Set Up Vitest

Goal

Add Vitest for fast test execution with native ESM and TypeScript support. If migrating from Jest, replace Jest + ts-jest with minimal code changes.

Why

• Native ESM support (no ts-jest transform overhead)
• Native TypeScript support (no configuration needed)
• 2-5x faster test execution for ESM TypeScript projects
• Jest-compatible API (minimal code changes when migrating)

Steps

1. Install Vitest

pnpm add -D vitest

2. Create vitest.config.ts

import { defineConfig } from 'vitest/config'

export default defineConfig({
  test: {
    globals: true,
    include: ['src/**/*.test.ts'],
  },
})

3. Update package.json Scripts

{
  "scripts": {
    "test": "vitest run",
    "test:watch": "vitest"
  }
}

4. Remove Jest (if migrating)

pnpm remove jest ts-jest @types/jest

Also delete jest.config.* if it exists.

5. Update Test Files (if migrating from Jest)

With globals: true, most Jest tests work unchanged. Key differences:

Mock changes (find and replace):

// jest.fn → vi.fn
// jest.mock → vi.mock
// jest.spyOn → vi.spyOn
// jest.clearAllMocks → vi.clearAllMocks
// jest.resetAllMocks → vi.resetAllMocks
// jest.restoreAllMocks → vi.restoreAllMocks
// jest.useFakeTimers → vi.useFakeTimers

Imports (if not using globals):

import { describe, test, expect, vi, beforeEach, afterEach } from 'vitest'

6. TypeScript Config

Add vitest types if using globals:

{
  "compilerOptions": {
    "types": ["vitest/globals"]
  }
}

Or create a separate tsconfig.test.json that extends the main one.

7. Verify

pnpm test           # All tests pass
pnpm test:watch     # Watch mode works

Gotchas

• Vitest uses esbuild for transforms (fast but no type-checking during tests — use tsc --noEmit separately)
• Snapshot files are compatible (.snap extension, same as Jest)
• If Jest config uses moduleNameMapper, use resolve.alias in vitest config instead