動態(tài)加載js腳本函數(shù)封裝
優(yōu)點：js代碼加載不阻塞瀏覽器其他的執(zhí)行

function loadScript(url,fn){
  var oScript = document.createElement('script');
  script.src=url;
  document.getElementsByTagName('head')[0].appendChild(oScript);
  if(window.addEventListener){//高版本瀏覽器
     oScript.onload = function(){
        fn && fn();
      }   
   }else{//IE低版本瀏覽器
      oScript.onreadystatechange = function(){
          if(oScript.readyState='loaded' || oScript.readyState='compltet'){
            oScript.onreadychange = null;
             fn && fn();
          }
        }
    }
}