---
title: Use Set/Map for O(1) Lookups
impact: LOW-MEDIUM
impactDescription: O(n) to O(1)
tags: 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):**

```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));
```