React Performance: JS Perf — Set Map Lookups. Keywords: set map includes has. Platform: React/Next.js. Description: Use Set/Map for O(1) lookups instead of array.includes(). Do: Convert to Set for membership checks. Don't: Use .includes() for repeated checks. Good Example: const allowed = new Set(['a','b']); allowed.has(id). Bad Example: const allowed = ['a','b']; allowed.includes(id). Severity: Low-Medium.