StateMachine.ts

type StateMap<TStates extends string, TActions extends string> = {
    [S in TStates]: { //for every string option in TStates:
        [A in TActions]?: TStates; //optionally map to the next state if it's a valid action there
    }
}; //AKA: Record<TStates, Partial<Record<TActions, TStates>>>;

class StateMachine<TStates extends string, TActions extends string>{
    constructor(private _currentState: TStates, private stateMap: StateMap<TStates, TActions>) { }

    get currentState() {
        return this._currentState;
    }

    advance(action: TActions) {
        let f = this.stateMap[this._currentState][action];
        let next = f || this._currentState;
        console.info(`${action}(${this._currentState}) => ${next}`);
        this._currentState = next;
    }
}


//client code

type TurnstileStates = "unlocked" | "locked" | "alarm";
type TurnstileActions = "coin" | "push" | "reset";

let s = new StateMachine<TurnstileStates, TurnstileActions>("unlocked",
    { // the point of all those generics:these action and state names are type checked and code-completed
        unlocked: {
            push: "locked"
        },
        locked: {
            coin: "unlocked",
            push: "alarm"
        },
        alarm: {
            reset: "locked"
        }
    });

s.advance("push");
s.advance("coin");
s.advance("push");
s.advance("push");
s.advance("push");
s.advance("coin");
s.advance("reset");
s.advance("reset");
s.advance("coin");

test it out at the typescript playground