Claude Skills Guide

Claude Code Jest to Vitest Migration Workflow Tutorial

Migrating test suites from Jest to Vitest is one of the most impactful upgrades you can make to your JavaScript development workflow. This transformation delivers dramatically faster test execution, native Vite integration, and a more intuitive API that aligns with modern frontend tooling. When you use Claude Code as your AI development assistant, the migration becomes a structured, low-friction process that handles the complexities of config translation, import rewrites, and test adaptation.

This tutorial walks you through a complete migration workflow using Claude Code, covering everything from initial assessment through final verification.

Why Migrate from Jest to Vitest

Vitest shares compatibility with Jest’s API, meaning most of your existing test patterns translate directly The performance gains stem from Vite’s native hot module replacement and intelligent watch mode. Teams report test suites running 10x faster after switching, particularly beneficial in large codebases with hundreds of test files.

The migration makes particular sense if your project already uses Vite for bundling. Vitest uses the same configuration, plugin ecosystem, and dependency resolution, eliminating duplicate tooling and reducing maintenance overhead.

Phase 1: Project Assessment with Claude Code

Begin by having Claude Code analyze your current Jest configuration. Prompt Claude with a request to examine your test setup:

Review my project's Jest configuration and identify potential migration challenges. Check jest.config.js, package.json test scripts, and any test utility files.

Claude Code examines your setup and produces a migration readiness report. This report highlights deprecated Jest-specific APIs, third-party dependencies that require Vitest equivalents, and configuration options that need translation.

For projects using TypeScript, note that Vitest provides excellent type inference out of the box. You may find that certain @types/jest packages become unnecessary after migration.

Phase 2: Dependency Installation and Configuration

The actual migration starts with replacing Jest dependencies. Create a new configuration file for Vitest:

// vitest.config.js
import { defineConfig } from 'vitest/config';
import path from 'path';

export default defineConfig({
  test: {
    environment: 'jsdom',
    globals: true,
    setupFiles: ['./test/setup.js'],
    include: ['**/*.test.js', '**/*.spec.js'],
    coverage: {
      provider: 'v8',
      reporter: ['text', 'json', 'html'],
    },
  },
  resolve: {
    alias: {
      '@': path.resolve(__dirname, './src'),
    },
  },
});

If you’re working with Vue projects, this is where invoking the frontend-design skill proves valuable. Vue test utilities work well with Vitest, and Claude can help you configure the proper environment.

For projects transitioning from Jest’s --coverage flag, Vitest’s coverage configuration works similarly but uses Vite’s coverage providers by default. The transition typically requires updating your CI pipeline’s coverage thresholds.

Phase 3: Test File Migration Patterns

Most Jest matchers work identically in Vitest, but several patterns require adjustment. Here’s a practical migration approach:

Original Jest test:

describe('UserService', () => {
  beforeEach(() => {
    jest.clearAllMocks();
  });

  it('should fetch user data', async () => {
    jest.spyOn(api, 'getUser').mockResolvedValue({ id: 1, name: 'Test' });
    
    const user = await UserService.fetchUser(1);
    
    expect(api.getUser).toHaveBeenCalledWith(1);
    expect(user).toEqual({ id: 1, name: 'Test' });
  });
});

Migrated Vitest version:

import { vi, describe, it, expect, beforeEach } from 'vitest';

describe('UserService', () => {
  beforeEach(() => {
    vi.clearAllMocks();
  });

  it('should fetch user data', async () => {
    vi.spyOn(api, 'getUser').mockResolvedValue({ id: 1, name: 'Test' });
    
    const user = await UserService.fetchUser(1);
    
    expect(api.getUser).toHaveBeenCalledWith(1);
    expect(user).toEqual({ id: 1, name: 'Test' });
  });
});

The primary changes involve importing test utilities explicitly and replacing jest global with vi. Vitest supports --globals flag if you prefer the Jest-style global API, but explicit imports align better with modern ESM workflows.

Phase 4: Handling Jest-Specific Features

Several Jest-specific features require targeted migration strategies:

Timer mocks: Vitest uses vi.useFakeTimers() instead of jest.useFakeTimers(). The API remains similar but returns a timer controller with slightly different methods.

Module mocking: Replace jest.mock() with vi.mock(). The syntax remains largely compatible, but you gain access to Vite’s module resolution during mocking.

// Vitest module mocking
vi.mock('./api', () => ({
  fetchData: vi.fn().mockResolvedValue({ mock: true }),
}));

Custom matchers: If you’ve built custom Jest matchers, migrate them to Vitest’s expect.extend. The API remains consistent, so this typically involves simple find-and-replace.

Phase 5: CI/CD Pipeline Updates

Your continuous integration configuration needs updating. Here’s a typical package.json transition:

{
  "scripts": {
    "test": "vitest",
    "test:run": "vitest run",
    "test:coverage": "vitest run --coverage"
  }
}

For GitHub Actions, update your workflow file to use Vitest commands. The test execution becomes significantly faster, reducing CI runtime:

- name: Run tests
  run: npm run test:run

Phase 6: Verification and Debugging

Run your migrated test suite and address any failures. Common issues include:

The tdd skill provides excellent guidance for maintaining test-driven development practices during this transition. Claude Code can help you write additional tests to cover edge cases exposed during migration.

Maintaining Your Migrated Suite

Post-migration, you’ll benefit from Vitest’s watch mode integration with Vite’s HMR. When editing source files, only related tests re-run automatically, dramatically speeding up development cycles.

Consider implementing snapshot management. Vitest handles snapshots similarly to Jest but stores them with .snap extension. You can migrate existing snapshots by running vitest --update once.

Summary

Migrating from Jest to Vitest with Claude Code as your migration assistant follows a structured path: assess your current setup, install Vitest dependencies, migrate test files using the provided patterns, update CI configuration, and verify everything works correctly. The performance improvements and simplified tooling make this migration worthwhile for any modern JavaScript project.

The key to success lies in taking incremental steps rather than attempting a complete migration overnight. Run the migration in stages, verify test coverage at each phase, and use Claude Code to handle the repetitive refactoring work.

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