/xstate

State machines and statecharts for the modern web.

Primary LanguageTypeScriptMIT LicenseMIT


XState
JavaScript state machines and statecharts

npm version

JavaScript and TypeScript finite state machines and statecharts for the modern web.

📖 Read the documentation

💙 Explore our catalogue of examples

📑 Adheres to the SCXML specification

💬 Chat on the Stately Discord Community

Packages

Templates

Get started by forking one of these templates on CodeSandbox:

Super quick start

npm install xstate
import { createMachine, interpret } from 'xstate';

// Stateless machine definition
// machine.transition(...) is a pure function used by the interpreter.
const toggleMachine = createMachine({
  id: 'toggle',
  initial: 'inactive',
  states: {
    inactive: { on: { TOGGLE: 'active' } },
    active: { on: { TOGGLE: 'inactive' } }
  }
});

// Machine instance with internal state
const toggleService = interpret(toggleMachine)
  .onTransition((state) => console.log(state.value))
  .start();
// => 'inactive'

toggleService.send('TOGGLE');
// => 'active'

toggleService.send('TOGGLE');
// => 'inactive'

Promise example

📉 See the visualization on xstate.js.org/viz

import { createMachine, interpret, assign } from 'xstate';

const fetchMachine = createMachine({
  id: 'Dog API',
  initial: 'idle',
  context: {
    dog: null
  },
  states: {
    idle: {
      on: {
        FETCH: 'loading'
      }
    },
    loading: {
      invoke: {
        id: 'fetchDog',
        src: (context, event) =>
          fetch('https://dog.ceo/api/breeds/image/random').then((data) =>
            data.json()
          ),
        onDone: {
          target: 'resolved',
          actions: assign({
            dog: (_, event) => event.data
          })
        },
        onError: 'rejected'
      },
      on: {
        CANCEL: 'idle'
      }
    },
    resolved: {
      type: 'final'
    },
    rejected: {
      on: {
        FETCH: 'loading'
      }
    }
  }
});

const dogService = interpret(fetchMachine)
  .onTransition((state) => console.log(state.value))
  .start();

dogService.send('FETCH');

Visualizer

Visualize, simulate, inspect, and share your statecharts in XState Viz

XState Viz

stately.ai/viz

Why?

Statecharts are a formalism for modeling stateful, reactive systems. This is useful for declaratively describing the behavior of your application, from the individual components to the overall application logic.

Read 📽 the slides (🎥 video) or check out these resources for learning about the importance of finite state machines and statecharts in user interfaces:

Finite State Machines

Finite states
Open in Stately Viz

import { createMachine } from 'xstate';

const lightMachine = createMachine({
  id: 'light',
  initial: 'green',
  states: {
    green: {
      on: {
        TIMER: 'yellow'
      }
    },
    yellow: {
      on: {
        TIMER: 'red'
      }
    },
    red: {
      on: {
        TIMER: 'green'
      }
    }
  }
});

const currentState = 'green';

const nextState = lightMachine.transition(currentState, 'TIMER').value;

// => 'yellow'

Hierarchical (Nested) State Machines

Hierarchical states
Open in Stately Viz

import { createMachine } from 'xstate';

const pedestrianStates = {
  initial: 'walk',
  states: {
    walk: {
      on: {
        PED_TIMER: 'wait'
      }
    },
    wait: {
      on: {
        PED_TIMER: 'stop'
      }
    },
    stop: {}
  }
};

const lightMachine = createMachine({
  id: 'light',
  initial: 'green',
  states: {
    green: {
      on: {
        TIMER: 'yellow'
      }
    },
    yellow: {
      on: {
        TIMER: 'red'
      }
    },
    red: {
      on: {
        TIMER: 'green'
      },
      ...pedestrianStates
    }
  }
});

const currentState = 'yellow';

const nextState = lightMachine.transition(currentState, 'TIMER').value;
// => {
//   red: 'walk'
// }

lightMachine.transition('red.walk', 'PED_TIMER').value;
// => {
//   red: 'wait'
// }

Object notation for hierarchical states:

// ...
const waitState = lightMachine.transition({ red: 'walk' }, 'PED_TIMER').value;

// => { red: 'wait' }

lightMachine.transition(waitState, 'PED_TIMER').value;

// => { red: 'stop' }

lightMachine.transition({ red: 'stop' }, 'TIMER').value;

// => 'green'

Parallel State Machines

Parallel states
Open in Stately Viz

const wordMachine = createMachine({
  id: 'word',
  type: 'parallel',
  states: {
    bold: {
      initial: 'off',
      states: {
        on: {
          on: { TOGGLE_BOLD: 'off' }
        },
        off: {
          on: { TOGGLE_BOLD: 'on' }
        }
      }
    },
    underline: {
      initial: 'off',
      states: {
        on: {
          on: { TOGGLE_UNDERLINE: 'off' }
        },
        off: {
          on: { TOGGLE_UNDERLINE: 'on' }
        }
      }
    },
    italics: {
      initial: 'off',
      states: {
        on: {
          on: { TOGGLE_ITALICS: 'off' }
        },
        off: {
          on: { TOGGLE_ITALICS: 'on' }
        }
      }
    },
    list: {
      initial: 'none',
      states: {
        none: {
          on: { BULLETS: 'bullets', NUMBERS: 'numbers' }
        },
        bullets: {
          on: { NONE: 'none', NUMBERS: 'numbers' }
        },
        numbers: {
          on: { BULLETS: 'bullets', NONE: 'none' }
        }
      }
    }
  }
});

const boldState = wordMachine.transition('bold.off', 'TOGGLE_BOLD').value;

// {
//   bold: 'on',
//   italics: 'off',
//   underline: 'off',
//   list: 'none'
// }

const nextState = wordMachine.transition(
  {
    bold: 'off',
    italics: 'off',
    underline: 'on',
    list: 'bullets'
  },
  'TOGGLE_ITALICS'
).value;

// {
//   bold: 'off',
//   italics: 'on',
//   underline: 'on',
//   list: 'bullets'
// }

History States

History state
Open in Stately Viz

const paymentMachine = createMachine({
  id: 'payment',
  initial: 'method',
  states: {
    method: {
      initial: 'cash',
      states: {
        cash: { on: { SWITCH_CHECK: 'check' } },
        check: { on: { SWITCH_CASH: 'cash' } },
        hist: { type: 'history' }
      },
      on: { NEXT: 'review' }
    },
    review: {
      on: { PREVIOUS: 'method.hist' }
    }
  }
});

const checkState = paymentMachine.transition('method.cash', 'SWITCH_CHECK');

// => State {
//   value: { method: 'check' },
//   history: State { ... }
// }

const reviewState = paymentMachine.transition(checkState, 'NEXT');

// => State {
//   value: 'review',
//   history: State { ... }
// }

const previousState = paymentMachine.transition(reviewState, 'PREVIOUS').value;

// => { method: 'check' }