Managing promise dependencies

Question

I\'m using Node.js and Bluebird to create some fairly complicated logic involving uncompressing a structured file, parsing JSON, creating and making changes to several Mongo

Answer 1

It is certainly not an antipattern to return promises within thens, flattening nested promises is a feature of the promise spec.

Here's a possible rewrite, though I'm not sure it's cleaner:

var doStuff = function () {

  promise1()
  .then(function (value1) {

    return promise2()
    .then(function (value2) {

      return promise3(value1)
      .then(successFunction)
      .finally(function() {
        cleanup(null, value1, value2);
      });

    })
    .finally(function() {
      cleanup(null, value1, null);
    });

  })
  .finally(function () {
    cleanup(null, null, null);
  });

};

Or another option, with atomic cleanup functions:

var doStuff = function () {

  promise1()
  .then(function (value1) {

    return promise2()
    .then(function (value2) {

      return promise3(value1)
      .then(successFunction)
      .finally(function() {
        cleanup3(value2);
      });

    })
    .finally(function() {
      cleanup2(value1);
    });

  })
  .finally(function (err) {
    cleanup1(err);
  });

};

Really, I feel like there's not much you can do to clean this up. Event with vanilla try/catches, the best possible pattern is pretty similar to these.

Answer 2

I find both answers currently provided nice but clumsy. They're both good but contain overhead I don't think you need to have. If you instead use promises as proxies you get a lot of things for free.

var doStuff = function () {
  var p1 = promise1();
  var p2 = p1.then(promise2);
  var p3 = p1.then(promise3); // if you actually need to wait for p2 here, do.
  return Promise.all([p1, p2, p3]).catch(function(err){
      // clean up based on err and state, can unwrap promises here
  });
};

Please do not use successFunction and such it is an anti-pattern and loses information. If you feel like you have to use successFunction you can write:

var doStuff = function () {
  var p1 = promise1();
  var p2 = p1.then(promise2);
  var p3 = p1.then(promise3); // if you actually need to wait for p2 here, do.
  Promise.join(p1, p2, p3, successFunction).catch(function(err){
      // clean up based on err and state, can unwrap promises here
  });
};

However, it is infinitely worse since it won't let the consumer handle errors they may be able to handle.

Answer 3

This question might be more appropriate for code review but here is how I'd approach it given this example:

var doStuff = function () {
  // Set up your promises based on their dependencies. In your example
  // promise2 does not use dependency1 so I left them unrelated.
  var dep1Promise = promise1();
  var dep2Promise = promise2();
  var dep3Promise = dependency1Promise.then(function(value){
    return promise3(value);
  });

  // Wait for all the promises the either succeed or error.
  allResolved([dep1Promise, dep2Promise, dep3Promise])
      .spread(function(dep1, dep2, dep3){

    var err = dep1.error || dep2.error || dep3.error;
    if (err){
      // If any errored, call the function you prescribed
      cleanupDependingOnSystemState(err, dep1.value, dep2.value);
    } else {
      // Call the success handler.
      successFunction(dep3.value);
    }
};

// Promise.all by default just fails on the first error, but since
// you want to pass any partial results to cleanupDependingOnSystemState,
// I added this helper.
function allResolved(promises){
  return Promise.all(promises.map(function(promise){
    return promise.then(function(value){
      return {value: value};
    }, function(err){
      return {error: err};
    });
  });
}

The use of allResolved is only because of your callback specifics, if you had a more general error handler, you could simply resolve using Promise.all directly, or even:

var doStuff = function () {
  // Set up your promises based on their dependencies. In your example
  // promise2 does not use dependency1 so I left them unrelated.
  var dep1Promise = promise1();
  var dep2Promise = promise2();
  var dep3Promise = dependency1Promise.then(function(value){
    return promise3(value);
  });

  dep3Promise.then(successFunction, cleanupDependingOnSystemState);
};