isNonEmptySet - API reference

API reference › @evolu/common › Set › isNonEmptySet

function isNonEmptySet<T>(set: ReadonlySet<T>): set is NonEmptyReadonlySet<T>;

Defined in: packages/common/src/Set.ts:133

Checks if a set is non-empty and narrows its type to NonEmptyReadonlySet.

Both mutable and readonly sets narrow to the branded NonEmptyReadonlySet type, which can be used with functions like firstInSet.

To check if a set is empty, use if (!isNonEmptySet(set)) — using the negated guard is better than .size === 0 for early returns because TypeScript narrows the type after the check.

Narrowing before access

import {
  assertEqual,
  assertType,
  isNonEmptySet,
  type NonEmptyReadonlySet,
} from "@evolu/common";

const set: ReadonlySet<number> = new Set([1, 2, 3]);
if (!isNonEmptySet(set)) throw new Error("Expected a non-empty set");

assertType<NonEmptyReadonlySet<number>, typeof set>();
assertEqual(set.size, 3);