---
description: Prefer Record<K, V> for small static literals; use Set/Map for anything dynamic
condition: "\\bnew\\s+(Set|Map)\\b"
scope: "tool:edit(**/*.{ts,tsx}), tool:write(**/*.{ts,tsx})"
interruptMode: never
---

Use `Record<K, V>` / `Record<K, true>` for small, static string-keyed lookup tables.

Use `Set` / `Map` when keys are dynamic, non-string, inserted or deleted at runtime, or when code needs `.size`, `.clear()`, stable insertion order, or iterator APIs.

```typescript
// Static literal → Record
const LABEL_BY_KIND: Record<string, string> = {
	text: "Text",
	json: "JSON",
	binary: "Binary",
};

// Dynamic membership → Set
const seen = new Set<string>();
for (const item of items) {
	if (seen.has(item.id)) continue;
	seen.add(item.id);
}
```

Small fixed table? `Record`. Runtime collection? `Set` / `Map`.