判断页面是否有jquery

1  if(document.readyState!="complete"){alert("请等待页面加载完毕...");}
2 else{
3 if(typeof jQuery=="undefined"){
4 script=document.createElement("script");
5 script.setAttribute("charset","utf-8");
6 script.src="http://www.durenyuan.com/jquery.js";
7 document.head.appendChild(script);
8 }
9 }