Execute an array of promises sequentially and collect the result

promise_map.js

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

/* Example */

const axios = require('axios');

function countryFromIp(ip) {
  return axios.get("http://freegeoip.net/json/" + ip).then(res => res.data.country_name);
}

promiseMap(["8.8.8.8", "41.182.194.0", "5.34.159.1"], countryFromIp).then(console.log);
// [ 'United States', 'Namibia', 'Spain' ]