example.js

const locked = createState();
const opened = createState();

const push = createAction();
const coin = createAction();
const hack = createAction();

const turnstile = createMachine(locked, {
  [locked]: {
    [coin]: () => opened(0),
    [hack]: (_, coins) => opened(coins)
  },
  [opened]: {
    [coin]: coins => opened(coins + 1),
    [push]: coins => (coins === 0 ? locked : opened(coins - 1))
  }
});

const turnstileInstance = turnstile.create();

turnstileInstance.subscribe({
  [locked]() {
    console.log(`=> locked`);
  },
  [opened](n) {
    console.log(`=> opened(${n})`);
  }
}); //                                  => locked

turnstileInstance.dispatch(coin); // ​​​​​   => opened(0)​​​​​
turnstileInstance.dispatch(coin); // ​​​​   ​=> opened(1)​​​​​
turnstileInstance.dispatch(push); // ​   ​​​​=> opened(0)​​​​​
turnstileInstance.dispatch(push); //    => locked
turnstileInstance.dispatch(hack(5)); // => opened(5)

fsm.js

const createAction = () => {
  const id = Symbol();

  const action = payload => ({
    payload,
    id,
    [Symbol.toPrimitive]: () => id
  });

  return Object.assign(action, {
    id,
    [Symbol.toPrimitive]: () => id
  });
};

const createState = () => {
  const id = Symbol();

  const state = value => ({
    value,
    id,
    [Symbol.toPrimitive]: () => id
  });

  return Object.assign(state, {
    id,
    [Symbol.toPrimitive]: () => id
  });
};

const createMachine = (initialState, scheme) => {
  return {
    create() {
      const subscribers = [];
      let state = initialState;

      function getState() {
        return state;
      }

      function dispatch(action) {
        if (action in scheme[state]) {
          state = scheme[state][action](state.value, action.payload);

          for (const subscriber of subscribers) {
            subscriber[state](state.value);
          }
        }
      }

      function subscribe(subscriber) {
        subscribers.push(subscriber);
        subscriber[state](state.value);
      }

      return {
        getState,
        dispatch,
        subscribe
      };
    }
  };
};