testing-patterns

Skill: Frontend Testing Patterns

When to load

When writing tests for components, hooks, or integration flows.

Philosophy

Test behavior, not implementation. A user doesn't care that you called setState; they care that clicking "Submit" shows a success message.

Component Test Template

import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { UserCard } from './UserCard';

const defaultUser: User = {
  id: '1', name: 'Jane Smith', email: '[email protected]', role: 'admin',
};

describe('UserCard', () => {
  it('renders user information', () => {
    render(<UserCard user={defaultUser} />);
    expect(screen.getByText('Jane Smith')).toBeInTheDocument();
  });

  it('calls onEdit when edit button is clicked', async () => {
    const onEdit = vi.fn();
    render(<UserCard user={defaultUser} onEdit={onEdit} />);
    await userEvent.click(screen.getByRole('button', { name: /edit/i }));
    expect(onEdit).toHaveBeenCalledWith(defaultUser.id);
  });
});

Custom Hook Testing

import { renderHook, act } from '@testing-library/react';
import { useCounter } from './useCounter';

it('increments count', () => {
  const { result } = renderHook(() => useCounter(0));
  act(() => { result.current.increment(); });
  expect(result.current.count).toBe(1);
});

MSW: Mock API Calls

import { http, HttpResponse } from 'msw';

export const handlers = [
  http.get('/api/users', () =>
    HttpResponse.json([{ id: '1', name: 'Jane' }])
  ),
];

Test Type Decision

Scenario	Test Type
Component renders	Unit (RTL)
User interaction	Unit (RTL + userEvent)
Data fetching states	Unit (RTL + MSW)
Full user journey	E2E (Playwright)