vitest

name	vitest
description	Use when writing or modifying Vitest tests to follow best practices for test structure, assertions, mocking, and coverage

Vitest Testing

Quick Start

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

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

  it('should handle basic case', () => {
    expect(myFunction('input')).toBe('expected');
  });

  it('should throw on invalid input', () => {
    expect(() => myFunction(null)).toThrow('Invalid input');
  });
});

Core Patterns

• Structure: describe for grouping, it for individual tests
• Assertions: expect(value).toBe(), .toEqual(), .toMatchObject(), .toThrow()
• Async: async/await or return Promise; use vi.waitFor() for polling
• Mocking: vi.fn(), vi.spyOn(), vi.mock('module')
• Setup/Teardown: beforeEach, afterEach, beforeAll, afterAll
• Snapshots: expect(value).toMatchSnapshot(), update with -u flag

Best Practices

• One assertion concept per test (multiple expects OK if testing same behavior)
• Use descriptive test names: "should [action] when [condition]"
• Avoid test interdependence; each test should be isolated
• Mock external dependencies (APIs, file system, time)
• Use vi.useFakeTimers() for time-dependent tests

Reference Files

• references/assertions.md - Complete assertion API
• references/mocking.md - Mocking strategies
• references/async.md - Async testing patterns