error-handling.md

Error Handling

Invalid Transitions

By default, if you try to fire a transition 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 onInvalidTransition handler:

const fsm = new StateMachine({
  init: 'A',
  transitions: [
    { name: 'step', from: 'A', to: 'B' },
    { name: 'reset', from: 'B', to: 'A' },
  ],
  lifecycles: {
    onInvalidTransition: function (transition, from, to) {
      throw new Exception('transition not allowed from that state');
    },
  },
});

fsm.state; // 'A'
fsm.can('step'); // true
fsm.can('reset'); // false

fsm.reset(); //  <-- throws "transition not allowed from that state"

Pending Transitions

By default, if you try to fire a transition during a Lifecycle Event for a pending transition, the state machine will throw an exception. If you prefer to handle the problem yourself, you can define a custom onPendingTransition handler:

const fsm = new StateMachine({
  init: 'A',
  transitions: [
    { name: 'step', from: 'A', to: 'B' },
    { name: 'step', from: 'B', to: 'C' },
  ],
  lifecycles: {
    onLeaveA: function () {
      this.step(); //  <-- uh oh, trying to transition from within a lifecycle event is not allowed
    },
    onPendingTransition: function (transition, from, to) {
      throw new Exception('transition already in progress');
    },
  },
});

fsm.state; // 'A'
fsm.can('step'), // true
  fsm.step(); //  <-- throws "transition already in progress"