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提示这样的错误:Uncaught TypeError: Cannot read property 'getElementsByTagName' of null

提示这样的错误:Uncaught TypeError: Cannot read property 'getElementsByTagName' of null

慕粉175552829 2019-03-17 15:00:40
alert明明有输出却还是显示这样的错误,ballball各位大佬教教我!!!<div class="form-group"> <div class="label"> <label>工号:</label> </div> <div class="field"> <input type="text" style="width:25%; float:left;" class="input w50" value="" name="userid" id ="userid" onblur="validate()" data-validate="required:请输入工号" /> <div id="usermsg"></div> </div> </div><script type="text/javascript"> var req; function validate() { var idField = document.getElementById("userid"); var url = "${pageContext.request.contextPath}/page/user/validate.jsp?id=" + escape(idField.value); // alert(url); if (window.XMLHttpRequest) { req = new XMLHttpRequest(); } else if (window.ActiveXObject) { req = new ActiveXObject("Microsoft.XMLHTTP"); } req.open("GET", url, true); req.onreadystatechange = callback; req.send(null); } function callback() { if (req.readyState == 4) { if (req.status == 200) { alert(req.responseText); var msg = req.responseXML.getElementsByTagName("msg")[0]; // alert(msg); setMsg(msg.childNodes[0].nodeValue); } } } function setMsg(msg) { // alert(msg); mdiv = document.getElementById("usermsg"); if (msg == "invalid") { mdiv.innerHTML = "<font color='red'>username exists</font>"; } else { mdiv.innerHTML = "<font color='green'>congratulations! you can use this username!</font>"; } }</script>validate.jsp<body> <% response.setContentType("text/xml"); response.setHeader("Cache-Control", "no-store"); //HTTP1.1 response.setHeader("Pragma", "no-cache"); //HTTP1.0 response.setDateHeader("Expires", 0); //prevents catching at proxy server response.setHeader("Cache-Control", "no-cache"); System.out.println(request.getParameter("id")); String userid = request.getParameter("id"); System.out.println(userid); User u = new User(); boolean userExist = u.CheckUserid(userid); System.out.println(userExist); if (userExist) { response.getWriter().write("<msg>invalid</msg>"); } else { response.getWriter().write("<msg>valid</msg>"); } %></body>
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