Unit testing with Jest and Vitest: complete guide

Jest vs Vitest

Jest has been the dominant JavaScript testing framework for years. Vitest is its modern competitor, compatible with Vite configuration and significantly faster. Both share a similar API, making migration straightforward.

Basic setup with Vitest

// vitest.config.ts
import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    globals: true,
    environment: 'node',
    coverage: {
      provider: 'v8',
      reporter: ['text', 'html'],
    },
  },
});

Test structure

describe('UserService', () => {
  it('should create a valid user', async () => {
    const user = await userService.create({ email: 'test@test.com' });
    expect(user.email).toBe('test@test.com');
    expect(user.id).toBeDefined();
  });
});

Mocks and spies

Mocks isolate the unit under test. Vitest provides vi.mock(), vi.spyOn(), and vi.fn() to simulate dependencies.

vi.mock('../database');
const mockDb = vi.mocked(database);
mockDb.save.mockResolvedValue({ id: '1' });

Coverage

Coverage measures what percentage of code runs during tests. Use it as a guide, not a target. 100% coverage doesn't guarantee correct code.

Need to set up testing in your project? At Vynta we establish quality pipelines with Jest and Vitest.