Discussion on: Async Iteration in Nodejs

[Robert Lippens]

Nice! This one has snuck up on me (and I'd wager most javascript developers) more often than I'd like to admit. Javascript's non-blocking I/O is nice but can be a bit counter-intuitive for async operations.

I've become a big fan of doing operations like this which can be done in parallel using something like Promise.all over callbacks. (I actually use generators most of the time, as the project I work on has issues upgrading past node v6 - but async/await would work great as Ryan mentioned)

I would wrap the findById in a Promise (I do this manually but something like BluebirdJS can handle entire libraries if you are dealing with old node packages):

const findById = (userId) => {
  return new Promise((resolve, reject) => {
    User.findById(userId, (err, user) => {
       if (err)  {
         reject(err);
       } else {
         resolve({
          firstName: user.firstName,
          lastName: user.lastName,
          email: user.email
         });
    });
  });
}

With this helper, we can just do something like:

return Promise.all(ids.map((id) => findById(id)));

(where our caller would use the result of the promise.)

Note that with Promise.all, a single rejection will terminate the entire call, so if you can tolerate partial results, I recommend just console.error(err) with a resolve() instead of reject(err). You can pick out the empty objects before returning.

[Kwabena Bio Berko]

Cool!