How to make one of the tabs of an open site "main"?

Question

Browsers

A browser can have multiple tabs open for the same site.
It is necessary to implement so that only one of them executes the conditional JS code. For example, once every 10 seconds, it sent an Ajax request and, depending on the response, showed a modal window.
If this tab is closed, another one should take its place and start processing the same code, etc.