loadScript. Async load a script and then fire a callback when loaded.

loadScript.js

//this function will work cross-browser for loading scripts asynchronously
function loadScript(src, callback) {
  var s,
    r,
    t;
  r = false;
  s = document.createElement('script');
  s.type = 'text/javascript';
  s.src = src;
  s.onload = s.onreadystatechange = function() {
    //console.log( this.readyState ); //uncomment this line to see which ready states are called.
    if (!r && (!this.readyState || this.readyState == 'complete')) {
      r = true;
      callback();
    }
  };
  t = document.getElementsByTagName('script')[0];
  t.parent.insertBefore(s, t);
}