World's Tiniest JavaScript State Machine

tiny-state-machine.js

var StateMachine = {
  // Register valid states for this object.
  __states__: {
    'success': ['failure'],
    'failure': ['success']
  },

  // Set initial state.
  __state__: 'success',

  // Get or set the state for this event.
  state: function (state) {
    var nowState = this.__state__;

    // If it's a state `get`, or there is no change, return the value.
    if (!state || nowState === state) return nowState;

    var states = this.__states__;
    var transitions = states[nowState];

    // Check if this is a valid transition.
    if (
      // If the state given does not show up in the registry of valid states
      !states[state] ||
      (
        // ...or if the transition requested is invalid
        transitions.indexOf(state) === -1 &&
        // ...and there is no wildcard operator (transition to any)
        transitions.indexOf('*') === -1
      )
    ) throw new Error('Invalid state transition');

    return this.__state__ = state;
  }
};