Promise.all implementations ... for fun

Promise.js

// Reduce Implementation
function promiseAllReduction(promises) {
  return promises.reduce(function(acc, i) {
    return acc.then(rs => i.then(r => [...rs, r]))
  }, Promise.resolve([]));
}

// Recursive Implementation
function promiseAllRecursive(promises) {
  if(promises.length === 0) {
    return Promise.resolve([]);
  }
  const [p, ...ps] = promises;
  return p.then(r => {
    return PromiseAllRecursive(ps).then(rs => [r, ...rs])
  })
}