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---
name: js-set-map-lookups
description: Use Set/Map for O(1) Lookups. Use when you need help with js set map lookups.
license: CC-BY-NC-SA-4.0
metadata:
risk: unknown
source: community
kind: mode
category: rules
---
## 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))
```
