Javascript Finite State Machine

NOTE: for a a sneak preview of what's coming next, read the documentation for v3.0.0-rc.1

A standalone library for finite state machines.

Download

Using npm:

npm install javascript-state-machine

Or download the source from state-machine.js, or the minified version

Usage

Include state-machine.js in your web application:

<script src='state-machine.js'></script>

Or for npm:

var StateMachine = require('javascript-state-machine');

In its simplest form, create a standalone state machine using:

var fsm = StateMachine.create({
  initial: 'green',
  events: [
    { name: 'warn',  from: 'green',  to: 'yellow' },
    { name: 'panic', from: 'yellow', to: 'red'    },
    { name: 'calm',  from: 'red',    to: 'yellow' },
    { name: 'clear', from: 'yellow', to: 'green'  }
]});

... will create an object with a method for each event:

• fsm.warn() - transition from 'green' to 'yellow'
• fsm.panic() - transition from 'yellow' to 'red'
• fsm.calm() - transition from 'red' to 'yellow'
• fsm.clear() - transition from 'yellow' to 'green'

along with the following members:

• fsm.current - contains the current state
• fsm.is(s) - return true if state s is the current state
• fsm.can(e) - return true if event e can be fired in the current state
• fsm.cannot(e) - return true if event e cannot be fired in the current state
• fsm.transitions() - return list of events that are allowed from the current state
• fsm.states() - return list of all possible states.

Multiple 'from' and 'to' states for a single event

If an event is allowed from multiple states, and always transitions to the same state, then simply provide an array of states in the from attribute of an event. However, if an event is allowed from multiple states, but should transition to a different state depending on the current state, then provide multiple event entries with the same name:

var fsm = StateMachine.create({
  initial: 'hungry',
  events: [
    { name: 'eat',  from: 'hungry',                                to: 'satisfied' },
    { name: 'eat',  from: 'satisfied',                             to: 'full'      },
    { name: 'eat',  from: 'full',                                  to: 'sick'      },
    { name: 'rest', from: ['hungry', 'satisfied', 'full', 'sick'], to: 'hungry'    },
]});

This example will create an object with 2 event methods:

• fsm.eat()
• fsm.rest()

The rest event will always transition to the hungry state, while the eat event will transition to a state that is dependent on the current state.

NOTE: The rest event could use a wildcard '*' for the 'from' state if it should be allowed from any current state.

NOTE: The rest event in the above example can also be specified as multiple events with the same name if you prefer the verbose approach.

Callbacks

4 types of callback are available by attaching methods to your StateMachine using the following naming conventions:

• onbeforeEVENT - fired before the event
• onleaveSTATE - fired when leaving the old state
• onenterSTATE - fired when entering the new state
• onafterEVENT - fired after the event

(using your specific EVENT and STATE names)

For convenience, the 2 most useful callbacks can be shortened:

• onEVENT - convenience shorthand for onafterEVENT
• onSTATE - convenience shorthand for onenterSTATE

In addition, 4 general-purpose callbacks can be used to capture all event and state changes:

• onbeforeevent - fired before any event
• onleavestate - fired when leaving any state
• onenterstate - fired when entering any state
• onafterevent - fired after any event

All callbacks will be passed the same arguments:

• event name
• from state
• to state
• (followed by any arguments you passed into the original event method)

Callbacks can be specified when the state machine is first created:

var fsm = StateMachine.create({
  initial: 'green',
  events: [
    { name: 'warn',  from: 'green',  to: 'yellow' },
    { name: 'panic', from: 'yellow', to: 'red'    },
    { name: 'calm',  from: 'red',    to: 'yellow' },
    { name: 'clear', from: 'yellow', to: 'green'  }
  ],
  callbacks: {
    onpanic:  function(event, from, to, msg) { alert('panic! ' + msg);               },
    onclear:  function(event, from, to, msg) { alert('thanks to ' + msg);            },
    ongreen:  function(event, from, to)      { document.body.className = 'green';    },
    onyellow: function(event, from, to)      { document.body.className = 'yellow';   },
    onred:    function(event, from, to)      { document.body.className = 'red';      },
  }
});

fsm.panic('killer bees');
fsm.clear('sedatives in the honey pots');
...

Additionally, they can be added and removed from the state machine at any time:

fsm.ongreen      = null;
fsm.onyellow     = null;
fsm.onred        = null;
fsm.onenterstate = function(event, from, to) { document.body.className = to; };

The order in which callbacks occur is as follows:

assume event go transitions from red state to green

• onbeforego - specific handler for the go event only
• onbeforeevent - generic handler for all events
• onleavered - specific handler for the red state only
• onleavestate - generic handler for all states
• onentergreen - specific handler for the green state only
• onenterstate - generic handler for all states
• onaftergo - specific handler for the go event only
• onafterevent - generic handler for all events

NOTE: the legacy onchangestate handler has been deprecated and will be removed in a future version

You can affect the event in 3 ways:

• return false from an onbeforeEVENT handler to cancel the event.
• return false from an onleaveSTATE handler to cancel the event.
• return ASYNC from an onleaveSTATE handler to perform an asynchronous state transition (see next section)

Asynchronous State Transitions

Sometimes, you need to execute some asynchronous code during a state transition and ensure the new state is not entered until your code has completed.

A good example of this is when you transition out of a menu state, perhaps you want to gradually fade the menu away, or slide it off the screen and don't want to transition to your game state until after that animation has been performed.

