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

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);
};