type TypesMap = {
    [0]: { foo: 'bar'; };
    [1]: { a: 'b'; };
};

type P<T extends keyof TypesMap> = { t: T; } & TypesMap[T];

type TypeHandlers = {
    [T in keyof TypesMap]?: (p: P<T>) => void;
};

const typeHandlers: TypeHandlers = {
    [0]: (p) => console.log(p.foo),
    [1]: (p) => console.log(p.a),
};

const onSomeEvent = <T extends keyof TypesMap>(p: P<T>) =>
    typeHandlers[p.t]?.(p);