XState Docs

# Packages

🤖 xstate - Core finite state machine and statecharts library + interpreter
🔬 @xstate/fsm (opens new window) - Minimal finite state machine library
📉 @xstate/graph (opens new window) - Graph traversal utilities for XState
⚛️ @xstate/react (opens new window) - React hooks and utilities for using XState in React applications
💚 @xstate/vue (opens new window) - Vue composition functions and utilities for using XState in Vue applications
🎷 @xstate/svelte (opens new window) - Svelte utilities for using XState in Svelte applications
🥏 @xstate/solid (opens new window) - Solid hooks and utilities for using XState in Solid applications
✅ @xstate/test (opens new window) - Model-Based-Testing utilities (using XState) for testing any software
🔍 @xstate/inspect (opens new window) - Inspection utilities for XState

# 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 stately.ai/viz (opens new window)

See the code

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
Why?
Finite State Machines
Hierarchical (Nested) State Machines
Parallel State Machines
History States

# Visualizer

Visualize, simulate, inspect, and share your statecharts in XState Viz (opens new window)

stately.ai/viz (opens new window)

# 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 (opens new window) (🎥 video (opens new window)) or check out these resources for learning about the importance of finite state machines and statecharts in user interfaces:

Statecharts - A Visual Formalism for Complex Systems (opens new window) by David Harel
The World of Statecharts (opens new window) by Erik Mogensen
Pure UI (opens new window) by Guillermo Rauch
Pure UI Control (opens new window) by Adam Solove
Spectrum - Statecharts Community (opens new window) (For XState specific questions, please use the GitHub Discussions (opens new window))

# Finite State Machines

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

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

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

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' }

# Sponsors

Special thanks to the sponsors who support this open-source project:

Transloadit

# SemVer Policy

We understand the importance of the public contract and do not intend to release any breaking changes to the runtime API in a minor or patch release. We consider this with any changes we make to the XState libraries and aim to minimize their effects on existing users.

# Breaking changes

XState executes much of the user logic itself. Therefore, almost any change to its behavior might be considered a breaking change. We recognize this as a potential problem but believe that treating every change as a breaking change is not practical. We do our best to implement new features thoughtfully to enable our users to implement their logic in a better, safer way.

Any change could affect how existing XState machines behave if those machines are using particular configurations. We do not introduce behavior changes on a whim and aim to avoid making changes that affect most existing machines. But we reserve the right to make some behavior changes in minor releases. Our best judgment of the situation will always dictate such changes. Please always read our release notes before deciding to upgrade.

# TypeScript changes

We also reserve a similar right to adjust declared TypeScript definitions or drop support for older versions of TypeScript in a minor release. The TypeScript language itself evolves quickly and often introduces breaking changes in its minor releases. Our team is also continuously learning how to leverage TypeScript more effectively - and the types improve as a result.

For these reasons, it is impractical for our team to be bound by decisions taken when an older version of TypeScript was its latest version or when we didn’t know how to declare our types in a better way. We won’t introduce declaration changes often - but we are more likely to do so than with runtime changes.

# Packages

Most of the packages in the XState family declare a peer dependency on XState itself. We’ll be cautious about maintaining compatibility with already-released packages when releasing a new version of XState, but each release of packages depending on XState will always adjust the declared peer dependency range to include the latest version of XState. For example, you should always be able to update xstate without @xstate/react. But when you update @xstate/react, we highly recommend updating xstate too.