/react-testing

name: react-testing
description: React component testing with React Testing Library, Vitest/Jest, MSW for network mocking, accessibility assertions with axe, and the decision boundary between component tests and Playwright/Cypress end-to-end runs. Use when writing or fixing tests for React components, hooks, or pages.
metadata:
  origin: ECC

React Testing

Comprehensive React testing patterns for behavior-focused component tests, custom hook tests, accessibility assertions, and network-level mocking.

When to Activate

• Writing tests for React components, custom hooks, or pages
• Adding test coverage to legacy untested components
• Migrating from Enzyme or class-component-era patterns to React Testing Library
• Setting up Vitest or Jest for a new React project
• Mocking HTTP requests in tests
• Asserting accessibility violations
• Deciding which tests belong in RTL vs Playwright Component Testing vs full E2E

Core Principle

Test what the user sees and does, not implementation details.

A test should:

• Render the component with the same providers it has in production
• Interact with it via accessible queries (role, label) and `userEvent`
• Assert visible output and observable side effects (callback fired, request sent)

A test should NOT:

• Inspect component state, props passed to children, or which hooks were called
• Mock React itself or framework hooks
• Assert on the number of renders or DOM structure beyond what affects users

Library Choice

| Runner | When | Note | |---|---|---| | **Vitest** | Vite, Remix, modern setups | Faster, native ESM, Jest-compatible API | | **Jest** | Next.js, CRA, established repos | Default for many React projects | | **Playwright Component Testing** | Real browser engine needed | Use when JSDOM lacks the required feature | | **Cypress Component Testing** | Real browser, Cypress already in use | Alternative to Playwright CT |

Pick one. Do not run RTL + Vitest AND Playwright CT in the same repo unless you have a clear lane separation.

Query Priority

React Testing Library exposes queries in three tiers — use top-down:

1. **Accessible to everyone**: `getByRole`, `getByLabelText`, `getByPlaceholderText`, `getByText`, `getByDisplayValue` 2. **Semantic**: `getByAltText`, `getByTitle` 3. **Test IDs (escape hatch)**: `getByTestId`

// Best
screen.getByRole("button", { name: /save/i });

// OK for inputs
screen.getByLabelText("Email");

// Last resort
screen.getByTestId("save-btn");

Variants:

• `getBy*` — throws if no match
• `queryBy*` — returns `null` (use for "assert absence")
• `findBy*` — async, returns a Promise (use for elements that appear after async work)

User Interaction with `userEvent`

import userEvent from "@testing-library/user-event";

test("submits the form", async () => {
  const user = userEvent.setup();
  const onSubmit = vi.fn();
  render(<UserForm onSubmit={onSubmit} />);

  await user.type(screen.getByLabelText("Email"), "user@example.com");
  await user.click(screen.getByRole("button", { name: /save/i }));

  expect(onSubmit).toHaveBeenCalledWith({ email: "user@example.com" });
});

• Always `await` userEvent calls
• Call `userEvent.setup()` once per test, reuse the returned `user`
• `userEvent` simulates a real browser sequence; `fireEvent` dispatches a single synthetic event — prefer `userEvent`

Async Patterns

// Element that appears after async work
expect(await screen.findByText("Loaded")).toBeInTheDocument();

// Side effect assertion
await waitFor(() => expect(saveSpy).toHaveBeenCalled());

// Element that should disappear
await waitForElementToBeRemoved(() => screen.queryByText("Loading"));

Never `setTimeout` + assertion — flaky. Use the matchers above.

Network Mocking with MSW

Mock Service Worker mocks at the network layer. The component, hooks, and fetch library all behave exactly as in production.

Setup

// test/setup.ts
import { setupServer } from "msw/node";
import { http, HttpResponse } from "msw";

export const handlers = [
  http.get("/api/users/:id", ({ params }) =>
    HttpResponse.json({ id: params.id, name: "Alice" }),
  ),
  http.post("/api/users", async ({ request }) => {
    const body = await request.json();
    return HttpResponse.json({ id: "new-id", ...body }, { status: 201 });
  }),
];

export const server = setupServer(...handlers);

beforeAll(() => server.listen({ onUnhandledRequest: "error" }));
afterEach(() => server.resetHandlers());
afterAll(() => server.close());

Configure `onUnhandledRequest: "error"` so any unmocked request fails the test loudly — silent passes are worse than red.

Per-test override

test("renders error on 500", async () => {
  server.use(
    http.get("/api/users/:id", () => new HttpResponse(null, { status: 500 })),
  );
  render(<UserPage id="1" />);
  expect(await screen.findByText(/something went wrong/i)).toBeInTheDocument();
});

Provider Wrapping

Wrap providers once in a `test-utils.tsx`:

// test-utils.tsx
import { render, RenderOptions } from "@testing-library/react";
import { QueryClient, QueryClientProvider } from "@tanstack/react-query";

export function renderWithProviders(
  ui: React.ReactElement,
  options?: RenderOptions,
) {
  const queryClient = new QueryClient({
    defaultOptions: { queries: { retry: false } },
  });

  return render(
    <QueryClientProvider client={queryClient}>
      <ThemeProvider theme={lightTheme}>
        <MemoryRouter>{ui}</MemoryRouter>
      </ThemeProvider>
    </QueryClientProvider>,
    options,
  );
}

export * from "@testing-library/react";

Then `import { renderWithProviders, screen } from "test-utils"` in every test file.

Custom Hook Testing

import { renderHook, act } from "@testing-library/react";

test("useCounter increments and decrements", () => {
  const { result } = renderHook(() => useCounter(0));

  expect(result.current.count).toBe(0);

  act(() => result.current.increment());
  expect(r