Iterator of promises: forEach

iterator-of-promises.js

function iteratorOfPromisesForEach(iterator, callback) {
    var snapshot = iterator.next();
    if (snapshot.done) {
        return Promise.resolve();
    }
    return snapshot.value.then(callback).then(() => iteratorOfPromisesForEach(iterator, callback));
}

var result = iteratorOfPromisesForEach(filesIterator, function (file) {
    // return immediately to just do sync processing
    // or return a promise to delay pulling new results until the promise fulfills
    // any rejections, either of the promises in the iterator or caused by `callback`
    //   throwing or caused by `callback` returning a rejected promise, flow to `result`
    // if there were no rejections `result` is fulfilled with `undefined`.
});

It's also possible to do it this way:

function iteratorOfPromisesForEach(iterator, callback) {
  let p = Promise.resolve()
  for (const i of iterator) {
    p = p.then(() => i).then(callback)
  }
  return p
}