Bacon.js Tutorial Part III : AJAX and Stuff

This is the next step in the Bacon.js tutorial series. I hope you've read Part II already! This time we're going to implement an "as you type" username availability check with AJAX. The steps are
  1. Create an EventStream of AJAX requests for use with jQuery.ajax()
  2. Create a stream of responses by issuing an AJAX request for each request event and capturing AJAX responses into the new stream.
  3. Define the usernameAvailable Property based on the results
  4. Some side-effects: disable the Register button if username is unavailable. Also, show a message.
I suggest you checkout the example code and switch to the tutorial-2 branch which will be the starting point for the coding part of this posting. If you just want to have a look at what we'ge got so far, have a peek.
So, at this point we've got a Property called username which represents the current value entered to the username text input field. We want to query for username availability each time this property changes. First, to get the stream of changes to the property we'll do this:
This will return an EventStream. The difference to the username Property itself is that there's no initial value (the empty string). Next, we'll transform this to a stream that provides jQuery compatible AJAX requests:
availabilityRequest = username.changes().map(function(user) { return { url: "/usernameavailable/" + user }})
The next step is extremely easy, using Bacon.UI.js:
availabilityResponse = availabilityRequest.ajax()
This maps the requests into AJAX responses. Looks very simple, but behind the scene it takes care of request/response ordering so that you'll only ever get the response of the latest issued request in that stream. This is where, with pure jQuery, we had to resort to keeping a counter variable for making sure we don't get the wrong result because of network delays.
So, what does the ajax() method in Bacon.UI look like? Does it do stuff with variables? Lets see.
Bacon.EventStream.prototype.ajax = function() {
  return this["switch"](function(params) { return Bacon.fromPromise($.ajax(params)) })
Not so complicated after all. But let's talk about flatMap now, for a while, so that you can build this kind of helpers yourself, too.

AJAX as a Promise

AJAX is asynchronous, hence the name. This is why we can't use map to convert requests into responses. Instead, each AJAX request/response should be modeled as a separate stream. And it can be done too. So, if you do
$.ajax({ url : "/usernameavailable/jack"})
you'll get a jQuery Deferred object. This object can be thought of as a Promise of a result. Using the Promise API, you can handle the asynchronous results by assigning a callback with the done(callback) function, as in
$.ajax({ url : "/usernameavailable/jack"}).done(function(result) { console.log(result)})
If you try this in you browser, you should see true printed to the console shortly. You can wrap any Promise into an EventStream using Bacon.fromPromise(promise). Hence, the following will have the same result:
Bacon.fromPromise($.ajax({ url : "/usernameavailable/jack"})).log()
This is how you make an EventStream of an AJAX request.

AJAX with flatMap

So now we have a stream of AJAX requests and the knowhow to create a new stream from a jQuery AJAX. Now we need to
  1. Create a response stream for each request in the request stream
  2. Collect the results of all the created streams into a single response stream
This is where flatMap comes in:
function toResultStream(request) {
  return Bacon.fromPromise($.ajax(request))
availabilityResponse = availabilityRequest.flatMap(toResultStream)
Now you'll have a new EventStream called availabilityResponse. What flatMap does is
  1. It calls your function for each value in the source stream
  2. It expects your function to return a new EventStream
  3. It collects the values of all created streams into the result stream
Like in this diagram.
So here we go. The only issue left is that flatMap doesn't care about response ordering. It spits out the results in the same order as they arrive. So, it may (and will) happen that
  1. Request A is sent
  2. Request B is sent
  3. Result of request B comes
  4. Result of request A comes
.. and your availabilityResponse stream will end with the wrong answer, because the latest response is not for the latest request. Fortunately there's a method for fixing this exact problem: Just replace flatMap with flatMapLatest (previously called "switch") and you're done.
Now that you know how it works, you may as well use the ajax() method that Bacon.UI provides:
availabilityResponse = availabilityRequest.ajax()

The easy part : side-effects

Let's show the "username not available" message. It's actually a stateful thing, so we'll convert the availabilityResponse stream into a new Property:
usernameAvailable = availabilityResponse.toProperty(true)
The boolean value is used to give the property a starting value. So now this property starts with the value true and after that reflects that value from the availabilityRequest stream. The visibility of the message element should actually equal to the negation of this property. So why not something like
usernameAvailable.not().onValue(setVisibility, unavailabilityLabel)
Once again, this is equivalent to
usernameAvailable.not().onValue(function(show) { setVisibility(unavailabilityLabel, show) })
... the idea in the former being that we partially-apply the setVisibility function: Bacon will call the function withunavailabilityLabel fixed as the first argument. The second argument to the function will be the value from theusernameAvailable.not() Property.
Finally, we'll also disable the Register button when the username is unavailable. This is done by changing
buttonEnabled = usernameEntered.and(fullnameEntered)
buttonEnabled = usernameEntered.and(fullnameEntered).and(usernameAvailable)
That's it for now. The result code can be found in the tutorial-3 branch.
Ideas for Part IV?