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JavaScript Promises and race conditions


As someone who usually promotes using promises, my suggestion is:

Do not use promises here

Promises represent one time events. They are an abstraction over values, one a promise changes its state it can no longer be changed. A promise starts off as pending and changes state once to either fulfilled or rejected.

You are facing a scenario where you have many users, each user joins and needs to add events, the users might 'go away', your scenario simply doesn't describe the same linear flow promises excel at. Promises are useful for a certain scenario - they are not for every concurrency problem. Using an event-emitter here is perfectly appropriate.

Your case (a user joining) does not really represent a resolved proxy operation. The code that "doesn't miss messages" is indeed more correct.

If you still choose to use promises here

There are a few things you can do:

  • You can use Q's progression events and add a progress handler in the creation phase. Note that Kris (Q's author) has called progression broken, and it is being removed in the next version of Q. I recommend against it.
  • You can wrap the message callback to only fire once a handler has been attached - accumulate items fired with a handler when it is created (in an array) and then trigger them all when the message handler is added (after you resolve, in a .then on the deferred you return.

Categories : Javascript

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