simple state machine

index.ts

interface Transition {
  name: string;
  from: string;
  to: string;
}

export interface Action {
  [actionName: string]: () => void;
}

interface State {
  init: string;
  transitions: Transition[];
  actions: Action;
}

interface Machine {
    definition: State,
    state: string,
    action: (actionName: string) => string
}

function createMachine<F extends State>(stateDefinition: F): Machine {
  const machine = {
    definition: stateDefinition,
    state: stateDefinition.init,
    action(actionName: string): string {
        const destinationState: string | undefined =
            this.definition.transitions.find(
            (transition: Transition) => transition.name === actionName)?.to;

      const action = this.definition.actions[actionName];
      typeof action === "function" && action();

      this.state = destinationState ?? this.state;
      return this.state;
    }
  };
  return machine;
}

const machine: Machine = createMachine<State>({
  init: "idle",
  transitions: [
    { name: "start", from: "idle", to: "pending" },
    { name: "success", from: "pending", to: "fulfilled" },
    { name: "failure", from: "pending", to: "rejected" }
  ],
  actions: {
    start: () => {
      console.log("action started");
    },
    success: () => {
      console.log("action successed");
    },
    failure: () => {
      console.log("action failured");
    }
  }
});

machine.state; // state: idle
machine.action("start"); // action started, state: pending 
machine.action("success"); // action successed, state: fulfilled
machine.action("failure"); // action failured, state: rejected