You can now return StateMachine.ASYNC from your onleavestate handler and the state machine will be 'put on hold' until you are ready to trigger the transition using the new transition() method.

For example, using jQuery effects:

var fsm = StateMachine.create({

  initial: 'menu',

  events: [
    { name: 'play', from: 'menu', to: 'game' },
    { name: 'quit', from: 'game', to: 'menu' }
  ],

  callbacks: {

    onentermenu: function() { $('#menu').show(); },
    onentergame: function() { $('#game').show(); },

    onleavemenu: function() {
      $('#menu').fadeOut('fast', function() {
        fsm.transition();
      });
      return StateMachine.ASYNC; // tell StateMachine to defer next state until we call transition (in fadeOut callback above)
    },

    onleavegame: function() {
      $('#game').slideUp('slow', function() {
        fsm.transition();
      };
      return StateMachine.ASYNC; // tell StateMachine to defer next state until we call transition (in slideUp callback above)
    }

  }
});

NOTE: If you decide to cancel the ASYNC event, you can call fsm.transition.cancel();

State Machine Classes

You can also turn all instances of a class into an FSM by applying the state machine functionality to the prototype, including your callbacks in your prototype, and providing a startup event for use when constructing instances:

MyFSM = function() {    // my constructor function
  this.startup();
};

MyFSM.prototype = {

  onpanic: function(event, from, to) { alert('panic');        },
  onclear: function(event, from, to) { alert('all is clear'); },

  // my other prototype methods

};

StateMachine.create({
  target: MyFSM.prototype,
  events: [
    { name: 'startup', from: 'none',   to: 'green'  },
    { name: 'warn',    from: 'green',  to: 'yellow' },
    { name: 'panic',   from: 'yellow', to: 'red'    },
    { name: 'calm',    from: 'red',    to: 'yellow' },
    { name: 'clear',   from: 'yellow', to: 'green'  }
  ]});

This should be easy to adjust to fit your appropriate mechanism for object construction.

NOTE: the startup event can be given any name, but it must be present in some form to ensure that each instance constructed is initialized with its own unique current state.

Initialization Options

How the state machine should initialize can depend on your application requirements, so the library provides a number of simple options.

By default, if you don't specify any initial state, the state machine will be in the 'none' state and you would need to provide an event to take it out of this state:

var fsm = StateMachine.create({
  events: [
    { name: 'startup', from: 'none',  to: 'green' },
    { name: 'panic',   from: 'green', to: 'red'   },
    { name: 'calm',    from: 'red',   to: 'green' },
]});
alert(fsm.current); // "none"
fsm.startup();
alert(fsm.current); // "green"

If you specify the name of your initial state (as in all the earlier examples), then an implicit startup event will be created for you and fired when the state machine is constructed.

var fsm = StateMachine.create({
  initial: 'green',
  events: [
    { name: 'panic', from: 'green', to: 'red'   },
    { name: 'calm',  from: 'red',   to: 'green' },
]});
alert(fsm.current); // "green"

If your object already has a startup method you can use a different name for the initial event

var fsm = StateMachine.create({
  initial: { state: 'green', event: 'init' },
  events: [
    { name: 'panic', from: 'green', to: 'red'   },
    { name: 'calm',  from: 'red',   to: 'green' },
]});
alert(fsm.current); // "green"

Finally, if you want to wait to call the initial state transition event until a later date you can defer it:

var fsm = StateMachine.create({
  initial: { state: 'green', event: 'init', defer: true },
  events: [
    { name: 'panic', from: 'green', to: 'red'   },
    { name: 'calm',  from: 'red',   to: 'green' },
]});
alert(fsm.current); // "none"
fsm.init();
alert(fsm.current); // "green"

Of course, we have now come full circle, this last example is pretty much functionally the same as the first example in this section where you simply define your own startup event.

So you have a number of choices available to you when initializing your state machine.

IMPORTANT NOTE: if you are using the pattern described in the previous section "State Machine Classes", and wish to declare an initial state in this manner, you MUST use the defer: true attribute and manually call the starting event in your constructor function. This will ensure that each instance gets its own unique current state, rather than an (unwanted) shared current state on the prototype object itself.

Handling Failures

By default, if you try to call an event method that is not allowed in the current state, the state machine will throw an exception. If you prefer to handle the problem yourself, you can define a custom error handler:

var fsm = StateMachine.create({
  initial: 'green',
  error: function(eventName, from, to, args, errorCode, errorMessage, originalException) {
    return 'event ' + eventName + ' was naughty :- ' + errorMessage;
  },
  events: [
    { name: 'panic', from: 'green', to: 'red'   },
    { name: 'calm',  from: 'red',   to: 'green' },
]});
alert(fsm.calm()); // "event calm was naughty :- event not allowed in current state green"

Contributing

> git clone git@github.com:jakesgordon/javascript-state-machine
> cd javascript-state-machine

> npm install      # install dev dependencies
> npm start        # run a local dev server

• Source code - state-machine.js
• Minified code - state-machine.min.js (build with npm run minify)
• Browse demo at /
• Run tests in browser at /test/
• Run tests in console with npm test
• Please include tests with pull requests.

Related Links

• You can find the code on github
• You can find a working demo here
• v2.3 release announcement
• v2.2 release announcement
• v2.1 release announcement
• v2.0 release announcement
• v1.2 release announcement
• v1.0 release announcement

Release Notes

See RELEASE NOTES file.

License

See LICENSE file.

Contact

If you have any ideas, feedback, requests or bug reports, you can reach me at jake@codeincomplete.com, or via my website: Code inComplete