Vitest vs Jest in 2026: Migration Guide for TypeScript Projects

npm uninstall jest ts-jest @types/jest babel-jest jest-environment-jsdom
npm install -D vitest @vitest/coverage-v8 jsdom @testing-library/jest-dom

import { defineConfig } from 'vitest/config';
import react from '@vitejs/plugin-react';
import path from 'path';

export default defineConfig({
  plugins: [react()],
  test: {
    globals: true,
    environment: 'jsdom',
    setupFiles: ['./src/test/setup.ts'],
    coverage: {
      provider: 'v8',
      reporter: ['text', 'html', 'lcov'],
      exclude: ['node_modules/', 'src/test/'],
    },
  },
  resolve: {
    alias: { '@': path.resolve(__dirname, './src') },
  },
});

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

// Before
import { jest } from '@jest/globals';
const mockFn = jest.fn();
jest.spyOn(module, 'method');
jest.mock('./module');

// After
import { vi } from 'vitest';
const mockFn = vi.fn();
vi.spyOn(module, 'method');
vi.mock('./module');

import { describe, it, expect, vi, beforeEach } from 'vitest';
import { GET } from './route';
import { db } from '@/lib/db';

vi.mock('@/lib/db', () => ({
  db: { user: { findMany: vi.fn() } },
}));

describe('GET /api/users', () => {
  beforeEach(() => vi.clearAllMocks());

  it('returns users', async () => {
    vi.mocked(db.user.findMany).mockResolvedValue([{ id: '1', email: 'test@example.com' }]);
    const response = await GET();
    expect(response.status).toBe(200);
  });
});

it('retries on failure', async () => {
  vi.useFakeTimers();
  const fn = vi.fn()
    .mockRejectedValueOnce(new Error('fail'))
    .mockResolvedValueOnce({ data: 'ok' });

  const result = retryFetch(fn, { maxRetries: 3, delay: 1000 });
  await vi.advanceTimersByTimeAsync(1000);

  expect(await result).toEqual({ data: 'ok' });
  expect(fn).toHaveBeenCalledTimes(2);
  vi.useRealTimers();
});

// src/test/setup.ts
import '@testing-library/jest-dom';