Personal Catalogue: React & Firebase — Chapter 5, Exercise 2
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TASK
Reproduce the lost-update scenario using two real sequential calls both
built from the same original tags snapshot, sent through the naive
read-modify-write approach — then repeat the same two-call scenario
using arrayUnion instead, and compare the two final results.

SOLUTION

Starting state: a real Book document with tags: ['Python', 'Programming'].

Naive approach, simulating two tabs both reading the same snapshot:

const snapshot = ['Python', 'Programming']; // captured once, "at t0"

// "Tab A", using the t0 snapshot
await updateDoc(doc(db, "items", bookId), { tags: [...snapshot, 'Web Development'] });

// "Tab B", using the SAME stale t0 snapshot (never re-read after Tab A's write)
await updateDoc(doc(db, "items", bookId), { tags: [...snapshot, 'FastAPI'] });

Result in Firestore after both calls:
tags: ['Python', 'Programming', 'FastAPI']

Tab A's own addition of 'Web Development' is gone — Tab B's write
overwrote the whole array with its own stale snapshot plus its own new
tag, with no awareness that Tab A had already changed anything.

Same two-call scenario using arrayUnion instead:

await addTag(bookId, 'Web Development'); // "Tab A"
await addTag(bookId, 'FastAPI');          // "Tab B"

Result in Firestore after both calls:
tags: ['Python', 'Programming', 'Web Development', 'FastAPI']

Both additions land. Neither call ever sent a full array — each one
told Firestore "add this one value to whatever the array currently
holds," so the second call's own effect was computed against the
result of the first, not against a stale snapshot from before either
call ran.

WHY THIS WORKS AS AN ANSWER
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It runs the identical two-step scenario through both approaches against
the same starting state, shows the real final array each one produces,
and correctly explains the difference (updating against a held snapshot
vs. updating against Firestore's own current value at write time)
rather than just asserting that one approach is "safer."