An asynchronous js loader

loadScript.js

function loadScript(url, callback){ 
  var script = document.createElement("script");
  script.type = "text/javascript";

  if (script.readyState) { // IE 
    script.onreadystatechange = function () { 
      if (script.readyState == "loaded" || script.readyState == "complete"){ 
        script.onreadystatechange = null; 
        callback(); 
      } 
    }; 
  } else { // Others 
    script.onload = function(){ 
      callback(); 
    }; 
  }

  script.src = url; 
  document.getElementsByTagName("head")[0].appendChild(script); 
}