Robust cross browser JS Loader for dependency management.

js-loader.js

/**
 * Author: Ben Gerrissen
 * Date: 9-3-11
 * License: MIT
 */

(function( global ){

    var undefined
    , preloads = false
    , queue = []
    , scriptNode = document.createElement( "script" )
    , anchorNode = document.getElementsByTagName( "script" )[0]
    , testNode
    , tid
    ;

    scriptNode.async = true;

    testNode = scriptNode.cloneNode( true);

    testNode.onload = testNode.onreadystatechange = function(){
        preloads = true;
        proceed();
    };
    // browser that preload scripts, will fire 'onreadystatechange:loaded'
    testNode.src = "javascript:void()";

    // 'onreadystatechange' fires immediatly after script execution, so we need to wait for it.
    tid = setTimeout( proceed , 9 );

    function proceed(){
        // now we should know if a browser preloads, lets proceed
        clearTimeout( tid );
        var q = queue;
        queue = undefined;
        while( q.length ) {
            loadScript.apply( null , q.pop() );
        }
        testNode = testNode.onload = testNode.onreadystatechange = null;
    }

    function append( node ){
        anchorNode.parentNode.insertBefore( node , anchorNode );
    }

    // src should be resolved before loadScript is called.
    // so loadScript expects complete and valid urls
    function loadScript( src , callback ){
        if ( queue ) {
            queue.push( arguments );
            return;
        }
        var node = scriptNode.cloneNode(true)
        , listener = function(){
            var node = listener.node;
            if(!node.readyState || !/in/.test( node.readyState ) ) {
                if ( preloads ) {
                    // after appending, script executes immediatly.
                    // and current program seems to be put on hold till script is done.
                    // cool.
                    append( node );
                    node.onload = node.onreadystatechange = null;
                }
                callback();
            }
            node = null;
        };
        listener.node = node;
        node.onload = node.onreadystatechange = listener;
        node.src = src;

        if ( !preloads ){
            append( node );
        }
        node = null;
    }

    global.loadScript = loadScript;

}( this ));