Dynamically loading an external script with call back

gistfile1.txt

var loadScript = function(url, callback) {
var d = document, 
    s = d.getElementsByTagName('script')[0], 
    done = false, 
    script = d.createElement("script");  
    script.type = "text/javascript", 
    script.src = url;	   
			
script.onload = script.onreadystatechange = function() {
    if (!done && (!this.readyState || this.readyState == 'loaded' || this.readyState == 'complete')) {
        done = true;
        script.onload = script.onreadystatechange = null;
        if (callback) {
            callback();
        }
    }
};

s.parentNode.insertBefore(script, s); // Find the <script> tag and insert new script above it
};

loadScript('URL', function(){
    //Call back code
});