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

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"), "[email protected]");
  await user.click(screen.getByRole("button", { name: /save/i }));

  expect(onSubmit).toHaveBeenCalledWith({ email: "[email protected]" });
});

• 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 });
  }),
];

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