Chaining promise results in Javascript

Question

I have two functions which return a Promise. The first one returns a list of values that I want to iterate through and make a second call on each, accumulating to the result to send back.

function firstCall() {
 return new Promise(returns list of items);
}

function secondCall(someVal) {
  return new Promise(return single value);
}

function doSomething() {
  firstCall().then((response1) => {
    response1.someResult.map((item) => {
      secondCall(item).then((response2) => {
        //how to collect all?
      });
   });
  });
}

Any suggestion on how I can chain these Promise returning call and returning list of final result?

Answer 1

I'm presuming that you are using ES6 or some polyfill (Bluebird, etc.) If that isn't the case, please update your question. If so, you can send an array of Promises to Promise.all() and get an array of answers back.

function firstCall() {
  return new Promise((resolve, reject) => {
    resolve([1, 2, 3, 4, 5, 6]);
  })
}

function secondCall(n) {
  return new Promise((resolve, reject) => {
    resolve(n * n);
  })
}

function doSomething() {
  firstCall()
    .then(response1 => {
      console.log(response1);
      let promises = response1.map(r => secondCall(r));

      Promise.all(promises)
        .then(response2 => {
          console.log(response2);
        })
    })
}

doSomething();