javascript - onreadystatechange 仅触发一次

Question

我的请求有以下代码:

     var req = new XMLHttpRequest();
  req.onreadystatechange = function() {
    if (req.readyState == 4)       // state of 4 is 'done'. The request has completed
    {
     callback(req.responseText); // The .responseText property of the request object
    } else {                       // contains the Text returned from the request.
      console.log(req.readyState);
    }
  };
  req.open("GET", url, true);
  req.send();

但是，readyState 更改为 1 并正确触发(我看到它在控制台中回显)，但它根本不会进展到 2。一段时间后，它超时，我在控制台中看到以下内容:

Failed to load resource: net::ERR_CONNECTION_TIMED_OUT

Uncaught SyntaxError: Unexpected end of input

有人知道为什么会这样吗？

Answer 1

放这个

req.open("GET", url, true);
  req.send();

此行上方

  req.onreadystatechange = function() {