Modifying the title of tab or window is an older practice. Gmail uses it to prompt users for new chat messages. When a new page is loaded through AJAX, this site also uses it to update the tab title. How is this done? At that time, it was by setting the document object.
A common mistake is: you will think that you should modify window.title. But in fact, you should use the document object, otherwise what you do will definitely be useless. Pay attention, you will see: many times setInterval is used to actually update document.title to attract the user's attention!