Display a confirm modal when closing the browser

Written byPhuoc Nguyen

#Adding an event listener to window

To detect when a user is attempting to close their browser, you'll need to add an event listener to the `window` object. You can use the `beforeunload` event to accomplish this. Here's an example:

window.addEventListener('beforeunload', (event) => {
    // ...
});

To display a confirm modal when the `beforeunload` event is triggered, use the `preventDefault()` method to cancel the default behavior of the `beforeunload` event. This prevents the browser from closing the tab or window as it normally would. For older browsers, you also need to set the `returnValue` property to an empty string.

window.addEventListener('beforeunload', (event) => {
    event.preventDefault();
    event.returnValue = '';
});

Once this is done, the browser displays a confirm modal. This is what it looks like in the Chrome browser:

Chrome confirmation modal when leaving a site

The message displayed on the modal changes based on the action the user takes to close the browser. For instance, when users try to reload the current page, the browser might modify the main message and label of buttons.

Chrome confirmation modal when reloading a site

It's important to note that the message on the confirm modal cannot be customized, as it's provided by the browser.

Good to know

On modern browsers like Chrome, you can set the `returnValue` property to any string without any impact on its behavior. But in older browsers like IE, `returnValue` is used to display a custom message for the default confirmation dialog.
// Old browsers
window.addEventListener('beforeunload', function(event) {
  var message = 'Are you sure you want to leave?';
  event.returnValue = message;
  return message;
});

#Conclusion

Adding a confirm modal when closing the browser can be a helpful feature for websites with unsaved changes or other crucial information. With JavaScript, you can effortlessly incorporate this feature into your website and enhance the user experience.

#See also

Display a modal when users leave website

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Phước Nguyễn