Test Runner

Write and run tests across languages and frameworks.

Core workflow

1. Detect the language, package manager, and existing test framework before changing anything.
2. Prefer the project's current test stack over introducing a new one.
3. Run the smallest relevant test scope first, then widen coverage after the failure is understood.
4. When fixing bugs, start with a failing test, make the smallest code change that passes, then refactor.
5. After changes, run the narrow test again and then a broader suite if the local workflow supports it.

Framework selection

Use the existing framework when one is already configured. If the project has no test stack, prefer:

Language	Unit tests	Integration	End-to-end
TypeScript / JavaScript	Vitest	Supertest	Playwright
Python	pytest	pytest + httpx	Playwright
Swift	XCTest	XCTest	XCUITest

Command guide

Vitest

bash

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

Use this baseline config when a project needs one:

typescript

import { defineConfig } from 'vitest/config'

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

Jest

bash

npm install -D jest @types/jest ts-jest
npx jest
npx jest --watch
npx jest --coverage
npx jest path/to/test

pytest

bash

uv pip install pytest pytest-cov pytest-asyncio httpx
pytest
pytest -v
pytest -x
pytest --cov=app
pytest tests/test_api.py -k "test_login"
pytest --tb=short

XCTest

bash

swift test
swift test --filter MyTests
swift test --parallel

Playwright

bash

npm install -D @playwright/test
npx playwright install
npx playwright test
npx playwright test --headed
npx playwright test --debug
npx playwright test --project=chromium
npx playwright show-report

Red-green-refactor

1. Red: write a failing test for the behavior you need.
2. Green: implement the smallest change that makes it pass.
3. Refactor: clean up without changing behavior.

Test patterns

Arrange, act, assert

typescript

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

  const total = cart.totalWithTax(0.08)

  expect(total).toBe(216)
})

Async tests

typescript

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

Mocking with Vitest

typescript

import { vi } from 'vitest'

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

vi.stubGlobal('fetch', mockFetch)

API tests in Python

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)

React component tests

typescript

import { fireEvent, render, screen } 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

bash

# JavaScript / TypeScript
npx vitest --coverage
npx jest --coverage

# Python
pytest --cov=app --cov-report=html
pytest --cov=app --cov-report=term
pytest --cov=app --cov-fail-under=80

What to test

Always test:

• Public APIs and exported functions
• Edge cases such as empty input, null, and boundary values
• Error handling such as invalid input or network failures
• Business logic such as calculations and state transitions

Usually skip:

• Private implementation details
• Framework internals
• Trivial getters and setters
• Third-party library behavior