Testing

Problem

You want to assert in unit tests that specific messages were logged at the right level — without writing to a real console, file, or external service.

Solution

import { Logit } from '@vielzeug/logit';
import { afterEach, expect, it, vi } from 'vitest';

afterEach(() => {
  vi.restoreAllMocks();
});

it('emits errors when enabled', () => {
  const log = Logit.child({ logLevel: 'error' });
  const spy = vi.spyOn(console, 'error').mockImplementation(() => {});

  log.error('failure');

  expect(spy).toHaveBeenCalled();
});

Pitfalls

• Asserting only on the final console payload can be brittle. Prefer checking the message and structured context arguments rather than the exact badge formatting.
• Remote handlers run asynchronously. If a test covers remote forwarding, await the microtask that delivers the handler call before asserting.
• Log level filtering applies in tests. If the test logger is created with logLevel: 'error', logger.debug() calls produce no output — ensure the test logger level matches the calls under test.

Related

• Child Logger Overrides
• Module Logger Pattern
• Production Setup