Explain Wait for multiple non nested callbacks?

Stoodq08 Nov, 2019News

Say I have two (or more) asynchronous operations option1 and option2 that call a callback function upon completion. How do I best wait for both to complete before starting option3? I am not able to nest them inside of each other (i.e. first starting option1, then starting option2 inside of option1callback()) (I of course also do not know which of these is going to complete first, but that doesn't really matter I guess)