Dynamically load a JavaScript file

You may create a script element dynamically, using Prototypes:

new Element("script", {src: "myBigCodeLibrary.js", type: "text/javascript"});

The problem here is that we do not know when the external script file is fully loaded.

We often want our dependant code on the very next line and like to write something like:

if (iNeedSomeMore) {
    Script.load("myBigCodeLibrary.js"); // includes code for myFancyMethod();
    myFancyMethod(); // cool, no need for callbacks!
}

There is a smart way to inject script dependencies without the need of callbacks. You simply have to pull the script via a synchronous AJAX request and eval the script on global level.

If you use Prototype the Script.load method looks like this:

var Script = {
    _loadedScripts: [],
    include: function(script) {
        // include script only once
        if (this._loadedScripts.include(script)) {
            return false;
        }
        // request file synchronous
        var code = new Ajax.Request(script, {
            asynchronous: false,
            method: "GET",
            evalJS: false,
            evalJSON: false
        }).transport.responseText;
        // eval code on global level
        if (Prototype.Browser.IE) {
            window.execScript(code);
        } else if (Prototype.Browser.WebKit) {
            $$("head").first().insert(Object.extend(
                new Element("script", {
                    type: "text/javascript"
                }), {
                    text: code
                }
            ));
        } else {
            window.eval(code);
        }
        // remember included script
        this._loadedScripts.push(script);
    }
};