React Testing

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

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

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

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" });
});

// 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"));

// 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());

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();
});

// 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";

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(result.current.count).toBe(1);

  act(() => result.current.decrement());
  expect(result.current.count).toBe(0);
});

test("useCounter accepts initial value", () => {
  const { result } = renderHook(() => useCounter(10));
  expect(result.current.count).toBe(10);
});

test("useUser fetches user data", async () => {
  // Instantiate QueryClient ONCE per test outside the wrapper so it survives re-renders.
  // Creating it inside the wrapper closure resets cache state on every render, producing flaky tests.
  const queryClient = new QueryClient({
    defaultOptions: { queries: { retry: false } },
  });
  const wrapper = ({ children }: { children: React.ReactNode }) => (
    <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
  );

  const { result } = renderHook(() => useUser("1"), { wrapper });

  await waitFor(() => expect(result.current.isSuccess).toBe(true));
  expect(result.current.data).toEqual({ id: "1", name: "Alice" });
});

import { axe, toHaveNoViolations } from "jest-axe"; // or vitest-axe
expect.extend(toHaveNoViolations);

test("UserCard has no a11y violations", async () => {
  const { container } = render(<UserCard user={mockUser} />);
  expect(await axe(container)).toHaveNoViolations();
});