Type-Level Testing: expectTypeOf and Static Type System Assertions

Complex TypeScript codebases often contain heavy type manipulations: generics, conditional types, and utility wrappers. Standard unit tests only verify runtime behavior, leaving static type regressions unchecked.

Vitest includes native type testing utilities via expectTypeOf() and assertType().

import { test, expectTypeOf } from 'vitest';
import type { UserResponse } from './types';

function createStore<T>(initial: T) {
  return { get: () => initial };
}

test('verifies type inference', () => {
  const store = createStore({ id: '101', active: true });

  // Type assertions (checked by TypeScript at build time)
  expectTypeOf(store.get()).toEqualTypeOf<{ id: string; active: boolean }>();
  expectTypeOf(store.get().id).toBeString();
  expectTypeOf(store.get().active).toBeBoolean();
});

Running Typechecks in CLI

You can execute type tests in CI/CD using Vitest’s typecheck command:

vitest typecheck

This runs tsc under the hood via Vitest’s runner, checking type assertions without spawning Node execution runtime.