Convert Jest to Vitest: A Complete Migration Guide
What is Jest?
Jest is a JavaScript testing framework known for its
simplicity, powerful mocking capabilities, and out-of-the-box setup.
It's been widely adopted by React and Node.js developers due to its snapshot
testing, fast assertions, and large community support.
What is Vitest?
Vitest is a modern testing framework designed to
integrate seamlessly with Vite. It's built for speed, offering
blazing-fast test execution thanks to Vite's bundling system. Vitest also
features native TypeScript support and a lightweight configuration,
making it a great option for modern JavaScript applications.
Why Switch from Jest to Vitest?
While Jest is still a powerful tool, Vitest brings some
unique advantages:
- Faster
Test Execution: Vitest utilizes Vite’s efficient bundling, reducing
test times.
- Seamless
TypeScript Integration: No need for extra setup for TypeScript
support.
- Better
Frontend Tooling Compatibility: Vitest is optimized for Vite-based
projects and modern frontend libraries.
- Improved
Developer Experience: Vitest supports live test updates with hot
module replacement (HMR), making development more intuitive.
Prerequisites for the Migration
Before you begin the migration, ensure that your project has
the following:
- Node.js
16+ installed.
- Existing
Jest tests configured and running successfully.
- Basic
knowledge of Vite and its ecosystem, especially if your project
uses Vite.
Step 1: Install Vitest and Dependencies
The first step is to install Vitest and any related
dependencies:
bash
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npm install vitest @vitest/ui --save-dev
You may also want to install plugins such as @testing-library/vue
or vitest-mock for additional capabilities based on your testing needs.
Step 2: Update Test Configuration Files
Vitest requires a vitest.config.ts configuration
file, similar to Vite’s config. Create this file if it doesn't already exist:
typescript
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// vitest.config.ts
import { defineConfig } from 'vitest/config'
export default defineConfig({
test: {
globals: true,
environment: 'jsdom',
// Use jsdom for browser-like tests
coverage: {
reporter: ['text',
'json', 'html'], // Optional: Add coverage reports
},
},
})
This configuration mirrors Jest’s capabilities, including global
variables and test environment setups.
Step 3: Update Test Scripts in package.json
Modify the scripts section in your package.json to
use Vitest instead of Jest:
json
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"scripts": {
"test": "vitest",
"test:watch":
"vitest --watch"
}
This will enable you to run tests using npm run test and
watch for file changes with npm run test:watch.
Step 4: Migrate Jest-Specific Code
While Vitest shares much of the same API as Jest, some key
differences need to be addressed:
- Mocking:
Replace jest.mock() with vi.mock().
javascript
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// From:
jest.mock('./module')
// To:
vi.mock('./module')
- Mock
Functions: Use vi.fn() instead of jest.fn().
- Setup
Files: Jest’s setupFiles can be replaced with hooks inside vitest.config.ts.
Step 5: Handle Differences in API Usage
Most Jest API functions are available in Vitest, but there
are slight differences:
- Snapshot
Testing: Vitest supports snapshots but may format them slightly
differently.
javascript
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expect(component).toMatchSnapshot()
- Global
Variables: Ensure any references to Jest’s globals (expect, beforeEach,
afterAll) are properly imported from Vitest.
Step 6: Run and Validate Your Tests
With your configuration and code changes complete, you can
now run your tests:
bash
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npm run test
Review the output for any errors or incompatibilities.
Debug any issues related to environment setup, such as missing modules
or slight API changes between Jest and Vitest.
Step 7: Update CI/CD Pipelines (If Applicable)
If you use CI/CD pipelines to automate your tests, you’ll
need to replace Jest with Vitest in your configuration files:
yaml
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# Example CI configuration (GitHub Actions)
jobs:
test:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v2
- uses: actions/setup-node@v2
with:
node-version:
'16'
- run: npm install
- run: npm run test
Ensure that your pipeline correctly identifies test
results and generates any required coverage reports.
Tools and Plugins to Enhance Vitest Testing
Vitest supports a variety of tools and plugins to enhance
testing:
- @testing-library/vue:
For testing Vue.js components.
- vitest-mock:
Adds enhanced mocking capabilities.
- @vitest/ui:
A web-based interface to view test results and debug errors.
Challenges You Might Encounter
While the migration from Jest to Vitest is generally
straightforward, you may encounter a few challenges:
- Custom
Matchers: If you use custom Jest matchers, you'll need to ensure
Vitest equivalents are available or create custom matchers.
- Mock
Behavior Differences: Some Jest mocks may behave slightly differently
in Vitest, especially for advanced mock setups.
- Snapshot
Format Differences: Snapshots may require re-generation due to format
differences between Jest and Vitest.
Best Practices for a Smooth Migration
- Migrate
Gradually: Move test suites in small increments to minimize
disruptions.
- Use
Version Control: Keep your migration changes in a separate branch to
avoid affecting the main codebase.
- Test
Thoroughly: Run tests in both Jest and Vitest during the
migration to ensure nothing breaks.
- Leverage
Community Resources: Refer to Vitest’s documentation and forums
for support during the transition.
Conclusion
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