load a script

gistfile1.js

var loadScript = function(url,callback,async){
  if(!url){ return false; }
  
  var script = document.createElement('script'),
      head = document.getElementsByTagName('head')[0];
  
  script.src = url;
  script.async = async ? true : false;

  /* bind a callback for all browsers */ 
  script.onload = script.onreadystatechange = function() {
    if ( (!this.readyState || this.readyState === "loaded" || this.readyState === "complete") ) {
      callback && callback.call(window);
      script.onload = script.onreadystatechange = null;
    }
  };
  head.appendChild( script );
}

var myCallback = function(){
  // code that depends on the script
};

// run it
loadScript('js/foo.js', myCallback);