set-state-action.ts

type SetStateAction<S> = S | (prevState: S) => S;

export const isStateTransition = <S>(
  x: SetStateAction<S>
): x is StateTransition<S> => {
  return typeof x === "function";
};

export const tapStateAction = <S>(
  s: SetStateAction<S>,
  f: (x: S) => void
): SetStateAction<S> => {
  if (isStateTransition(s)) {
    return (prevState: S) => {
      const newState = s(prevState);
      f(newState);
      return newState;
    };
  }
  f(s);
  return s;
};