用于检查url是否存在的Javascript

我用JavaScript编写了一个脚本,检查单击的链接是否有效 脚本基本上执行以下操作

• 拦截所有链接点击
• 检查它们是否嵌套在 a[href] 标签
• 检查url中是否有特定文本,否则允许链接
• 检查该URL是否有效

如果URL 404s或其他内容,我希望阻止点击

function iClicked(event) {
  var link = event.target;
  //go up the family tree until A tag
  while (link && link.tagName != 'A') {
    link = link.parentNode;
  }

  if (link) {
    var url = link.href;
    var ajaxurl = link.getAttribute('href');
    var needToCheck = url.indexOf('speed') != -1;
    //check if the url has the string 'speed' in it
    if (needToCheck) {
      var reader = new XMLHttpRequest();
      //asynchronous is true
      reader.open('get', ajaxurl, true);
      //check each time the ready state changes
      //to see if the object is ready
      reader.onreadystatechange = checkReadyState;

      function checkReadyState() {
        if (reader.readyState === 4) {
          //check to see whether request for the file failed or succeeded
          if ((reader.status == 200) || (reader.status === 0)) {
            //page exists - redirect to the clicked url
            document.location.href = url;

          } else {
            //if the url does not exist
            alert("No use going there!");
            return false;
          }
        }
      }
    }
  }
  return true;
}

//intercept link clicks
document.onclick = iClicked;

现在它不起作用了,我感觉有点不对劲 ajaxurl init和 reader.open 具有 半旋转 也许 return false 部分。但我还是看不清整件事。我对JavaScript很陌生,所以你们能帮我吗?

编辑/结束问题 多亏了@Louy和@epascarello,代码才完整。

// ==UserScript==
// @name        Check before Click
// @namespace   CheckbeforeClick
// @include     *
// @version     1
// @grant       none
// ==/UserScript==


function iClicked(event) {
    var link = event.target;
    //go up the family tree until A tag
    while (link && link.tagName != 'A') {
        link = link.parentNode;
    }

    if (!link) return true;

    var url = link.href;
    var ajaxurl = link.getAttribute('href');
    //change the following to apply on other links, maybe regex
    var needToCheck = url.indexOf('speed') != -1;
    //check if the url has the string 'speed' in it
    if (!needToCheck) return true;

    var reader = new XMLHttpRequest();
    //asynchronous is true
    reader.open('get', ajaxurl, true);
    //check each time the ready state changes
    //to see if the object is ready
    reader.onreadystatechange = checkReadyState;
    function checkReadyState() {
      if (reader.readyState === 4) {
        //check to see whether request for the file failed or succeeded
        if ((reader.status == 200) || (reader.status === 0)) {
          //page exists - redirect to the clicked url
          document.location.href = url;
          // or 
          // window.open(url)
        } else {
          //if the url does not exist
          alert("No use going there!");
        }
      }
    }
    reader.send(null);

    return false;
}

//intercept link clicks
document.onclick = iClicked;