Personal Catalogue: React & Firebase — Chapter 2, Exercise 2
=====================================================================

TASK
Extend `validateItem()` to also require a `director` field on Dvd and
Bluray documents, then run it against one valid and one invalid
document of each new type to confirm the new rule actually fires.

SOLUTION

const REQUIRED_BY_TYPE = {
  Book: ['author'],
  Cd: ['artist'],
  Dvd: ['director'],
  Bluray: ['director'],
};

const VALID_ITEM_TYPES = Object.keys(REQUIRED_BY_TYPE);

export function validateItem(data) {
  if (!data.title) {
    throw new Error('Every item needs a title.');
  }
  if (!VALID_ITEM_TYPES.includes(data.itemType)) {
    throw new Error(`itemType must be one of ${VALID_ITEM_TYPES.join(', ')}, got "${data.itemType}".`);
  }
  for (const field of REQUIRED_BY_TYPE[data.itemType]) {
    if (!data[field]) {
      throw new Error(`A ${data.itemType} needs a ${field}.`);
    }
  }
  if (data.itemType === 'Bluray' && data.resolution && !['1080p', '4K'].includes(data.resolution)) {
    throw new Error(`resolution must be "1080p" or "4K", got "${data.resolution}".`);
  }
}

Only `REQUIRED_BY_TYPE` needed to change — the loop that actually
enforces the requirement already reads from that object generically, so
adding `'director'` to the Dvd and Bluray arrays is the entire fix.

Run:
validateItem({ itemType: 'Dvd', title: 'Spirited Away', director: 'Hayao Miyazaki' });
// passes, no error

validateItem({ itemType: 'Dvd', title: 'Spirited Away' });
// throws: "A Dvd needs a director."

validateItem({ itemType: 'Bluray', title: 'Blade Runner 2049', director: 'Denis Villeneuve', resolution: '4K' });
// passes, no error

validateItem({ itemType: 'Bluray', title: 'Blade Runner 2049', resolution: '4K' });
// throws: "A Bluray needs a director."

Output confirms the new rule fires for both new types and stays silent
when the field is actually present, exactly the same behavior the
original Book/Cd checks already had.

WHY THIS WORKS AS AN ANSWER
----------------------------
It extends the validator through its own existing generic mechanism
(the `REQUIRED_BY_TYPE` lookup) rather than hand-writing a new special
case for Dvd and Bluray, and it demonstrates the new rule actually
firing with real before/after calls rather than just editing the code
and assuming it works.