Javascript Finite State Machine

fsm.js

function FiniteStateMachine(states, transitions, initialState) {
  var state = initialState;

  this.getState = function() {
    return state;
  };

  this.transition = function(newState) {
    if (_.detect(transitions, function(transition) { return (transition[0] == state) && (transition[1] == newState); })) {
      if (('onExit' in states[state]) && (typeof states[state].onExit == 'function')) {
	states[state].onExit();
      }

      state = newState;

      if (('onEnter' in states[state]) && (typeof states[state].onEnter == 'function')) {
	states[state].onEnter();
      }
      return state;
    } else {
      throw "invalid transition";
    }
  };

  return this;
}