I have an unmanned task that is executed in browser, by JS

I would need to know on my phone when it terminates, since I left the browser open maybe for hours and go away.

I don't care about the specific method: SMS, tweet, alert, telegram would be the same, but it should be:

• free of charge
• fast and easy to setup few lines of code, possibly without server side

which solution can I implement?

NOTE: I already know is weird and inefficient to run unmanned tasks in a browser, it should be nicer on server side, but is ok for my circumstances