[eldude]

Unfortunately, in practice promises end up making your code more difficult to reason about by adding cruft and unnecessary abstraction. They're also very limiting from a control-flow perspective.

This is especially noticeable when you have branching behavior / want to resolve a promise early[1]:

Branching with promises:

  function doTask(task, callback) {
    return Q.ncall(task.step1, task)
    .then(function(result1) {
      if (result1) {
        return result1;
      } else {
        return continueTasks(task);
      }
    })
    .nodeify(callback)
  }

  function continueTasks(task) {
    return Q.ncall(task.step2, task);
    .then(function(result2) {
      return Q.ncall(task.step3, task);
    })
  }

As opposed to with stepdown[2]:

  function doTask(task, callback) {
    $$([
      $$.stepCall(task.step1),
      function($, result1) {
        if (result1) return $.end(null, result1)
      },
      $$.stepCall(task.step2),
      $$.stepCall(task.step3)
    ], callback)
  }

I would really love for a post to include a non-trivial problem implemented with promises, vanilla callbacks, and async (and I'd be happy to add a stepdown equivalent), and allow people to see for themselves (how in my opinion promises make code harder to read).

[1] http://stackoverflow.com/questions/11302271/how-to-properly-...

[2] https://github.com/Schoonology/stepdown (docs need updating, view tests for documentation)