prefer-set-has

Declare the constant list as a Set, for example const ALLOWED = new Set(["a", "b", "c"]), then test membership with ALLOWED.has(value). This changes lookups from O(n) to O(1) and makes membership intent explicit.

Use Set.has() when checking membership in a constant list of known values such as allowlists, blocklists, or enum-like sets. It communicates membership-test intent and avoids accidental mutation of a backing array.

Yes, .includes() is fine for dynamic or short-lived arrays such as function parameters or user input, where constructing a Set would add unnecessary noise without meaningful performance gain.

Set.has() runs in O(1) constant time because Sets use hash-based lookup, while Array.includes() scans elements linearly in O(n) time. The difference matters most for large or frequently queried constant lists.

The guidance targets TypeScript files specifically, both when writing new membership checks and when reviewing existing ones. The underlying O(1) versus O(n) principle generalizes, but the examples and scope are TypeScript-focused.