What is the best practice for long promise chainings within Promise.All

Question

Suppose I have several main promises, some of which return promise inside. I want to retrieve results from sort of promise functions that why I think about Promise.all. First off, let take a look at my example code below

var Promise = require('bluebird');
promise = []
function promise1(){
  return new Promise(function(resolve, reject){
    console.log('promise1');
    resolve('promise1');
  });
}
function promise2(){
  return new Promise(function(resolve, reject) {
    console.log('promise2');
    resolve('promise2');
  });
}
function promise3(){
  promise2().then(function(){
    return new Promise(function(resolve, reject) {
      console.log('promise3');
      resolve('promise3')
    })
  })
}

Upper piece of code, 2 main promises are promise1 and promise3. promise2 will be returned by promise3. I'm demonstating promise3 as a long promise chaning. To run, I initialized

promise = [promise1(), promise3()];
Promise.all(promise).then(function(data){
  console.log('done', data);
})

Outcome was

promise1

promise2

promise3

done [ 'promise1', undefined ]

but I am expecting that

done [ 'promise1', 'promise3' ]

My question is what is the best practice in this case?

Answer 1

promise3 is missing a return. With this it works as expected.

function promise3(){
  return promise2().then(function(){
  // ^^^ missing return here
    return new Promise(function(resolve, reject) {
      console.log('promise3');
      resolve('promise3')
    })
  })
}

---

Update:

If you simplify your case you're doing:

var a = new Promise(function(resolve) {
  resolve("a");
});
var b = a.then(function () {
  return new Promise(function(resolve) {
    resolve("b");
  }));
});

And then your question is: "why is the resolved value of a not equal to b?". Well, they're two different promises.

With return a you return the original promise2. In a.then you eventually return promise3.