sequentialPromise.js

// Kinda pseudo code
// This script will download and resize each image to half
// Because the images can be RRREAAALLY big
// the download and resize is done sequentially (in my case, inside a child_process)
// to ensure that memory is released after each image.

// We dont know how many images there are
let images = [
  'https://flickr.com/photo/1',
  'https://flickr.com/photo/2'
  'https://flickr.com/photo/3'
  'https://flickr.com/photo/4'
];

// Fetch and resize (it usually forks a new chil process to do this);
let resize = (image) => {
  return () => {
    return fetch(url).then((image) => {
      image.width = image.width / 2;
      image.width = image.width / 2;
      return image;
    });
  };
};

// Chain the resize functions into a sequential promise
// it should only fetch one image after it has completed the last one
let process = images.reduce((promise, image) => {
  return promise.then(resize(image));
}, Promise.resolve(true)); // Create an empty promise

process.then(() => {
  // All images should have been downloaded and resized
})