my-snippet.ts

export interface Action<
  T extends string
> {
  type: T;
}

export interface ActionWithPayload<
  T extends string,
  P
> extends Action<T> {
  payload: P;
}

export function createAction<
  T extends string
>(type: T): Action<T>;

export function createAction<
  T extends string,
  P
>(
  type: T,
  payload: P
): ActionWithPayload<T, P>;

export function createAction<
  T extends string,
  P
>(type: T, payload?: P) {
  return payload === undefined
    ? { type }
    : { type, payload };
}