javascript - resolving an array of promises from within a parent promise

Question

This is my first shot at nested promises. I'm using the bluebird library but I think the idea is the same for all of the promise libraries.

At a high level, this is what I'm trying to do:

myService.getSomeData(url)
 .then((data) => {
   myOtherService.getMoreData(data.uniqueId)
   .then((thisDataIsAnArray) => {
      //loop over the data above and do something
   });
 });

getMoreData() is supposed to make X service calls and store the results in an array X elements long. This is where I start getting lost, in that I'm not sure how to craft this method and what I should be returning from it. I've taken a few stabs at bluebird's Promise.all and Promise.map but am floundering and thought I'd solicit suggestions.

Answer 1

Promises are just return values you attach callbacks to, instead of passing callbacks into functions. Unless you return all of them, there's no way for the callbacks to chain, or catch all their errors.

Also, return from all the .then's the instant you have another promise. This flattens things.