由于我声誉不佳，我无法评论Derek***的回答。
我已经尝试过该代码，但效果不佳。 Derek***的代码有三个问题。

Due to my low reputation I couldn't comment on Derek ***'s answer. I've tried that code as it is and it didn't work well. There are three issues on Derek ***'s code.

1. 第一个是异步发送请求并更改其属性status的时间比执行下一个请求要慢表达式 - if（request.status ===404）。因此，由于互联网频段，request.status最终将保持在状态0（零），如果是，它将无法在下面实现代码。要解决这个问题很简单：在ajax请求的方法打开时将'true'更改为'false'。这将导致代码短暂（或不是这样）阻塞（由于同步调用），但会在到达if上的测试之前更改请求的状态。

1. The first is that the time to async send the request and change its property 'status' is slower than to execute the next expression - if(request.status === "404"). So the request.status will eventually, due to internet band, remain on status 0 (zero), and it won't achieve the code right below if. To fix that is easy: change 'true' to 'false' on method open of the ajax request. This will cause a brief (or not so) block on your code (due to synchronous call), but will change the status of the request before reaching the test on if.

第二个是状态是整数。使用'==='javascript比较运算符，您试图比较左侧对象是否与右侧对象相同。为了完成这项工作，有两种方法：

The second is that the status is an integer. Using '===' javascript comparison operator you're trying to compare if the left side object is identical to one on the right side. To make this work there are two ways:

• 删除围绕404的引号，使其成为整数;


• 使用javascript的运算符'=='，这样您就可以测试两个对象是否相似。

第三，对象XMLHttpRequest仅适用于较新的浏览器（Firefox，Chrome和IE7 +）。如果您希望该片段适用于W3Schools建议的所有浏览器： w3schools ajax

The third is that the object XMLHttpRequest only works on newer browsers (Firefox, Chrome and IE7+). If you want that snippet to work on all browsers you have to do in the way W3Schools suggests: w3schools ajax

真正适合我的代码是：

var request;
if(window.XMLHttpRequest)
    request = new XMLHttpRequest();
else
    request = new ActiveXObject("Microsoft.XMLHTTP");
request.open('GET', 'http://www.mozilla.org', false);
request.send(); // there will be a 'pause' here until the response to come.
// the object request will be actually modified
if (request.status === 404) {
    alert("The page you are trying to reach is not available.");
}