await...defer in forEach loop

Problem

should this code return 12334 or 12433 ?

I expect 12334 but it gives 12433...

console.log '1'
anArray.forEach (info, index, array)->
  console.log '2'

  await model.findOne info, defer(err, doc)
  console.log '3'

console.log '4'
Problem courtesy of: Charles

Solution

Your intuition is incorrect: IcedCoffeeScript's await...defer cannot block a synchronous forEach loop.

Remember that IcedCoffeeScript compiles to JavaScript, which does not support blocking. Once you've called forEach on an array, you've committed to iterating through that entire array before any events can fire.

The good news is that you can get the behavior you want by using IcedCoffeeScript's own loop constructs. In your case,

for info, index in anArray ->
  ...

For more information on dealing with JavaScript's non-blocking event model, check out my new book, Async JavaScript.

Solution courtesy of: Trevor Burnham

Discussion

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