javascript - 为什么我的浏览器在这里显示 "uncaught referenceerror"？

Question

我只是在浏览器中进行一些测试，以了解发生了什么 - 直到这一行为止，一切似乎都正常工作:

responseJson = JSON.parse(localReq.responseText);

当我评估这部分时: JSON.parse(localReq.responseText);我得到了适当的值。但是当我评估“responseJson”时，它给了我一个 Uncaught ReferenceError ，我不明白为什么。

function login()
{
   userName = document.getElementById("_name").value;
   password = document.getElementById("_password").value;
   data = "userName=" + userName + "&" + "password=" + password;
   localReq = new XMLHttpRequest();

   localReq.open("POST", "http://universe.tc.uvu.edu/cs2550/assignments/PasswordCheck/check.php", true);
   localReq.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
   localReq.send(data);
   response = document.getElementById("_login");

   if (localReq.status == 200)
   {
      responseJson = JSON.parse(localReq.responseText);
   }



}

Answer 1

您需要在 AJAX 请求上使用事件监听器，通过回调异步处理来自服务器的响应。如果不这样做，您将在服务器响应之前检查响应代码 200。

function login(){
   userName = "username";
   password = "password";
   data = "userName=" + userName + "&" + "password=" + password;
   localReq = new XMLHttpRequest();

    // use an event handler here
    localReq.addEventListener("load", function(evt){
        if (localReq.status == 200) {
            responseJson = JSON.parse(localReq.responseText);
            alert("Success: " + localReq.responseText);
        } else {
            alert("Not Success!= :(");
            console.log(localReq);
        }
    });

    localReq.open("POST", "http://universe.tc.uvu.edu/cs2550/assignments/PasswordCheck/check.php", true);
    localReq.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    localReq.send(data);
}

查看这个jsFiddle:http://jsfiddle.net/wwsj3r4q/