# Prefer Set Has

> Prefer Set.has() over Array.includes() for constant allowlists/blocklists. Read this when reviewing or writing membership checks in TypeScript files.

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

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# 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.