promise-sequences

A small library to control a sequence of promises with concurrency limiting. There are zero dependencies, it expects you are using a javascript runtime with Promise support or a polyfill.

Why?

When working with promise apis the standard Promise.all will allow you to run promises in parallel. Controlling the limit of how many can run concurrently is not part of the api, this library offers a simple solution. For example say you want to download 100 images, 3 at a time and continue downloading if one download task throws an error. Lets see how you can do it:

First, install the package from npm

npm i promise-sequences

import { seriesSettled } from 'promise-sequences'

const imageDownloadTasks = [
  fetch('http://image/0.png'),
  ... // the 100 images, you could generate this array from some datasource
]

seriesSettled(imageDownloadTasks, 3) // 3 at a time
  .then(results => {
    console.log(results)
    // now you can do what you need to with the response of the urls
  })

There are two main apis, series and seriesSettled.

series(sequence[() => Promise], concurrent, stepCallback)

Just like standard Promise.all a series will reject on the first failure. You can easily control the concurrency with the second parameter.

import {series} from 'promise-sequences'

const fetchImages = [
  fetch('http://openclipart.org/image/2400px/svg_to_png/271178/old-man.png'),
  fetch('https://openclipart.org/image/300px/svg_to_png/271966/doctorsandsurgery.png'),
]

series(fetchImages, 1)
  .then(results => {
    let statusCodes = results.map(response => response.status)
    console.log(statusCodes) // [200, 200]
  })

allSettled(sequence[() => Promise], concurrent, stepCallback)

This sequence will continue a even if one of the items in the sequence rejects. So that you can easily process the results, each item will have an object with the the state ('resolved'|'rejected') and the result (the rejected or resolved value). The parameters for concurrency are the same.

import { seriesSettled } from 'promise-sequences'

const fetchImages = [
  fetch('http://openclipart.org/image/2400px/svg_to_png/271178/old-man.png'),
  fetch('https://invalid-image-url.png'),
  fetch('https://openclipart.org/image/300px/svg_to_png/271966/doctorsandsurgery.png'),
]

seriesSettled(fetchImages, 1)
  .then(results => {
    console.log(results)
    // results will be
      [
        {
            result: {response},
            state: 'resolved'
        },
        {
            result: {FetchError},
            state: 'rejected'
        },
        {
            result: {response},
            state: 'resolved'
        },
      ]
  })