Async Callbacks in javascript

async-callbacks.js

function fetchSports(prefix, callback) {
  console.log("prefix", prefix);
  const sports = ["vikings", "vipers"];
  setTimeout(() => callback(sports), 500);
}

function fetchNews(prefix, callback) {
  const news = ["victory", "votes"];
  setTimeout(() => {
    callback(news);
  }, 1000);
}

const fetchAll = combine([fetchSports, fetchNews]);
fetchAll("v", (results) => console.log("My fetch all results", results));

function combine(fetchers) {
  const promises = fetchers.map((fn) => promisify(fn));
  return (prefix, callback) => {
    Promise.all(promises.map((promise) => promise(prefix))).then((results) => {
      callback([].concat(...results));
    });
  };
}

function promisify(fn) {
  return (...args) => {
    return new Promise((resolve) => {
      const callback = (results) => {
        resolve(results);
      };
      args.push(callback);
      fn(...args);
    });
  };
}