jest to vitest migration: common breakages

TL;DR

Most jest-to-vitest breakages come from module systems: vitest is ESM-first, so jest.mock hoisting, require usage, and CJS interop behave differently. The usual fix list: replace jest.* APIs with vi.* (or the compat layer), rewrite jest.mock calls to vi.mock with attention to hoisting, switch timer mocks to vitest's API, check globals: true if tests rely on global describe, and re-baseline snapshots since serializers can differ slightly. Migrate one directory at a time with both runners configured.

The query

jest to vitest migration: common breakages

Use this when

• Planning a jest to vitest migration.
• Debugging failures after switching runners.

Not for

• Choosing a test runner for a new project.
• Migrating from vitest back to jest.

Steps

1. Get both runners working side by side on the repo first. Expected output: jest green as before, vitest configured.
2. Migrate one directory; replace jest.mock with vi.mock and jest.fn with vi.fn. Expected output: the migrated directory passes under vitest.
3. Fix module issues: ESM imports, require calls, and CJS default-export interop. Expected output: no more module-not-found or undefined-default errors.
4. Fix timers and fake-date usage with vitest's timer API. Expected output: time-dependent tests pass deterministically.
5. Re-baseline snapshots and diff them carefully before accepting. Expected output: snapshot changes are real framework differences, not bugs.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_6iXINWNE3JVlhhvpB3qSHA