How to show the “Are you sure you want to navigate away from this page?” when changes committed?

Question

Here in stackoverflow, if you started to make changes then you attempt to navigate away from the page, a javascript confirm button shows up and asks: \"Are you sure you want

Answer 1

The standard states that prompting can be controlled by canceling the beforeunload event or setting the return value to a non-null value. It also states that authors should use Event.preventDefault() instead of returnValue, and the message shown to the user is not customizable.

As of 69.0.3497.92, Chrome has not met the standard. However, there is a bug report filed, and a review is in progress. Chrome requires returnValue to be set by reference to the event object, not the value returned by the handler.

It is the author's responsibility to track whether changes have been made; it can be done with a variable or by ensuring the event is only handled when necessary.

window.addEventListener('beforeunload', function (e) {
    // Cancel the event as stated by the standard.
    e.preventDefault();
    // Chrome requires returnValue to be set.
    e.returnValue = '';
});
    
window.location = 'about:blank';