Writing Vitest Tests

This skill teaches test structure and patterns for writing effective Vitest tests.

Quick Start

Basic Test

import { test, expect } from 'vitest';

test('adds numbers correctly', () => {
  expect(1 + 2).toBe(3);
});

Test Suite

import { describe, test, expect } from 'vitest';

describe('Math utilities', () => {
  test('adds numbers', () => {
    expect(1 + 2).toBe(3);
  });

  test('subtracts numbers', () => {
    expect(5 - 2).toBe(3);
  });
});

For complete test organization patterns, see [references/test-organization.md](./references/test-organization.md)

Essential Assertions

Basic Matchers

expect(value).toBe(expected);
expect(value).toEqual(expected);
expect(value).toBeTruthy();
expect(value).toBeFalsy();

Arrays and Objects

expect(array).toContain(item);
expect(array).toHaveLength(3);
expect(object).toHaveProperty('key');
expect(object).toMatchObject({ key: 'value' });

Strings and Numbers

expect(string).toMatch(/pattern/);
expect(number).toBeGreaterThan(3);
expect(number).toBeCloseTo(0.3, 5);

For complete assertion API, see [references/assertions.md](./references/assertions.md)

Mocking with vi

Mock Functions

import { vi, test, expect } from 'vitest';

test('calls callback', () => {
  const callback = vi.fn();

  callback('arg1', 'arg2');

  expect(callback).toHaveBeenCalledWith('arg1', 'arg2');
  expect(callback).toHaveBeenCalledTimes(1);
});

Mock Return Values

const mockFn = vi.fn();
mockFn.mockReturnValue('result');

expect(mockFn()).toBe('result');

Mock Implementation

const mockFn = vi.fn();
mockFn.mockImplementation((a, b) => a + b);

expect(mockFn(1, 2)).toBe(3);

For testing runtime type validation with Zod, use the using-runtime-checks skill for patterns on schema validation, error handling, and type inference.

Mock Promises

const mockFn = vi.fn();
mockFn.mockResolvedValue('data');

const result = await mockFn();
expect(result).toBe('data');

For testing code that validates external data with Zod, use the validating-external-data skill for patterns on API response validation, error handling, and type-safe validation flows.

For complete mocking patterns, see [references/mocking.md](./references/mocking.md)

Module Mocking

Basic Module Mock

vi.mock('./module', () => ({
  method: vi.fn(),
  constant: 'mocked',
}));

Partial Module Mock

vi.mock(import('./module'), async (importOriginal) => {
  const mod = await importOriginal();
  return {
    ...mod,
    specificMethod: vi.fn(),
  };
});

For testing Zod validation of database inputs in Vitest, use the validating-query-inputs skill for patterns on Zod schema validation before Prisma operations.

Async Testing

Async/Await

test('fetches data', async () => {
  const data = await fetchData();
  expect(data).toBeDefined();
});

Promise Assertions

test('promise resolves', async () => {
  await expect(fetchData()).resolves.toBe('data');
});

test('promise rejects', async () => {
  await expect(fetchBadData()).rejects.toThrow('error');
});

Parameterized Tests

test.each with Arrays

test.each([
  [1, 1, 2],
  [1, 2, 3],
  [2, 1, 3],
])('add(%i, %i) -> %i', (a, b, expected) => {
  expect(a + b).toBe(expected);
});

test.each with Objects

test.each([
  { a: 1, b: 1, expected: 2 },
  { a: 1, b: 2, expected: 3 },
  { a: 2, b: 1, expected: 3 },
])('$a + $b should equal $expected', ({ a, b, expected }) => {
  expect(a + b).toBe(expected);
});

For testing Zod schema validation in Vitest, use the testing-zod-schemas skill for patterns testing validation logic, error messages, and transformations.

Lifecycle Hooks

import { describe, test, beforeEach, afterEach, beforeAll, afterAll } from 'vitest';

describe('User service', () => {
  beforeAll(() => {
  });

  afterAll(() => {
  });

  beforeEach(() => {
  });

  afterEach(() => {
  });

  test('creates user', () => {
  });
});

Test Modifiers

test.skip('skipped test', () => {});
test.only('run only this test', () => {});
test.todo('implement later');
test.fails('should fail', () => {});
test.concurrent('runs in parallel', async () => {});

For testing Server Actions in isolation with Vitest, use the testing-server-actions skill for patterns on mocking authentication, database, and form data.

For testing form validation in React components with Vitest, use the validating-forms skill for patterns on form validation, useActionState, and error handling.

Timer Mocking

Fake Timers

import { vi } from 'vitest';

vi.useFakeTimers();

const callback = vi.fn();
setTimeout(callback, 1000);

vi.advanceTimersByTime(1000);
expect(callback).toHaveBeenCalled();

vi.useRealTimers();

Date Mocking

vi.setSystemTime(new Date(2022, 0, 1));

expect(Date.now()).toBe(new Date(2022, 0, 1).getTime());

vi.useRealTimers();

Snapshot Testing

test('matches snapshot', () => {
  const result = generateOutput();
  expect(result).toMatchSnapshot();
});

Update snapshots: vitest -u

Error Testing

test('throws error', () => {
  expect(() => {
    throw new Error('failed');
  }).toThrow('failed');
});

test('async throws', async () => {
  await expect(async () => {
    throw new Error('failed');
  }).rejects.toThrow('failed');
});

Fixtures

import { test as baseTest } from 'vitest';

const test = baseTest.extend({
  todos: async ({}, use) => {
    const todos = [];
    await use(todos);
    todos.length = 0;
  },
});

test('adds item', ({ todos }) => {
  todos.push('item');
  expect(todos).toHaveLength(1);
});

For testing custom React hooks with Vitest, use the testing-hooks skill for renderHook patterns and hook-specific assertions.

Mock Cleanup

Per-Test Cleanup

import { afterEach, vi } from 'vitest';

afterEach(() => {
  vi.restoreAllMocks();
});

Config-Based Cleanup

export default defineConfig({
  test: {
    restoreMocks: true,
    clearMocks: true,
  },
});

Component Testing

For testing React 19 components with Vitest browser mode, use the testing-components skill for patterns using @testing-library/react and handling useActionState, Server Actions, and React 19 hooks.

Best Practices

1. One assertion per test: Keep tests focused
2. Descriptive test names: Explain what is being tested
3. AAA pattern: Arrange, Act, Assert
4. Clean up mocks: Restore after each test
5. Test behavior, not implementation: Focus on public API
6. Avoid test interdependence: Tests should run independently
7. Use fixtures for shared setup: Reduce duplication

Common Mistakes

1. Not awaiting async operations: Use async/await
2. Not cleaning up mocks: Use afterEach or config options
3. Testing implementation details: Test public behavior
4. Overly complex tests: Break into smaller tests
5. Not using parameterized tests: Reduce duplication

References

For detailed testing patterns:

• [Test Organization](./references/test-organization.md) - Structure, hooks, fixtures, patterns
• [Assertions](./references/assertions.md) - Complete matcher API reference
• [Mocking](./references/mocking.md) - vi, modules, timers, globals

For configuration options, see @vitest-4/skills/configuring-vitest-4

For browser testing, see @vitest-4/skills/using-browser-mode

For complete API reference, see @vitest-4/knowledge/vitest-4-comprehensive.md