Exit a function if it's called again externally in Javascript

Exit a function if it's called again externally in Javascript

I would like to have one function being called and executed only once at a
time. So the function will exit if it's called again externally, while the
previous execution is not finished yet.
How would I do that? I don't have an approach in mind. Your help is highly
appreciated.
Please pure JavaScript, no library, Thanks.