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Version: XState v5

Eventless (always) transitions

Eventless transitions are transitions that happen without an explicit event. These transitions are always taken when the transition is enabled.

Eventless transitions are specified on the always state property and often referred to as “always” transitions.

import { createMachine } from 'xstate';
const machine = createMachine({
  states: {
    form: {
      initial: 'valid',
      states: {
        valid: {},
        invalid: {},
      },
      always: {
        guard: 'isValid',
        target: 'valid',
      },
    },
  },
});

Eventless transitions and guards​

Eventless transitions are taken immediately after normal transitions are taken. They are only taken if enabled, for example, if their guards are true. This makes eventless transitions helpful in doing things when some condition is true.

Avoid infinite loops​

Eventless transitions with no target nor guard will cause an infinite loop. Transitions using guard and actions may run into an infinite loop if its guard keeps returning true.

You should define eventless transitions either with:

  • target
  • guard + target
  • guard + actions
  • guard + target + actions

If target is declared, the value should differ from the current state node.

When to use​

Eventless transitions can be helpful when a state change is necessary, but there is no specific trigger for that change.

import { createMachine } from 'xstate';

const machine = createMachine({
  id: 'kettle',
  initial: 'lukewarm',
  context: {
    temperature: 80,
  },
  states: {
    lukewarm: {
      on: {
        boil: { target: 'heating' },
      },
    },
    heating: {
      always: {
        guard: ({ context }) => context.temperature > 100,
        target: 'boiling',
      },
    },
    boiling: {
      entry: ['turnOffLight'],
      always: {
        guard: ({ context }) => context.temperature <= 100,
        target: 'heating',
      },
    },
  },
  on: {
    'temp.update': {
      actions: ['updateTemperature'],
    },
  },
});

Eventless transitions and TypeScript​

Eventless transitions can potentially be enabled by any event, so the event type is the union of all possible events.

const machine = createMachine({
  types: {} as {
    events: { type: 'greet'; message: string } | { type: 'submit' };
  },
  // ...
  always: {
    actions: ({ event }) => {
      event.type; // 'greet' | 'submit'
    },
    guard: ({ event }) => {
      event.type; // 'greet' | 'submit'
      return true;
    },
  },
});

Eventless transitions cheatsheet​

Cheatsheet: root eventless (always) transition​

import { createMachine } from 'xstate';

const machine = createMachine({
  always: {
    guard: 'isValid',
    actions: ['doSomething'],
  },
  // ...
});

Cheatsheet: state eventless (always) transition​

const machine = createMachine({
  initial: 'start',
  states: {
    start: {
      always: {
        guard: 'isValid',
        target: 'otherState',
      },
    },
    otherState: {
      /* ... */
    },
  },
});