Execute promises sequentially (one at at a time) and return array with the results

promise_map.js

function promiseMap(inputValues, mapper) {
  const reducer = (acc$, inputValue) =>
    acc$.then(acc => mapper(inputValue).then(result => acc.push(result) && acc));
  return inputValues.reduce(reducer, Promise.resolve([]));
}

/* Example */

const axios = require('axios');

function countryFromIp(ip) {
  return axios.get(`http://ip-api.com/json/${ip}`)
    .then(res => res.data.country);
}

promiseMap(["8.8.8.8", "41.182.194.0", "5.34.159.1"], countryFromIp).then(console.log);

Thanks! What does "ysAcc" stands for?

Exactly what I was looking for. Thanks so much for sharing 👍

This was really useful. I needed the result to be a Map rather than an Array, so I modified it slightly like this:

function promiseMap(xs, f) {
  const reducer = (ysAcc$, x) =>
    ysAcc$.then(ysAcc => f(x).then(y => { ysAcc[x] = y; return ysAcc; }));
  return xs.reduce(reducer, Promise.resolve({}));
}

works perfectly, I was in a promise chain hell-hole (currently this is the 6th in a 7 stage promise chain), but now it works :)