代码：【复制】 
var loadJs = function(url, callback) {
  var script = document.createElement('script');
  script.type = 'text/javascript';
  if (callback)
   script.onload = script.onreadystatechange = function() {
    if (script.readyState && script.readyState != 'loaded' && script.readyState != 'complete') return;
    script.onreadystatechange = script.onload = null;
    callback();
   };
  script.src = url;
  document.getElementsByTagName('head')[0].appendChild (script);
 }