Load a script

loadScript.js

var loadScript = function(url, cb) {
  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;
        cb();
      }
    };
  } else { //Others
    script.onload = cb;
  }
  script.src = url;
  document.getElementsByTagName("head")[0].appendChild(script);
};