Example of a type safe finite-state-machine API in TypeScript.

example.ts

interface StateOne {
  type: "one";
  prevName?: string;
}

interface StateTwo {
  type: "two";
  name?: string;
}

type State = StateOne | StateTwo;

const two: StateTwo = { type: "two" };

const createWrapper = fsm<State>()({
  one: {
    toStateTwo: () => ({ type: "two" }),
    toStateTwoWithName: (_, opts: { name: string }) => ({ type: "two", name: opts.name })
  },
  two: {
    toStateOne: prevState => ({ type: "one", prevName: prevState.name })
  }
});

const wrappedTwo = createWrapper({ type: "two", name: "davina" });

const wrappedOne = wrappedTwo.to.toStateOne();
wrappedOne.to.toStateTwo();
wrappedOne.to.toStateTwoWithName({ name: "brad" });


const wrappedState = createWrapper({ type: "one" } as State);
type WrappedState = typeof wrappedState;

if (wrappedState.type === "two") {
  wrappedState.to.toStateOne()
}