Example of creating a 'sync' loop from async work

gistfile1.txt

let Promise = require('es6-promise').Promise;

function iterateStreamAsync(nextOpFn) {
  function loop(prior) {
    if (! prior) {
      return;
    }
    return prior.then(() => loop(nextOpFn()));
  }
  return loop(nextOpFn());
}

let workItems = [
  { letter: 'A', time: 1000 },
  { letter: 'D', time: 300 },
  { letter: 'C', time: 100 },
  { letter: 'B', time: 2000 },
];

let nextOpFn = () => {
  if (workItems.length !== 0) {
    let workItem = workItems.shift();
    return new Promise((resolve, reject) => {

      // Fill this in with your own async work.
      setTimeout(() => {
        console.log(workItem.letter);
        // Call resolve when the work is done.
        resolve();
      }, workItem.time);

    })
  } else {
    return undefined;
  }
}

iterateStreamAsync(nextOpFn);