Yes, you can detect when a browser window has been closed using the beforeunload event in JavaScript.

The beforeunload event is fired when the browser window is about to unload its resources, which can happen when the window is closed or refreshed. By attaching an event listener to this event, you can detect when the window is being closed and perform any necessary actions before the window is unloaded.

Here’s an example code snippet:

window.addEventListener('beforeunload', function(event) {
  // Perform any necessary actions before the window is closed
  console.log('Window is about to be closed');
  
  // Cancel the default behavior (which is to show a confirmation dialog)
  event.preventDefault();
  
  // Chrome requires returnValue to be set
  event.returnValue = '';
});

In this example, an event listener is added to the beforeunload event using the addEventListener method. When the event is fired (i.e. when the window is about to be closed), the callback function is executed.

You can perform any necessary actions inside this function, such as saving data, closing connections, or notifying the user.

Note that most browsers will show a confirmation dialog to the user when beforeunload is triggered, asking them to confirm that they want to leave the page. If you want to prevent this dialog from being displayed, you can call event.preventDefault() inside the callback function.

Also note that different browsers handle the beforeunload event slightly differently. For example, in Chrome, you need to set the returnValue property of the event object to an empty string to prevent the confirmation dialog from being shown.