Promise-based Sequential Queue

sequence.js

// Queues multiple asynchronous processes to run sequentially via promises
// Process can be added to the queue while other processes are running

var Sequence = function() {};
Sequence.prototype.enqueue = function(fn) {
  this.tail = this.tail ? this.tail.finally(fn) : fn();
};

// Usage

var sequence = new Sequence();

sequence.enqueue(function() {
  var deferred = $q.defer();
  // do some async stuff
  deferred.resolve();
  return deferred.promise;
});

sequence.enqueue(function() {
  return doSomethingAsyncThatReturnsAPromise();
});

That's a nice idea, how do you use it? What happens when the last promise is fulfilled, are new enqueued items called directly? Does it all work even with then instead of finally?

I'm not expecting you to answer all these questions, just saving my questions for future use.