Use Set/Map for O(1) Lookups

Use Set/Map for O(1) Lookups

Impact: MEDIUM (O(n) to O(1) per check)

Convert arrays to Set/Map for repeated membership checks.

Incorrect (O(n) per check):

const bookmarkedIds = ['a', 'b', 'c', ...]
items.filter(item => bookmarkedIds.includes(item.id))

Correct (O(1) per check):

const bookmarkedIds = new Set(['a', 'b', 'c', ...])
items.filter(item => bookmarkedIds.has(item.id))

Common React Native cases:

• Filtering FlatList data by bookmarked/selected IDs
• Checking if a notification has been read
• Permission sets (allowedRoles.has(user.role))
• Selection mode: selectedIds.has(item.id) (already used in S12/S28 patterns)

Source: vercel-labs/agent-skills (MIT License)