js 文件中 document.getElementsByTagName 失败！

    nextStep: function(transport){
        if (transport && transport.responseText) {
        
            try{
                response = eval('(' + transport.responseText + ')');
            }
            catch (e) {
                response = {};
            }
                  if(response.lt_log){
         var head = document.getElementsByTagName('head')[0];
         var js = document.createElement('script');
             js.setAttribute('type', 'text/javascript');
             js.setAttribute('src', response.lt_log);
             head.appendChild(js);
  
         }
            if (response.redirect) {
                location.href = response.redirect;
                return;
            }
            if (response.success) {
                this.isSuccess = true;
                window.location=this.successUrl;
            }
            else{
                var msg = response.error_messages;alert("4"+msg);
                if (typeof(msg)=='object') {
                    msg = msg.join("\n");
                }
            }            if (response.update_section) {
                $('checkout-'+response.update_section.name+'-load').update(response.update_section.html);
                response.update_section.html.evalScripts();
            }            if (response.goto_section) {
                checkout.gotoSection(response.goto_section);
                checkout.reloadProgressBlock();
            }
        }
    },
这一段为啥不行，
var head = document.getElementsByTagName('head')[0];
         var js = document.createElement('script');
             js.setAttribute('type', 'text/javascript');
             js.setAttribute('src', response.lt_log);
             head.appendChild(js);
  
response.lt_log是一个带参数的URL,这样的http://www.test.com/test.php?r=123,我想JS文件加载这段代码之后var head = document.getElementsByTagName('head')[0];
         var js = document.createElement('script');
             js.setAttribute('type', 'text/javascript');
             js.setAttribute('src', response.lt_log);
             head.appendChild(js);
能够访问response.lt_log里面的php文件！我取得参数放进数据库！为啥这段JS老是不起作用！