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Async Iteration in Nodejs

Kwabena Bio Berko on October 17, 2017

Node.js is an amazing JavaScript run-time environment, no doubt. Its non-blocking IO model makes applications built with it amazingly fast and high...
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Robert Lippens • • Edited

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.

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Kwabena Bio Berko •

Cool!

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Ryan •

Hey Kwabena, it’s funny that you posted this today because I posted my solution to the async problem as well today 😂 I like the creative counter approach that you used. You might be able to benefit from Promises and Async/Await! I talk about them in my last article: dev.to/ryhenness/the-path-to-conqu...

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Manuel Romero • 
if(error) return callback(error)

If there is no error:

return callback(null, userInfoArr)

When working with callbacks is important error first management