Question

Someone told me that when you use setTimeout you must clear it with clearTimeout. I can understand before the timeout runs out, but why after? Or is it untrue?

Answer 1

It’s not true – there’s no harm in clearing a timeout after it has finished, but it’s not necessary.

Per the specification:

If handle does not identify an entry in the list of active timers of the WindowOrWorkerGlobalScope object on which [clearTimeout] was invoked, the method does nothing.

In other words, it’s a no-op; nothing happens, and no error will be thrown.