Use a context’s fallback value.

Use a context’s fallback value with a small React component and explicit browser behavior.

import { useContext, createContext } from 'react';

const ThemeContext = createContext("light");
function Theme() { return <output>{useContext(ThemeContext)}</output>; }

export default function App() {
    return (
        <main data-ready>
            <Theme />
        </main>
    );
}

Expected output EXAMPLE

The output displays “light”.

Details

Use this App.jsx in a React 19 project with JSX support. Mount the default App component with createRoot in your existing entry point. The test mounts this exact component and checks its rendered behavior.

✓ Verified

Tested on Ubuntu 24.04.3 LTS · 2026-11-02 18:46:54 UTC · Node.js v24.15.0; Chromium 141.0.7390.37 · esbuild 0.28.2; playwright 1.56.0; react 19.3.1; react-dom 19.3.1

Test inputs, results and environment

The exact published code ran in a dedicated Linux test container with prepared inputs. Dedicated test container, external networking disabled, read-only root, temporary per-example working directory. Browser network requests are blocked. Fixtures and output records are isolated from the application database. Testing covers this example and its stated checks, not every input, operating system, or runtime version. The Linux container shares its host's kernel; this is not a native Windows test.

Architecture / host kernel: x86_64 / 6.18.54.1-microsoft-standard-WSL2
Snippet identity / locale: uid=65534, gid=65534 (unprivileged) / C.UTF-8
Interpreter invocation: bundle the exact published component with esbuild; mount it in Chromium and execute browser assertions
Container image ID: sha256:c9bce90110f636fe04907469a37e9c8ac888171fd569cbf87101743ec7cdf099
Code SHA-256: 8ea4d675f2d79613ee94949e5140784df77570ec9870a850e07d9146dc55b3eb
Dependencies: esbuild 0.28.2; playwright 1.56.0; react 19.3.1; react-dom 19.3.1

Test input setup

These are the prepared inputs used by the test harness. See Details above for the prerequisites and how to run this example.

The harness supplies index.html and mounts the published component with locally installed, pinned dependencies.

Observed standard output

Expected output describes the rendered page. The standard-output record below is the browser test harness receipt; the additional assertions check the result and interactions.

Use a context’s fallback value: browser checks passed.

Exit code: 0. Standard error: empty. Validation: Shape/content assertions. System-dependent values above are one recorded run.

Additional assertions that passed

Checks recorded by the test harness. Where Python assertions are shown, browser cases use Playwright and simulator cases may inspect pin transitions: result.stdout is captured output, work is the temporary directory, and Path is pathlib.Path. The harness also imports json and csv.

await text('output','light');