Stateful is a simple implementation of the State Pattern for JavaScript.

stateful.js

// Stateful is a simple implementation of the state pattern for javascript.
//
// Read more on this design pattern here:
// -> http://sourcemaking.com/design_patterns/state
//
// Initialize Stateful by passing it an object, the name of the initial state
// (defaults to "default"), and an optional hash of interfaces that will be
// applied for each state. If these interfaces are not passed, they default to
// the object's constructor's States property. So, for example:
//
// Example:
//
//     function TrafficLight() {
//       this.state = new Stateful(this, "stop");
//     }
//
//     TrafficLight.States = {
//       stop: {
//         color: "red",
//         time:  8,
//
//         next: function() {
//           this.state.transition("go");
//         },
//
//         onEnterState: function() {
//           // Turn on traffic camera to see who crosses on a red light
//         },
//
//         onExitState: function() {
//           // Turn off traffic camera
//         }
//       },
//
//       go: {
//         color: "green",
//         time:  10,
//
//         next: function() {
//           this.state.transition("caution");
//         }
//       },
//
//       caution: {
//         color: "yellow",
//         time:  2,
//
//         next:  function() {
//           this.state.transition("stop");
//         }
//       }
//     }
//
//     var light = new TrafficLight();
//     light.color //=> "red"
//     light.next()
//     light.color //=> "green"
//     light.next()
//     light.color //=> "yellow"
//     light.next()
//     light.color //=> "red"
//
// Each state interface can have "special" `onEnterState` / `onExitState`
// methods that get called automatically whenever you switch states.
//

function Stateful(object, initialState, interfaces, onInitialize) {
  var currentState;
  this.interfaces = interfaces = interfaces || object.constructor.States;

  if (typeof interfaces == "undefined") {
    throw "An object with the set of interfaces for each state is required";
  }

  function trigger() {
    if (typeof object.trigger == "function") {
      object.trigger.apply(object, arguments);
    }
  }

  function applyState(state) {
    var previousInterface = interfaces[currentState];
    var newInterface = interfaces[state];

    if (typeof newInterface == "undefined") {
      throw "Invalid state: " + state;
    }

    if (previousInterface) {
      trigger("state:exit", currentState);

      if (typeof previousInterface.onExitState == "function") {
        object.onExitState();
      }

      for (property in previousInterface) {
        delete object[property];
      }
      delete object["onEnterState"];
      delete object["onExitState"];

      trigger("state:exited", currentState);
    }

    trigger("state:enter", state);

    for (property in newInterface) {
      object[property] = newInterface[property]
    }

    if (typeof newInterface.onEnterState == "function") {
      object.onEnterState();
    }

    trigger("state:entered", state);
  }

  function transitionTo(state) {
    applyState(state);
    currentState = state;
    trigger("state:change");
  }

  function isCurrentState(state) {
    return currentEvents === state;
  }

  if (typeof onInitialize == "function") {
    onInitialize(this, object);
  }

  transitionTo(initialState || "default");

  return {
    is: isCurrentState,
    transition: transitionTo
  };
}