— name: js-set-map-lookups description: Use Set and Map for O(1) membership lookups instead of array.includes(). Apply when checking membership repeatedly or performing frequent lookups against a collection. — ## Use Set/Map for O(1) Lookups Convert arrays to Set/Map for repeated membership checks. **Incorrect (O(n) per check):** “`typescript const allowedIds = ['a', 'b', 'c', …] items.filter(item => allowedIds.includes(item.id)) “` **Correct (O(1) per check):** “`typescript const allowedIds = new Set(['a', 'b', 'c', …]) items.filter(item => allowedIds.has(item.id)) “`