        });
    }              
}

ie6PNGFixLoader(["/Global/ICIS/Scripts/DD_belatedPNG_0.0.8a-min.js",
                 "/Global/ICIS/Scripts/DD_PNG_listing.js"]);

EDIT

To make it clear, timing the loop is useless, the request is async, all you do is timing a loop and a call to $.ajax there’s no point in optimizing here, especially not for two entries. Even in IE6, doing the Ajax call itself (even just calling $.ajax) will be way slower then the loop.