几乎所有主流浏览器现在都支持window.navigator. online属性，以及相应的在线和离线窗口事件。运行以下代码片段来测试它:

console.log('Initially ' + (window.navigator.onLine ? 'on' : 'off') + 'line'); window.addEventListener('online', () => console.log('Became online')); window.addEventListener('offline', () => console.log('Became offline')); document.getElementById('statusCheck').addEventListener('click', () => console.log('window.navigator.onLine is ' + window.navigator.onLine)); <button id="statusCheck">Click to check the <tt>window.navigator.onLine</tt> property</button><br /><br /> Check the console below for results:

尝试将您的系统或浏览器设置为离线/在线模式，并检查日志或window.navigator.onLine属性的值变化。

请注意Mozilla文档中的这段话:

In Chrome and Safari, if the browser is not able to connect to a local area network (LAN) or a router, it is offline; all other conditions return true. So while you can assume that the browser is offline when it returns a false value, you cannot assume that a true value necessarily means that the browser can access the internet. You could be getting false positives, such as in cases where the computer is running a virtualization software that has virtual ethernet adapters that are always "connected." Therefore, if you really want to determine the online status of the browser, you should develop additional means for checking. In Firefox and Internet Explorer, switching the browser to offline mode sends a false value. Until Firefox 41, all other conditions return a true value; since Firefox 41, on OS X and Windows, the value will follow the actual network connectivity.

(重点是我自己的)

这意味着如果window.navigator.onLine为false(或者您得到一个脱机事件)，则保证您没有Internet连接。

然而，如果它是真的(或者你得到一个在线事件)，它充其量只意味着系统连接到某个网络。这并不意味着你可以上网。要检查这一点，您仍然需要使用其他答案中描述的解决方案之一。

我最初打算把这篇文章作为格兰特·瓦格纳(Grant Wagner)的回答的更新，但这似乎太过编辑了，尤其是考虑到2014年的更新已经不是他的了。

我认为这是一个非常简单的方法。

var x = confirm("Are you sure you want to submit?");
if (x) {
  if (navigator.onLine == true) {
    return true;
  }
  alert('Internet connection is lost');
  return false;
}
return false;

我不得不为一个与学校有很多合作的客户制作一个web应用程序(基于ajax)，这些学校的互联网连接经常不好，我使用这个简单的功能来检测是否有连接，工作得非常好!

我使用CodeIgniter和Jquery:

function checkOnline() {
    setTimeout("doOnlineCheck()", 20000);
}

function doOnlineCheck() {
    //if the server can be reached it returns 1, other wise it times out
    var submitURL = $("#base_path").val() + "index.php/menu/online";

    $.ajax({
        url : submitURL,
        type : "post",
        dataType : "msg",
        timeout : 5000,
        success : function(msg) {
            if(msg==1) {
                $("#online").addClass("online");
                $("#online").removeClass("offline");
            } else {
                $("#online").addClass("offline");
                $("#online").removeClass("online");
            }
            checkOnline();
        },
        error : function() {
            $("#online").addClass("offline");
            $("#online").removeClass("online");
            checkOnline();
        }
    });
}

您可以使用$.ajax()的错误回调，它在请求失败时触发。如果textStatus等于字符串"timeout"，这可能意味着连接中断:

function (XMLHttpRequest, textStatus, errorThrown) {
  // typically only one of textStatus or errorThrown 
  // will have info
  this; // the options for this ajax request
}

医生说:

错误:一个函数被调用，如果请求 失败。函数被传递了3个 参数:XMLHttpRequest对象 描述错误类型的字符串 这是可选的 异常对象，如果发生。 第二个可能的值 参数(除null外)为"timeout"， "error"， "notmodified"和 “parsererror”。这是一个Ajax事件

例如:

 $.ajax({
   type: "GET",
   url: "keepalive.php",
   success: function(msg){
     alert("Connection active!")
   },
   error: function(XMLHttpRequest, textStatus, errorThrown) {
       if(textStatus == 'timeout') {
           alert('Connection seems dead!');
       }
   }
 });

window.navigator.onLine

是你要找的，但这里要添加的东西很少，首先，如果它是你想要持续检查的应用程序上的一些东西(比如看看用户是否突然脱机，在这种情况下大多数时候是正确的，那么你也需要监听变化)，为此你添加事件监听器到窗口来检测任何变化，为了检查用户是否脱机，你可以这样做:

window.addEventListener("offline", 
  ()=> console.log("No Internet")
);

检查是否在线:

window.addEventListener("online", 
  ()=> console.log("Connected Internet")
);

您可以通过发出失败的XHR请求来确定连接已丢失。

标准的方法是重试请求几次。如果没有通过，请提醒用户检查连接，然后优雅地失败。

旁注:将整个应用程序置于“脱机”状态可能会导致大量易出错的处理状态工作。无线连接可能来来去去，等等。因此，最好的办法可能是优雅地失败，保存数据，并提醒用户。允许他们最终解决连接问题(如果有的话)，并在一定程度上原谅他们继续使用你的应用。

旁注:您可以检查像谷歌这样的可靠站点的连接性，但这可能并不完全有用，因为只是尝试发出自己的请求，因为虽然谷歌可能可用，但您自己的应用程序可能不可用，并且您仍然必须处理自己的连接问题。尝试向谷歌发送ping信息是确认网络连接本身已中断的好方法，因此如果该信息对您有用，那么可能值得费心。

Sidenote: Sending a Ping could be achieved in the same way that you would make any kind of two-way ajax request, but sending a ping to google, in this case, would pose some challenges. First, we'd have the same cross-domain issues that are typically encountered in making Ajax communications. One option is to set up a server-side proxy, wherein we actually ping google (or whatever site), and return the results of the ping to the app. This is a catch-22 because if the internet connection is actually the problem, we won't be able to get to the server, and if the connection problem is only on our own domain, we won't be able to tell the difference. Other cross-domain techniques could be tried, for example, embedding an iframe in your page which points to google.com, and then polling the iframe for success/failure (examine the contents, etc). Embedding an image may not really tell us anything, because we need a useful response from the communication mechanism in order to draw a good conclusion about what's going on. So again, determining the state of the internet connection as a whole may be more trouble than it's worth. You'll have to weight these options out for your specific app.