1---
2title: Use Set/Map for O(1) Lookups
3impact: LOW-MEDIUM
4impactDescription: O(n) to O(1)
5tags: javascript, set, map, data-structures, performance
6---
7 
8## Use Set/Map for O(1) Lookups
9 
10Convert arrays to Set/Map for repeated membership checks.
11 
12**Incorrect (O(n) per check):**
13 
14```typescript
15const allowedIds = ['a', 'b', 'c', ...]
16items.filter(item => allowedIds.includes(item.id))
17```
18 
19**Correct (O(1) per check):**
20 
21```typescript
22const allowedIds = new Set(['a', 'b', 'c', ...])
23items.filter(item => allowedIds.has(item.id))
24```
25