Callbacks And Promises

The idea of nonblocking execution is sweet for performance, but there are a couple competing styles. One is with continuations (callbacks), and the other with promises.

I find promises much easier to conceptualize and work with. You can combine collections of promises into super-promises, have them promised in series, etc. However, node libraries and many of the ORMs and other node modules use callbacks.

Callbacks To Promise

It's not too difficult to wrap callback-expecting functions to make promise-returning functions. My favorite promise library is when, but any module that implements the Promises/A+ spec should work.

// callback -> promise

var when = require('when');

function withCallbacks(args, error, success) {
  // module logic
}

function returnPromise(args) {
  var deferred = when.defer();
  withCallbacks(args, deferred.reject, deferred.resolve);
  return deferred.promise;
}

Promise To Callbacks

If you want to use a module that returns promises, but you prefer callbacks, you can wrap the functions to do that as well:

// promise -> callback

function returnPromise(args) {
  // module logic
}

function withCallbacks(args, error, success) {
  var promise = returnPromise(args);
  promise.then(success, error);
}