Promise.eachLimit

promise.js

function createArrayIterator(coll) {
  var i = -1;
  var len = coll.length;
  return function next() {
    return ++i < len ? {value: coll[i], key: i} : null;
  }
}

function eachLimit(coll, limit, iteratee) {
  if (limit <= 0 || !coll) return Promise.resolve();

  var done = false;
  var running = 0;
  var nextElem = createArrayIterator(coll);

  function replenish() {
    return new Promise(function(resolve, reject) {
      while (running < limit && !done) {
        var elem = nextElem();
        if (elem === null) {
          done = true;
          if (running <= 0) {
            resolve();
          }
          return;
        }

        running++;
        iteratee(elem.value, elem.key).then(function() {
          running--;
          if (done && running <= 0) {
            resolve();
          } else {
            replenish().then(resolve, reject)
          }
        }).catch(function(e) {
          running--;
          done = true;
          reject(e);
        });
      }
    });
  }

  return replenish();
}