Detect the Internet connection is offline?

Question

How to detect the Internet connection is offline in JavaScript?

Answer 1

IE 8 will support the window.navigator.onLine property.

But of course that doesn't help with other browsers or operating systems. I predict other browser vendors will decide to provide that property as well given the importance of knowing online/offline status in Ajax applications.

Until that happens, either XHR or an Image() or <img> request can provide something close to the functionality you want.

Update (2014/11/16)

Major browsers now support this property, but your results will vary.

Quote from Mozilla Documentation:

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. All other conditions return a true value.

Answer 2

There are 2 answers forthis for two different senarios:-

1. If you are using JavaScript on a website(i.e; or any front-end part) The simplest way to do it is:

   <h2>The Navigator Object</h2>
   
   <p>The onLine property returns true if the browser is online:</p>
   
   <p id="demo"></p>
   
   <script>
     document.getElementById("demo").innerHTML = "navigator.onLine is " + navigator.onLine;
   </script>
   

2. But if you're using js on server side(i.e; node etc.), You can determine that the connection is lost by making failed XHR requests.

   The standard approach is to retry the request a few times. If it doesn't go through, alert the user to check the connection, and fail gracefully.

Answer 3

an ajax call to your domain is the easiest way to detect if you are offline

$.ajax({
      type: "HEAD",
      url: document.location.pathname + "?param=" + new Date(),
      error: function() { return false; },
      success: function() { return true; }
   });

this is just to give you the concept, it should be improved, check for return http status codes etc

Answer 4

As olliej said, using the navigator.onLine browser property is preferable than sending network requests and, accordingly with developer.mozilla.org/En/Online_and_offline_events, it is even supported by old versions of Firefox and IE.

Recently, the WHATWG has specified the addition of the online and offline events, in case you need to react on navigator.onLine changes.

Please also pay attention to the link posted by Daniel Silveira which points out that relying on those signal/property for syncing with the server is not always a good idea.

Answer 5

You can use $.ajax()'s error callback, which fires if the request fails. If textStatus equals the string "timeout" it probably means connection is broken:

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

From the doc:

Error: A function to be called if the request fails. The function is passed three arguments: The XMLHttpRequest object, a string describing the type of error that occurred and an optional exception object, if one occurred. Possible values for the second argument (besides null) are "timeout", "error", "notmodified" and "parsererror". This is an Ajax Event

So for example:

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

Answer 6

request head in request error

$.ajax({
    url: /your_url,
    type: "POST or GET",
    data: your_data,
    success: function(result){
      //do stuff
    },
    error: function(xhr, status, error) {

      //detect if user is online and avoid the use of async
        $.ajax({
            type: "HEAD",
            url: document.location.pathname,
            error: function() { 
              //user is offline, do stuff
              console.log("you are offline"); 
              }
         });
    }   
});