Best way to detect when a user leaves a web page?

Try the onbeforeunload event: It is fired just before the page is unloaded. It also allows you to ask back if the user really wants to leave. See the demo onbeforeunload Demo.

Alternatively, you can send out an Ajax request when he leaves.

In the case you need to do some asynchronous code (like sending a message to the server that the user is not focused on your page right now), the event beforeunload will not give time to the async code to run. In the case of async I found that the visibilitychange and mouseleave events are the best options. These events fire when the user change tab, or hiding the browser, or taking the courser out of the window scope.

document.addEventListener('mouseleave', e=>{
     //do some async code
})

document.addEventListener('visibilitychange', e=>{
     if (document.visibilityState === 'visible') {
   //report that user is in focus
    } else {
     //report that user is out of focus
    }  
})

Thanks to Service Workers, it is possible to implement a solution similar to Adam's purely on the client-side, granted the browser supports it. Just circumvent heartbeat requests:

// The delay should be longer than the heartbeat by a significant enough amount that there won't be false positives
const liveTimeoutDelay = 10000
let liveTimeout = null

global.self.addEventListener('fetch', event => {
  clearTimeout(liveTimeout)
  liveTimeout = setTimeout(() => {
    console.log('User left page')
    // handle page leave
  }, liveTimeoutDelay)
  // Forward any events except for hearbeat events
  if (event.request.url.endsWith('/heartbeat')) {
    event.respondWith(
      new global.Response('Still here')
    )
  }
})

One (slightly hacky) way to do it is replace and links that lead away from your site with an AJAX call to the server-side, indicating the user is leaving, then use that same javascript block to take the user to the external site they've requested.

Of course this won't work if the user simply closes the browser window or types in a new URL.

To get around that, you'd potentially need to use Javascript's setTimeout() on the page, making an AJAX call every few seconds (depending on how quickly you want to know if the user has left).

What you can do, is open up a WebSocket connection when the page loads, optionally send data through the WebSocket identifying the current user, and check when that connection is closed on the server.

For What its worth, this is what I did and maybe it can help others even though the article is old.

PHP:

session_start();

$_SESSION['ipaddress'] = $_SERVER['REMOTE_ADDR'];

if(isset($_SESSION['userID'])){
    if(!strpos($_SESSION['activeID'], '-')){
        $_SESSION['activeID'] = $_SESSION['userID'].'-'.$_SESSION['activeID'];
    }
}elseif(!isset($_SESSION['activeID'])){
    $_SESSION['activeID'] = time();
}

JS

window.setInterval(function(){
            var userid = '<?php echo $_SESSION['activeID']; ?>';
            var ipaddress = '<?php echo $_SESSION['ipaddress']; ?>';
            var action = 'data';

            $.ajax({
                url:'activeUser.php',
                method:'POST',
                data:{action:action,userid:userid,ipaddress:ipaddress},
                success:function(response){
                     //alert(response);                 
                }
            });
          }, 5000);

Ajax call to activeUser.php

if(isset($_POST['action'])){
    if(isset($_POST['userid'])){
        $stamp = time();
        $activeid = $_POST['userid'];
        $ip = $_POST['ipaddress'];

        $query = "SELECT stamp FROM activeusers WHERE activeid = '".$activeid."' LIMIT 1";
        $results = RUNSIMPLEDB($query);

        if($results->num_rows > 0){
            $query = "UPDATE activeusers SET stamp = '$stamp' WHERE activeid = '".$activeid."' AND ip = '$ip' LIMIT 1";
            RUNSIMPLEDB($query);
        }else{
            $query = "INSERT INTO activeusers (activeid,stamp,ip)
                    VALUES ('".$activeid."','$stamp','$ip')";
            RUNSIMPLEDB($query);
        }
    }
}

Database:

CREATE TABLE `activeusers` (
  `id` int(11) NOT NULL,
  `activeid` varchar(20) NOT NULL,
  `stamp` int(11) NOT NULL,
  `ip` text
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

Basically every 5 seconds the js will post to a php file that will track the user and the users ip address. Active users are simply a database record that have an update to the database time stamp within 5 seconds. Old users stop updating to the database. The ip address is used just to ensure that a user is unique so 2 people on the site at the same time don't register as 1 user.

Probably not the most efficient solution but it does the job.

Page Visibility API

✅ The Page Visibility API provides events which can be watch to know when a document becomes visible or hidden.

✅ When the user minimizes the window or switches to another tab, API triggers a visibilitychange event.

✅ We can perform the actions based on the visibilityState

function onVisibilityChange() {
  if (document.visibilityState === 'visible') {
     console.log("user is focused on the page")
  } else {
     console.log("user left the page")
  }
}

document.addEventListener('visibilitychange', onVisibilityChange);