Basically ALL adverts are javascript.

So, it could be as inane as a bugged do-loop.
It just runs and runs and runs.

It could be a bitcoin miner, there have been many imbedded in adverts, games and even wifi access buttons.

It could be trying to hack my computer, or hack someone else's computer.

You can do all kinds of things with javascript and you are relying on your browser to keep it from affecting your computer.