Replace Jest with Vitest

[Starefossen]

Summary

Migrate the test framework from Jest to Vitest for better performance, native ESM/TypeScript support, and tighter alignment with the Vite ecosystem.

Motivation

• Performance: Vitest runs significantly faster thanks to native ESM and built-in TypeScript/JSX transformation — no ts-jest or Babel needed.
• Configuration: Vitest reuses vite.config.ts conventions, reducing config surface (no separate jest.config.ts, jest.setup.ts, path alias duplication).
• ESM-first: Jest's ESM support is still experimental and requires workarounds (jest.resetModules(), dynamic imports). Vitest handles ESM natively.
• Watch mode: Vitest's watch mode is instant and uses the same HMR pipeline as Vite dev, making TDD faster.
• API compatibility: Vitest is largely API-compatible with Jest (describe, it, expect, vi.fn(), vi.mock()), so migration is mostly mechanical.

Scope

Files to update/remove

• jest.config.ts → remove, replace with vitest config (can live in vite.config.ts or vitest.config.ts)
• jest.setup.ts → migrate to Vitest setup file
• package.json scripts: test, test:debug, test:watch
• All 41 test suites in __tests__/ and src/**/ — update imports from @jest/globals to vitest

Key changes per test file

1. Replace import { describe, it, expect, jest, ... } from '@jest/globals' with import { describe, it, expect, vi, ... } from 'vitest'
2. Replace jest.fn() → vi.fn(), jest.mock() → vi.mock(), jest.spyOn() → vi.spyOn(), etc.
3. Remove /** @jest-environment node */ directives (Vitest uses environment in config or per-file // @vitest-environment node)
4. Remove jest.resetModules() / dynamic import() patterns where they were workarounds for Jest's module caching

Dependencies

• Remove: jest, ts-jest, @jest/globals, jest-environment-jsdom (if present), @types/jest
• Add: vitest, @vitest/coverage-v8 (optional)

Acceptance criteria

• All 41 test suites pass under Vitest
• pnpm run test, pnpm run test:watch, pnpm run test:debug work
• No Jest dependencies remain in package.json
• CI (if applicable) uses Vitest
• pnpm run check passes cleanly