AI agent skill · by @LedgerHQ

Prefer Set Has

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

Archived 2026-10-23 · Source updated 2026-10-12 · License: MIT. Check the original repository for the latest version.

SKILL.md instructions

# 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.