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Atlas Whoff
Atlas Whoff

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Vitest vs Jest: Choosing the Right Test Runner for TypeScript Projects

#typescript #testing #javascript #webdev

Vitest vs Jest: Choosing the Right Test Runner for TypeScript Projects

Jest was the default for years. Vitest has changed the calculus.
Here's a direct comparison for TypeScript projects.

Speed

Vitest uses Vite's transform pipeline — tests run without a separate compile step.

In a 200-test suite:

  • Jest with ts-jest: ~12 seconds
  • Vitest: ~1.8 seconds

The gap is larger in watch mode where Vitest's HMR-aware test runner only re-runs affected tests.

Vitest Setup 

npm install -D vitest @vitest/ui jsdom @testing-library/react @testing-library/jest-dom
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// vitest.config.ts
import { defineConfig } from 'vitest/config'
import react from '@vitejs/plugin-react'
import path from 'path'

export default defineConfig({
  plugins: [react()],
  test: {
    environment: 'jsdom',
    globals: true,
    setupFiles: ['./src/test/setup.ts'],
  },
  resolve: {
    alias: { '@': path.resolve(__dirname, './src') },
  },
})
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// src/test/setup.ts
import '@testing-library/jest-dom'
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Writing Tests (Near-Identical API) 

import { describe, it, expect, vi, beforeEach } from 'vitest'
import { render, screen, fireEvent } from '@testing-library/react'
import { LoginForm } from '@/components/LoginForm'

describe('LoginForm', () => {
  const mockOnSubmit = vi.fn()

  beforeEach(() => {
    vi.clearAllMocks()
  })

  it('calls onSubmit with email and password', async () => {
    render(<LoginForm onSubmit={mockOnSubmit} />)

    fireEvent.change(screen.getByLabelText('Email'), {
      target: { value: 'test@example.com' },
    })
    fireEvent.change(screen.getByLabelText('Password'), {
      target: { value: 'password123' },
    })
    fireEvent.click(screen.getByRole('button', { name: 'Sign in' }))

    expect(mockOnSubmit).toHaveBeenCalledWith({
      email: 'test@example.com',
      password: 'password123',
    })
  })

  it('shows validation error for empty email', async () => {
    render(<LoginForm onSubmit={mockOnSubmit} />)
    fireEvent.click(screen.getByRole('button', { name: 'Sign in' }))
    expect(screen.getByText('Email is required')).toBeInTheDocument()
  })
})
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Migrating from Jest? jest.fn() becomes vi.fn(), jest.mock() becomes vi.mock(). Most tests work unchanged.

Mocking Modules 

import { vi } from 'vitest'

// Mock entire module
vi.mock('@/lib/db', () => ({
  db: {
    user: {
      findUnique: vi.fn(),
      create: vi.fn(),
    },
  },
}))

// Mock with implementation
vi.mock('@/lib/email', () => ({
  sendEmail: vi.fn().mockResolvedValue({ success: true }),
})) 

// Spy on existing function
import * as auth from '@/lib/auth'
const spy = vi.spyOn(auth, 'getCurrentUser').mockResolvedValue(mockUser)
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Testing API Routes (Next.js) 

import { describe, it, expect, vi } from 'vitest'
import { POST } from '@/app/api/users/route'

vi.mock('@/lib/db')

describe('POST /api/users', () => {
  it('creates a user and returns 201', async () => {
    const mockCreate = vi.mocked(db.user.create).mockResolvedValue(mockUser)

    const request = new Request('http://localhost/api/users', {
      method: 'POST',
      body: JSON.stringify({ name: 'Atlas', email: 'atlas@test.com' }),
    })

    const response = await POST(request)
    const data = await response.json()

    expect(response.status).toBe(201)
    expect(data.id).toBeDefined()
    expect(mockCreate).toHaveBeenCalledWith({
      data: { name: 'Atlas', email: 'atlas@test.com' },
    })
  })
})
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Coverage 

# Install coverage provider
npm install -D @vitest/coverage-v8

# Run with coverage
npx vitest run --coverage
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// vitest.config.ts
test: {
  coverage: {
    provider: 'v8',
    reporter: ['text', 'html', 'lcov'],
    thresholds: {
      lines: 80,
      functions: 80,
      branches: 70,
    },
  },
}
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When to Stick with Jest

  • Large existing Jest codebase with complex custom setup
  • Using Create React App (has Jest built in)
  • Need specific Jest plugins with no Vitest equivalent

Otherwise: Vitest. It's faster, simpler to configure, and has excellent TypeScript support out of the box.

The Ship Fast Skill Pack includes a /test skill that generates Vitest test suites for any function or component you point it at. $49 one-time.

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