What does it do?

• Adds vitest-config in packages/utils/vitest-config/ as a private shared Vitest config:

  • Unit preset with globals: false (explicit describe/it/expect/vi imports), environment: 'node', include: ['**/*.vitest.test.ts'], testTimeout: 5000, and exclusions aligned with the Jest unit setup.

• Integrates Vitest in packages/core/core/:

  • vitest.config.ts that merges the unit preset with package-specific.
  • Scripts: test:unit:vitest (run once) and test:unit:vitest:watch (watch).
  • Dev deps: vitest, vitest-config.

• Root-level scripts (in root package.json):

  • test:unit:vitest and test:unit:vitest:watch that run Vitest via packages/core/core/vitest.config.ts.

• Centralized Vitest version:

  • Root package.json: vitest in devDependencies, and vitest: "4.0.18" in resolutions so the whole monorepo uses the same version.

• CI (.github/workflows/tests.yml):

  • New job unit_back_vitest (Node 20/22/24), same conditions as unit_back, runs yarn test:unit:vitest.

• Docs:

  • VITEST_MIGRATION.md with setup overview, migration steps (Jest → Vitest, *.vitest.test.ts), and dependency/version strategy.

Why is it needed?

• Modernize the test stack by introducing Vitest (faster, ESM-friendly, Vite-based) alongside Jest.
• Allow incremental migration via the *.vitest.test.ts pattern so existing *.test.ts Jest tests keep running while new or migrated tests use Vitest.
• Pin a single Vitest version (4.0.18) via resolutions so every package uses the same runner and upgrades are done in one place.

How to test it?

1. Install and run Vitest
   From repo root:

   yarn install
   yarn test:unit:vitest

   Expected: Vitest runs and exits successfully

2. Run from core package

   cd packages/core/core && yarn test:unit:vitest

   Expected: Vitest runs and exits successfully

3. Watch mode

   yarn test:unit:vitest:watch

   And

   cd packages/core/core && yarn test:unit:vitest:watch

   Confirm Vitest starts in watch mode (Ctrl+C to stop).

4. Vitest version

   yarn vitest --version

   Expected: vitest/4.0.18 ….

5. Jest still works

   yarn test:unit

   Existing Jest unit tests should run unchanged.

Environment: Node 20–24, macOS/Linux. No extra env vars needed for the above.

Related issue(s)/PR(s)

None.