JavaScript Information

A JavaScript engine is the software inside your browser that parses and executes JavaScript code. The three major engines are V8 (used by Chrome, Edge, Opera, Brave, and Node.js), SpiderMonkey (used by Firefox), and JavaScriptCore (used by Safari). Different engines optimize code differently, so the same script can run at very different speeds depending on which engine is running it.

Each engine produces a distinctive Error.stack format. V8 prefixes every frame with 'at ', SpiderMonkey and JavaScriptCore use 'funcName@url:line:col', and we use a handful of engine-specific globals (InternalError, etc.) to disambiguate the last two. This is a heuristic check — the user agent header is authoritative — but the stack-shape check confirms the actual runtime executing the page, which is useful when a browser ships a custom engine or when the UA is spoofed.

hardwareConcurrency reports the number of logical CPU cores your browser can schedule threads onto. Web apps use it to size Web Worker pools, to tune WASM SIMD batch sizes, and to decide whether to run heavy algorithms in parallel. It is also a strong fingerprinting signal — combined with device memory, screen resolution, and GPU it can narrow your browser to a small crowd of users.

SharedArrayBuffer lets multiple threads read and write the same memory, which is essential for high-performance WASM. After the Spectre and Meltdown CPU vulnerabilities, browsers restricted it to pages served with Cross-Origin Opener Policy (COOP) and Cross-Origin Embedder Policy (COEP) headers that isolate the page from other origins. If your page does not send those headers, SharedArrayBuffer appears as undefined.

Client Hints are the modern replacement for the User-Agent string. Instead of sending one long string on every request, the browser exposes navigator.userAgentData with low-entropy fields (brand, mobile, platform) by default and a getHighEntropyValues() method that resolves high-entropy fields (architecture, bitness, full version list, model, platform version) on demand. Chromium ships them today; Firefox and Safari have declined to implement them, so the API shows as unsupported on those browsers.

Web Audio is a fingerprinting signal, not just an audio feature. Even without playing a sound, a tracker can create an AudioContext, read destination.maxChannelCount and the default sample rate, and compute a device-specific hash from analyser-node output. The values reported here are the same ones fingerprinting libraries sample; changing your audio device or OS audio settings can shift them.