gistfile1.js

// Returns a single promise that emits success or error when the given promises
// are complete.
//
//   var promises = require('promise-group')
//   promises.group(promise1, promise2).addCallback(...)
//
exports.group = function() {
  return new PromiseGroup(Array.prototype.slice.call(arguments)).promise;
}
 
function PromiseGroup(promises) {
  var promise    = new process.Promise(),
      promiseCount = 0,
      started = false,
      responses  = {} // holds values and their name, 
                      // we don't know what order they will finish

  for (name in promises) {

    var p = promises[name];
    promiseCount++;

    p.addCallback(function(value) {
      promiseCount--;
      responses[name] = value;
      if (started && promiseCount == 0) {
        promise.emitSuccess(responses);
      }
    })
    .addErrback(function () {
      promise.emitError();
    });
  }

  // Don't allow promises to start emitting success on the
  // parent promise unless we've queued them all up.
  started = true;

  // check in case all the promises finished before we got here.
  if (promiseCount == 0) {
    promise.emitSuccess(responses);
  }

  this.promise = promise;
}