Js Set Map Lookups

## 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)) ```