react derived state mistake

追加: 2026-05-16

[00:00:00]React beginners often store a value that can already be calculated from existing state.

[00:00:05]Now every item update also needs a count update.

[00:00:08]Forget once and the UI becomes wrong.

[00:00:11]If the value comes from current data, calculated during render instead.

[00:00:16]The rule is simple.

[00:00:17]Store source of truth, not values you can derive from it.