How to prevent closing browser window?

Another implementation is the following you can find it in this webpage: http://ujap.de/index.php/view/JavascriptCloseHook

<html>
  <head>
    <script type="text/javascript">
      var hook = true;
      window.onbeforeunload = function() {
        if (hook) {
          return "Did you save your stuff?"
        }
      }
      function unhook() {
        hook=false;
      }
    </script>
  </head>
  <body>
    <!-- this will ask for confirmation: -->
    <a href="http://google.com">external link</a>

    <!-- this will go without asking: -->
    <a href="anotherPage.html" onClick="unhook()">internal link, un-hooked</a>
  </body>
</html>

What it does is you use a variable as a flag.

Keep your code as is and use jQuery to handle links:

$(function () {
  $("a").click(function {
    window.onbeforeunload = null;
  });
});

You can detect hyperlinks clicks, but you can't determine whether user:

Attempted to refresh page.
Attempted to close browser tab.
Attempted to close browser window.
Inputted another URL in the URL bar and hit enter.

All these actions generate beforeunload event on window, without any more exact information about the event.

In order to display confirmation dialog when doing above actions, and not display it when a hyperlink was clicked, follow these steps:

Assign beforeunload event listener to window, which returns confirmation text as a string, unless a specific variable (flag) is set to true.
Assign click event to document. Check if a element has been clicked (event.target.tagName). If yes, set flag to true.

You should also handle form submissions by assigning a submit event listener to document.

Your code could look like this:

let disableConfirmation = false;
window.addEventListener('beforeunload', event => {
  const confirmationText = 'Are you sure?';
  if (!disableConfirmation) {
    event.returnValue = confirmationText; // Gecko, Trident, Chrome 34+
    return confirmationText;              // Gecko, WebKit, Chrome <34
  } else {
    // Set flag back to false, just in case
    // user stops loading page after clicking a link.
    disableConfirmation = false;
  }
});
document.addEventListener('click', event => {
  if (event.target.tagName.toLowerCase() === 'a') {
    disableConfirmation = true;
  }
});
document.addEventListener('submit', event => {
  disableConfirmation = true;
});

<p><a href="https://stacksnippets.net/">google.com</a></p>
<form action="https://stacksnippets.net/"><button type="submit">Submit</button></form>
<p>Try clicking the link or the submit button. The confirmation dialog won't be displayed.</p>
<p>Try reloading the frame (right click -> "Reload frame" in Chrome). You will see a confirmation dialog.</p>

Note that in some browsers you have to use event.returnValue in beforeunload listener, and in others you use return statement.

See also beforeunload event docs.

I have added an additional code that will to handle further process of if window is closed

let disableConfirmation = false;
window.addEventListener('beforeunload', event => {
  const confirmationText = 'Are you sure?';
  if (!disableConfirmation) {
    event.returnValue = confirmationText; // Gecko, Trident, Chrome 34+
    return confirmationText;              // Gecko, WebKit, Chrome <34
  } else {
    // Set flag back to false, just in case
    // user stops loading page after clicking a link.
    disableConfirmation = false;
  }
});

After Confimation if you want to send any ajax request and you have used jquery

 $(window).on('unload', function() {
   // async: false will make the AJAX synchronous in case you're using jQuery
   axios
     .get('ajax_url')
     .then(response => {});
 });

And if no jquery is used you can use navigator, it require navigator library

navigator.sendBeacon(url, data);

you can download this library from here : https://github.com/miguelmota/Navigator.sendBeacon

How to prevent closing browser window?

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