state-machine-basic.ts

interface Machine<State extends string, Event extends string> {
  initialState: State;
  transition(currentState: State, event: Event): State;
}

interface MachineDefinition<State extends string, Event extends string> {
  initialState: State;
  states: Record<State, Record<Event, State>>;
}

function createMachine<State extends string, Event extends string>(
  definition: MachineDefinition<State, Event>
): Machine<State, Event> {
  return {
    initialState: definition.initialState,
    transition(currentState, event) {
      const currentStateDefinition = definition.states[currentState];
      const destinationState = currentStateDefinition[event];

      if (!destinationState) {
        return currentState;
      }

      return destinationState;
    },
  };
}

const machine = createMachine({
  initialState: "green",
  states: {
    green: {
      TIME_ELAPSED: "yellow",
    },
    yellow: {
      TIME_ELAPSED: "red",
    },
    red: {
      TIME_ELAPSED: "green",
    },
  },
});

let state = machine.initialState;
console.log("current state:", state);
state = machine.transition(state, "TIME_ELAPSED");
console.log("current state:", state);
state = machine.transition(state, "TIME_ELAPSED");
console.log("current state:", state);
state = machine.transition(state, "TIME_ELAPSED");
console.log("current state:", state);