davila7--claude-code-templates/cli-tool/components/skills/development/react-best-practices/rules/js-set-map-lookups.md

title, impact, impactDescription, tags

title	impact	impactDescription	tags
Use Set/Map for O(1) Lookups	LOW-MEDIUM	O(n) to O(1)	javascript, set, map, data-structures, performance

Use Set/Map for O(1) Lookups

Convert arrays to Set/Map for repeated membership checks.

Incorrect (O(n) per check):

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

Correct (O(1) per check):

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