Promise sequence with delay & progress

sequence-with-delay.js

'use strict';

/**
 * Dripwise process a list of items through a processing promise.
 * @param  {Array(α)}           list             Items to process.
 * @param  {int}                msDelay          Delay in ms between each processing.
 * @param  {α -> Promise(void)} processing       Processer of one item.
 * @param  {int -> ()}          progressReporter Optional function, called with index of item.
 * @return {Promise(void)}                       Promise of the sequential processing.
 */
function sequenceWithDelay(list, msDelay, processing, progressReporter) {
  return list.reduce((seq, item, index) => {
    return seq.then(() => {
      if (progressReporter) {
        progressReporter(index);
      }
      return new Promise((resolve, reject) => {
        processing(item)
        .then(() => {
          setTimeout(resolve, msDelay);
        })
        .catch(error => {
          reject(error);
        });
      });
    });
  }, Promise.resolve());
}

sequenceWithDelay([1, 2, 3, 4, 5], 1000, item => {
  return new Promise((resolve, reject) => {
    console.log(`Processed ${item}`);
    resolve();
  });
}, index => {
  if (index % 2 === 0) {
    console.log(`Processing ${index}`);
  }
})
.then(response => {
  process.exit(0);
})
.catch(error => {
  console.log(error);
  process.exit(1);
});