Sequential promise processing

sequential-promise.js

'use strict';

/**
* Run through promises sequentially. Useful when breaking 1000s records into manageable chunks
*/

function processFile (filename) {

  return new Promise((superResolve, superReject) => {

    let promises = [];

    for (let i = 10; i > 0; i--) {
      promises.push(new Promise((resolve, reject) => {
        setTimeout(() => {
          resolve(10 - i + 1);
        }, i * 10);
      }));
    }

    let last = promises.reduce((curr, next) => {
      return curr.then(value => {
        console.log(next, value);
        return next;
      });
    }, Promise.resolve('First'));

    last.then(result => {
      superResolve('Totally done');
    });

  });

}

let filenames = [1, 2, 3, 4, 5, 6];

let last = filenames.reduce((curr, next) => {
  return curr.then(result => {
    return new Promise((resolve, reject) => {
      processFile(next + '.json')
        .then(result => {
          resolve('Resolved: ' + next + '.json');
        })
    });
  });
}, Promise.resolve('First'));

last
  .then(result => {
    console.log('Finished! ' + result);
  });