“Waiting” on either a promise or regular object using native(ES6) JavaScript promises

Question

I played with promises a few times a few years ago using either jQuery or Q. I'm now quite rusty and want to learn and use the new native ES6 promises.

I seem to remember one neat concept where you can "wait" on something and not care whether it's a plain object or a promise. If it's a promise the callback is called when it asynchronously completes, if it's anything else the callback is called immediately - maybe the next tick.

But I can't recall how this is done. I'm not sure if it has a name so it's proving difficult to Google for. I'm not sure if it's a standard feature across all JS promise implementations, or if it was just something only jQuery had.

What is this called? Can I still do this with native promises? Where can I read up on it?

Answer 1

Both jQuery's $.when() and ES6's Promise.all() exhibit the behaviour you refer to. Provide a promise and the function waits for the promise to resolve, but for any other value it returns immediately.