prefer-set-has

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().

// ❌ 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.