---
name: prefer-set-has
description: |
  Prefer Set.has() over Array.includes() for constant allowlists/blocklists.
  Read this when reviewing or writing membership checks in TypeScript files.
---

# Prefer Set.has() over Array.includes()

When checking membership in a **constant list** of known values (allowlists, blocklists, enum-like sets), use a `Set` with `.has()` instead of an array with `.includes()`.

```typescript
// ❌ BAD
const ALLOWED = ["a", "b", "c"];
if (ALLOWED.includes(value)) { … }

// ✅ GOOD
const ALLOWED = new Set(["a", "b", "c"]);
if (ALLOWED.has(value)) { … }
```

**Why:** `Set.has()` is O(1) vs `.includes()` O(n), communicates "membership test" intent more clearly, and avoids accidental mutation of the backing array.

**When .includes() is fine:** Searching within a dynamic or short-lived array (e.g. function parameters, user input) where creating a Set would add noise.