[SEMW]

> what is the threshold that I can assume that my code will run sync, and when it cant?

Generally you won't have a choice. If something in your function calls another async function, the function as a whole is forced to be async. If not, then it won't be.

It's possible to make a function fake-async even if it doesn't call any async functions, by just calling the callback at the end. But there's not much point -- and callers may make assumptions about the the callback being called in a separate iteration of the event loop, so doing that could cause subtle bugs. That latter problem is solvable by calling the callback in a setImmediate callback, but again, no point unless you have a good reason to.

(Exception: there are a few apis where you do have a choice, e.g node's randomBytes can be either sync or async. Generally they're things that might block, where the sync version returns an error if it can't satisfy immediately. But generally you don't.)