Javascript - How to check if user has revisited the website on an open tab?

I want to add analytics to my website every time a user open my website. I can detect if a user loads my website on a tab by checking initialization of my web app.

But what if a user has left the website open on a tab and revisited again after say 30 min or so to purchase again.

In that case is it possible to detect any event which fires when a tab becomes active again?