Alerts when navigating away from a web page

By the browser. It’s the beforeunload event handler that returns the customized text of the dialog, which is only the middle of the three paragraphs – the other two paragraphs as well as the text of the buttons cannot be customized or otherwise changed.

window.onbeforeunload = function(){ return 'Testing...' }

// OR

var unloadListener = function(){ return 'Testing...' };
window.addEventListener('beforeunload', unloadListener);

Will yield a dialog that says

Are you sure you want to navigate away from this page?

Testing...

Press OK to continue, or Cancel to stay on the current page.

You can nullify this by setting the handler to null

window.onbeforeunload = null;

// OR

window.removeEventListener('beforeunload', unloadListener);