Resolve an array of promises, one after the other

synchronize.js

/**
 * For each item in the array, call the callback, passing the item as the argument.
 * However, only call the next callback after the promise returned by the previous
 * one has resolved.
 *
 * @param {Array<*>} items the elements to iterate over
 * @param {Function} cb called for each item, with the item as the parameter. Should return a promise
 * @return {Promise}
 */
function synchronize(items, cb) {
  return items.reduce(function(promise, item) {
    return promise.then(function() {
      return cb.call(this, item);
    });
  }, RSVP.resolve());
}