output stream does not preserve order of input stream

[nlacasse]

If the mapper function is asynchronous, the output stream is not guaranteed to have the same order as the input stream. This violates the semantics of "map".

Here is a test to illustrate the problem:

exports ['stream comes back in the correct order'] = function (test) {
  var input = [3, 2, 1]

  var delayer = map(function(data, cb){
    setTimeout(function () {
      cb(null, data)
    }, 100 * data)
  })

  readStream(delayer, function (err, output) {
    it(output).deepEqual(input)
    test.done()
  })

  writeArray(input, delayer)
}

And the resulting error:

test/
  stream comes back in the correct order
    AssertionError: [1,2,3] deepEqual [3,2,1]

[dominictarr]

you are correct.

I would happily take a pull request to fix this.

When I need this functionality, i use this https://github.com/dominictarr/pull-paramap
because pull-streams are simpler than node streams (for streams of objects) and handle back pressure better.

[parshap]

What if you don't need this behavior? This adds overhead and delays streaming in those cases. Could this be an option?

[dominictarr]

@parshap it's really easy to make a thing that does that with https://github.com/dominictarr/through
it should be a separate module, if you write it, I'll link it from the map-stream readme.