test-runner

Write and run tests across languages and frameworks.

Framework Selection

Quick Start by Framework

Vitest (TypeScript / JavaScript)

npm install -D vitest @testing-library/react @testing-library/jest-dom

// vitest.config.ts
import { defineConfig } from 'vitest/config'
export default defineConfig({
  test: {
    globals: true,
    environment: 'jsdom',
    setupFiles: './tests/setup.ts',
  },
})

npx vitest              # Watch mode
npx vitest run          # Single run
npx vitest --coverage   # With coverage

Jest

npm install -D jest @types/jest ts-jest

npx jest                # Run all
npx jest --watch        # Watch mode
npx jest --coverage     # With coverage
npx jest path/to/test   # Single file

pytest (Python)

uv pip install pytest pytest-cov pytest-asyncio httpx

pytest                          # Run all
pytest -v                       # Verbose
pytest -x                       # Stop on first failure
pytest --cov=app                # With coverage
pytest tests/test_api.py -k "test_login"  # Specific test
pytest --tb=short               # Short tracebacks

XCTest (Swift)

swift test                      # Run all tests
swift test --filter MyTests     # Specific test suite
swift test --parallel           # Parallel execution

Playwright (E2E)

npm install -D @playwright/test
npx playwright install

npx playwright test                    # Run all
npx playwright test --headed           # With browser visible
npx playwright test --debug            # Debug mode
npx playwright test --project=chromium # Specific browser
npx playwright show-report             # View HTML report

TDD Workflow

1. Red — Write a failing test that describes the desired behavior.
2. Green — Write the minimum code to make the test pass.
3. Refactor — Clean up the code while keeping tests green.

┌─────────┐     ┌─────────┐     ┌──────────┐
│  Write   │────▶│  Write  │────▶│ Refactor │──┐
│  Test    │     │  Code   │     │  Code    │  │
│  (Red)   │     │ (Green) │     │          │  │
└─────────┘     └─────────┘     └──────────┘  │
     ▲                                          │
     └──────────────────────────────────────────┘

Test Patterns

Arrange-Act-Assert

test('calculates total with tax', () => {
  // Arrange
  const cart = new Cart([{ price: 100, qty: 2 }]);

  // Act
  const total = cart.totalWithTax(0.08);

  // Assert
  expect(total).toBe(216);
});

Testing Async Code

test('fetches user data', async () => {
  const user = await getUser('123');
  expect(user.name).toBe('Colt');
});

Mocking

import { vi } from 'vitest';

const mockFetch = vi.fn().mockResolvedValue({
  json: () => Promise.resolve({ id: 1, name: 'Test' }),
});
vi.stubGlobal('fetch', mockFetch);

Testing API Endpoints (Python)

import pytest
from httpx import AsyncClient
from app.main import app

@pytest.mark.asyncio
async def test_get_users():
    async with AsyncClient(app=app, base_url="http://test") as client:
        response = await client.get("/users")
    assert response.status_code == 200
    assert isinstance(response.json(), list)

Testing React Components

import { render, screen, fireEvent } from '@testing-library/react';
import { Button } from './Button';

test('calls onClick when clicked', () => {
  const handleClick = vi.fn();
  render(<Button onClick={handleClick}>Click me</Button>);
  fireEvent.click(screen.getByText('Click me'));
  expect(handleClick).toHaveBeenCalledOnce();
});

Coverage Commands

# JavaScript/TypeScript
npx vitest --coverage          # Vitest (uses v8 or istanbul)
npx jest --coverage            # Jest

# Python
pytest --cov=app --cov-report=html    # HTML report
pytest --cov=app --cov-report=term    # Terminal output
pytest --cov=app --cov-fail-under=80  # Fail if < 80%

# View HTML coverage report
open coverage/index.html       # macOS
open htmlcov/index.html        # Python

What to Test

Always test:

• Public API / exported functions
• Edge cases: empty input, null, boundary values
• Error handling: invalid input, network failures
• Business logic: calculations, state transitions

Don't bother testing:

• Private implementation details
• Framework internals (React rendering, Express routing)
• Trivial getters/setters
• Third-party library behavior