没有头绪 document.getElementById报“尚未实现”错误

求思路或者提示
大概意思是这样的.
<html>
<head>
       function get_price_and_others($site_name,$isbn){
              var request;
              request = new XMLHttpRequest();
              request.onreadystatechange=function(){
                     if (request.readyState == 4 && request.status == 200 ){
                            var infos=eval(request.responseText);
                            if(infos){
                                   new_child=document.createElement("div");
                                   parent=document.getElementById('price');//这一行说尚未实现。头痛。
                                   indicator=document.getElementById('indicator');
                                   parent.insertBefore(new_child,indicator);
                            }
                            delete request;
                            request=null
                     }
              }
              request.open('GET', url, true);
              request.send('');
       }</script>
</head>
<body>
......
<div id='price'><div id='indicator'>indicator</div></div><script>
get_price_and_others("site",xxx);
</script>
...
</body>
</html>