Run JavaScript code on window close or page refresh?

Question

There is both window.onbeforeunload and window.onunload, which are used differently depending on the browser. You can assign them either by setting the window properties to functions, or using the .addEventListener:

window.onbeforeunload = function(){
   // Do something
}
// OR
window.addEventListener("beforeunload", function(e){
   // Do something
}, false);
   

Usually, onbeforeunload is used if you need to stop the user from leaving the page (ex. the user is working on some unsaved data, so he/she should save before leaving). onunload isn't supported by Opera, as far as I know, but you could always set both.

Ok, I found a working solution for this, it consists of using the beforeunload event and then making the handler return null. This executes the wanted code without a confirmation box popping-up. It goes something like this:

window.onbeforeunload = closingCode;
function closingCode(){
   // do something...
   return null;
}

Sometimes you may want to let the server know that the user is leaving the page. This is useful, for example, to clean up unsaved images stored temporarily on the server, to mark that user as "offline", or to log when they are done their session.

Historically, you would send an AJAX request in the beforeunload function, however this has two problems. If you send an asynchronous request, there is no guarantee that the request would be executed correctly. If you send a synchronous request, it is more reliable, but the browser would hang until the request has finished. If this is a slow request, this would be a huge inconvenience to the user.

Later came navigator.sendBeacon(). By using the sendBeacon() method, the data is transmitted asynchronously to the web server when the User Agent has an opportunity to do so, without delaying the unload or affecting the performance of the next navigation. This solves all of the problems with submission of analytics data: the data is sent reliably, it's sent asynchronously, and it doesn't impact the loading of the next page.

Unless you are targeting only desktop users, sendBeacon() should not be used with unload or beforeunload since these do not reliably fire on mobile devices. Instead you can listen to the visibilitychange event. This event will fire every time your page is visible and the user switches tabs, switches apps, goes to the home screen, answers a phone call, navigates away from the page, closes the tab, refreshes, etc.

Here is an example of its usage:

document.addEventListener('visibilitychange', function() {
    if (document.visibilityState == 'hidden') { 
        navigator.sendBeacon("/log.php", analyticsData);
    }
});

When the user returns to the page, document.visibilityState will change to 'visible', so you can also handle that event as well.

sendBeacon() is supported in:

• Edge 14
• Firefox 31
• Chrome 39
• Safari 11.1
• Opera 26
• iOS Safari 11.4

It is NOT currently supported in:

• Internet Explorer
• Opera Mini

Here is a polyfill for sendBeacon() in case you need to add support for unsupported browsers. If the method is not available in the browser, it will send a synchronous AJAX request instead.

Update:

It might be worth mentioning that sendBeacon() only sends POST requests. If you need to send a request using any other method, an alternative would be to use the fetch API with the keepalive flag set to true, which causes it to behave the same way as sendBeacon(). Browser support for the fetch API is about the same.

fetch(url, {
   method: ..., 
   body: ...,            
   headers: ...,       
   credentials: 'include',
   mode: 'no-cors',
   keepalive: true,
})

jQuery version:

$(window).unload(function(){
    // Do Something
});

Update: jQuery 3:

$(window).on("unload", function(e) {
    // Do Something
});

Thanks Garrett

The documentation here encourages listening to the onbeforeunload event and/or adding an event listener on window.

window.addEventListener('beforeunload', function(event) {
  //do something here
}, false);

You can also just populate the .onunload or .onbeforeunload properties of window with a function or a function reference.

Though behaviour is not standardized across browsers, the function may return a value that the browser will display when confirming whether to leave the page.

You can use window.onbeforeunload.

window.onbeforeunload = confirmExit;
function confirmExit(){
    alert("confirm exit is being called");
    return false;
}

The event is called beforeunload, so you can assign a function to window.onbeforeunload.