我是模仿MDN上面这个页面的step3中的例子写的：
链接描述

这个是例子的代码：

<button id="ajaxButton" type="button">Make a request</button>

<script>
(function() {
  var httpRequest;
  document.getElementById("ajaxButton").addEventListener('click', makeRequest);

  function makeRequest() {
    httpRequest = new XMLHttpRequest();

    if (!httpRequest) {
      alert('Giving up :( Cannot create an XMLHTTP instance');
      return false;
    }
    httpRequest.onreadystatechange = alertContents;
    httpRequest.open('GET', 'test.html');
    httpRequest.send();
  }

  function alertContents() {
    if (httpRequest.readyState === XMLHttpRequest.DONE) {
      if (httpRequest.status === 200) {
        alert(httpRequest.responseText);
      } else {
        alert('There was a problem with the request.');
      }
    }
  }
})();
</script>

我运行这个例子没有问题，但是我模仿它的代码提示我未定义变量，我的代码部分：

function loadPage(userType, id){
    if (userType==="普通用户") {
        var req = new XMLHttpRequest();
        if (!req) {
            alert('Giving up :( cannot create an XMLHTTP instance');
            return false;
        }
        req.onreadystatechange = alertContents;
        req.open("GET", "edit.php?id="+id);
        req.send();
    }else{
        var req = new XMLHttpRequest();
        if (!req) {
            alert('Giving up :( cannot create an XMLHTTP instance');
            return false;
        }
        req.onreadystatechange = alertContents;
        req.open("GET", "readonly.php?id"+id);
        req.send();
    }
}
function alertContents(){
    if (req.readyState === XMLHttpRequest.DONE) {                  
        if (req.status === 200) {
            document.getElementById('sectionDetail').innerHTML = req.requestText;
        }else{
            alert('there was a problem with the request.');
        }
    }else{
        alert('there was a problem');
    }
}

然后console提示：

29行是函数alertContents 里的第一行，我看了半天，没找到的代码和例子差在哪里，求大佬指教！