Run a set of async fns in sequence

promise-sequence.js

/**
 * Run a series of async functions in sequence,
 * so that each fn waits to run until the last one has completed.
 * 
 * @param {function():Promise<T>} fns
 * @returns {Promise<T[]>
 */
function sequence(fns) {
  return fns
    .reduce((_prevResults, fn) => (
      _prevResults.then(
        prevResults => fn().then(res => prevResults.concat([res]))
      )
    ), Promise.resolve([]));
}

promise-sequence.ts

/**
 * Run a series of async functions in sequence,
 * so that each fn waits to run until the last one has completed.
 */
function sequence<TRes>(fns:IAsyncFn<TRes>[]):Promise<TRes[]> {
  return fns
    .reduce((_prevResults:Promise<TRes[]>, fn) => (
      _prevResults.then(
        prevResults => fn().then(res => prevResults.concat([res]))
      )
    ), Promise.resolve([]));
}

export interface IAsyncFn<TRes> {
  ():Promise<TRes>
}

export default sequence;

eg

sequence([
  () => Promise.resolve(1),
  () => Promise.resolve(2),
  () => Promise.resolve(3),
  () => Promise.resolve(4),
])
  .then(res => {
     // res === [1, 2, 3, 4]
  })