How to know if a function is async?

Question

I have to pass a function to another function, and execute it as a callback. The problem is that sometimes this function is async, like:

async function() {
          


        

Answer 1

In case you're using NodeJS 10.x or later

Use the native util function.

   util.types.isAsyncFunction(function foo() {});  // Returns false
   util.types.isAsyncFunction(async function foo() {});  // Returns true

Do keep all the concerns in mind from above ansers though. A function that just returns by accident a promise, will return a false negative.

And on top of that (from the docs):

Note that this only reports back what the JavaScript engine is seeing; in particular, the return value may not match the original source code if a transpilation tool was used.

But if you use async in NodeJS 10 and no transiplation. This is a nice solution.