How to add a confirmation before the closing of the tab/browser with JavaScript?

To add a confirmation before the closing of the tab/browser with JavaScript, we set the window.onbeforeunload property to a function that shows the alert.

For instance, we write

window.onbeforeunload = (e) => {
  return "Sure?";
};

to set window.onbeforeunload to a function that returns a non-empty string.

Then the browser will show an alert box before we close the tab or browser window.