Asynchronous Script Loading

async.js

function asyncLoad(scriptURL,callback)
{
    if(!scriptURL){return;}
    var firstScriptTag=document.getElementsByTagName('script')[0];
    var js=document.createElement('script');
    js.type='text/javascript';
    js.src=scriptURL;
    js.async=true;

    if(callback && typeof(callback)===typeof(Function))
    {
        //Modern browsers (IE9+)
        if(js.addEventListener)
        {
            js.addEventListener('load',callback,false);
        }
        else//(IE8-)
        {
            js.onreadystatechange=function()
            {
                if(js.readyState in {loaded:1,complete:1})
                {
                    js.onreadystatechange=null;
                    callback();
                }
            };
        }
    }

    firstScriptTag.parentNode.insertBefore(js,firstScriptTag);
}

usage.js

asyncLoad('some/local/script.js',someFunction);
function someFunction()
{
    //script.js loaded
}

asyncLoad('http://code.jquery.com/jquery-1.8.0.min.js',function()
{
    //$===jQuery
});

Source http://coderwall.com/p/2yacmq