vitest

name	vitest
description	Use when writing or configuring Vitest tests - assertions, mocking, coverage, and workspace-aware testing for TypeScript projects

Vitest Testing Framework

Quick Start

import { describe, it, expect, vi, beforeEach } from 'vitest';

describe('Calculator', () => {
  beforeEach(() => {
    vi.clearAllMocks();
  });

  it('adds numbers correctly', () => {
    expect(add(2, 3)).toBe(5);
  });

  it('handles async operations', async () => {
    const result = await fetchData();
    expect(result).toMatchObject({ id: 1 });
  });

  it('mocks dependencies', () => {
    const mockFn = vi.fn().mockReturnValue(42);
    expect(mockFn()).toBe(42);
    expect(mockFn).toHaveBeenCalledOnce();
  });
});

Core Assertions

Assertion	Purpose
toBe(value)	Strict equality (===)
toEqual(value)	Deep equality
toMatchObject(obj)	Partial object match
toContain(item)	Array/string contains
toThrow(error?)	Function throws
toMatchSnapshot()	Snapshot testing
toHaveBeenCalledWith()	Mock call verification

Mocking

// Mock module
vi.mock('./api', () => ({ fetchUser: vi.fn() }));

// Spy on method
const spy = vi.spyOn(object, 'method');

// Mock implementation
mockFn.mockImplementation((x) => x * 2);
mockFn.mockResolvedValue({ data: [] });
mockFn.mockRejectedValue(new Error('fail'));

Configuration (vitest.config.ts)

import { defineConfig } from 'vitest/config';

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

CLI Commands

• vitest - Watch mode
• vitest run - Single run
• vitest run --coverage - With coverage
• vitest --workspace - Monorepo mode