returning a promise inside then doesn't work

Question

I'm using bluebird as my Promise libaray. I have the following chain:

function a() {
  return Promise.all([promise1, promise2]).then((results) => {      
      return promise3(results)
  })
}

But I couldn't get the result of promise3, I mean the following doesn't work:

a().then((result) => {
    console.log(result)
}).catch((err) => {
    console.error(err)
})

I searched and read about promises but I couldn't understand where the problem is?

Answer 1

This could be for several reasons and it is impossible to determine which from the post. So to help future readers - let's figure out what can go wrong in the above scenario:

Let's make the following adjustments to the code:

function a() {
  console.log("a called"); // if this doesn't log, you're redefining 'a'
  return Promise.all([promise1, promise2]).then((results) => {
      console.log("In .all"); // if this isn't called, promise1 or promise2 never resolve      
      return promise3(results);
  }).tap(() => 
    console.log("Promise 3 fn resolved") // if this doesn't log, promise3 never resolves
  });
}

Then, our then handler:

let r = a().then((result) => {
    console.log('result', result); // this logs together with promise 3 fn
}); // bluebird automatically adds a `.catch` handler and emits unhandledRejection

setTimeout(() => {
  // see what `r` is after 30 seconds, if it's pending the operation is stuck
  console.log("r is", r.inspect()); 
}, 30000);

That should give you full coverage for all cases that could possible go wrong.

If I had to guess - one of the promises never resolves since you have a Promise constructor somewhere that is never calling resolve or reject.

(Bluebird will warn you against this in certain scenarios - make sure you have warnings on)

Another scenario which could cause this is cancellation - but I'm assuming you don't have that turned on.

Happy coding.