Promises in Sequence

If you have multiple promises, an array of promises, and you want to perform an action after they all have resolved you can use Promise.resolve(). However if you'd like the same result, whilst having the promises called in order you will need to come up with a custom solution because there is no api for that.

Let's create a promise factory to generate our promises that we want to execute in sequence:

function createP(id) {
  return new Promise((resolve, reject) => {
    console.log(id);
    const timeout = Math.random() * 100;
    setTimeout(() => {
      resolve({ id, timeout });
    }, timeout);
  });
}

The inSequence function which takes an array of promises as an argument:

function inSequence(promises) {
  return promises.reduce((prev, next) => {
    return prev.then((prevRes => {
        return next.then(nextRes => {
            prevRes.push(nextRes); // take the previous result and append the next to it
            return prevRes;
        });
    }));
  }, Promise.resolve([]));
}

Usage:

const promises = [
  createP(7), createP(5), createP(3)
];
inSequence(promises).then(res => {
    console.log(res);
});

Result:

[
  {
    "id": 7,
    "timeout": 56.985111228065755
  },
  {
    "id": 5,
    "timeout": 25.835860521855423
  },
  {
    "id": 3,
    "timeout": 13.25073904955656
  }
]

tjunghans/promises-in-sequence.md

Promises in Sequence