# JS Set Map Lookups

> 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.

- Skill: `theorcdev/js-set-map-lookups` (Agent Skill)
- Install (CLI): `npx skillmds@latest add theorcdev/js-set-map-lookups`
- Raw SKILL.md: https://api.skillmd.com/api/skills/theorcdev/js-set-map-lookups/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: theorcdev (https://skillmd.com/u/theorcdev)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/theorcdev/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))
```