xstate

Functional, stateless JavaScript finite state machines and statecharts.

📖 Read the documentation!

Why?

In short, 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:

• Statecharts - A Visual Formalism for Complex Systems by David Harel
• The World of Statecharts by Erik Mogensen
• Pure UI by Guillermo Rauch
• Pure UI Control by Adam Solove

Installation

1. npm install xstate --save
2. import { Machine } from 'xstate';

Finite State Machines

import { Machine } from 'xstate';

const lightMachine = Machine({
  key: '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

import { Machine } from 'xstate';

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

const lightMachine = Machine({
  key: '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 States

const wordMachine = Machine({
  parallel: true,
  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

To provide full flexibility, history states are more arbitrarily defined than the original statechart specification. To go to a history state, use the special key $history.

const paymentMachine = Machine({
  initial: 'method',
  states: {
    method: {
      initial: 'cash',
      states: {
        cash: { on: { SWITCH_CHECK: 'check' } },
        check: { on: { SWITCH_CASH: 'cash' } }
      },
      on: { NEXT: 'review' }
    },
    review: {
      on: { PREVIOUS: 'method.$history' }
    }
  }
});

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' }