app.js

// This file is responsible for actually creating the objects, and kicking things off.
// We create a simple object with a run method (which Poller expects) that when called,
// calls checker.check().
define('MyApp', ['Poller, StatusChecker'], function(Poller, StatusChecker) {
  var checker = new StatusChecker({ url: 'http://localhost:3000/foobar', limit: 5000 }),
      poller  = new Poller({
        run: function() {
          return checker.check(function(status) {
              if (status == 200) {
                return true;
              } else {
                return false;
              }
          });
        }
      }, { delay: 5000 });

  poller.poll();
});

poller.js

// Poller knows nothing about ajax requests. It only know how to maintain state
// over its internal data, which it encapsulates. It expects a 'Pollable' with
// a `run` method that returns true or false. If true, we reset delay, else we
// double it.
define('Poller', function() {
    function Poller(pollable, config) {
      this.pollable = pollable;
      this.originalDelay = config.delay;
      this.threshold = config.threshold;

      this.delay = this.originalDelay;
    }

    Poller.prototype.get = function() {
      return this.delay;
    };

    Poller.prototype.increment = function() {
      this.delay = (this.delay < this.threshold) ? this.delay * 2 : this.delay;
    };

    Poller.prototype.reset = function() {
      this.delay = this.originalDelay;
    };

    Poller.prototype.poll = function() {
      window.setTimeout($.proxy(function() {
        if (this.pollable.run()) {
          this.reset();
        } else {
          this.increment();
        }
      }, this), this.get());
    };

    Poller.defaults = {
      delay: 5000,
      threshold: 20000
    };

    return function(pollable, config) {
      return new Poller(pollable, $.extend(Poller.defaults, config));
    };
});

status_checker.js

// StatusChecker is a very simple object that takes a url, and everytime
// you call .check(callback), makes a request to the url, yielding the status
define('StatusChecker', ['module/bootstrap'], function(news) {
  var $ = news.$;

  function StatusChecker(config) {
    this.config = config;
  }

  StatusChecker.prototype.check = function(callback) {
    $.ajax(this.config.url, { timeout: this.config.limit })
     .done(function(data, status, xhr) { callback(xhr.status); });
     .fail(function(xhr, status, error) { callback(xhr.status); });
  };

  StatusChecker.defaults = {
    limit: 5000
  };

  return function(config) {
    return new StatusChecker($.extend(StatusChecker.defaults, config));
  };
});

I thought that by initially thinking in an OO way, I could work out where the responsibilities were, so that I could better approach looking at it functionally. Turns out, it didn't really help me with the functional stuff at all!