assertType

• Type: <T>(value: T): void

You can use this function as an alternative for expectTypeOf to easily assert that the argument type is equal to the generic provided.

import { assertType } from 'vitest';

function concat(a: string, b: string): string;
function concat(a: number, b: number): number;
function concat(a: string | number, b: string | number): string | number;

assertType<string>(concat('a', 'b'));
assertType<number>(concat(1, 2));
// @ts-expect-error wrong types
assertType(concat('a', 2));