What is the JavaScript engine in React Native? Compare Hermes, JSC, and V8.

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React Native runs your JavaScript inside an engine on the device. The choice of engine affects startup time, memory, bundle size, and debugging . Hermes (default since RN 0.70): Built by Meta specifically for React Native on mobile. Precompiles JS to bytecode at build time → drastically faster app startup (no parse cost at launch). Lower memory footprint, smaller APK/IPA size, garbage collector tuned for mobile. Built-in sampling profiler and source-map support for Chrome DevTools. JavaScriptCore (JSC) — legacy fallback: Apple's JS engine bundled with WebKit; was the RN default before 0.70 . On iOS, JSC is provided by the OS (no extra binary cost); on Android, RN had to bundle it (larger APK). Slower startup than Hermes (no bytecode precompilation). V8 — NOT a stock option: V8 is the Chrome/Node.js engine — not a built-in RN engine. Some teams used react-native-v8 historically for parity with Node; rare in 2026. The wrong answer is 'V8' — V8 is the Chrome / Node engine and is NOT used in React Native. **Hermes** has been the default since RN 0.70 (2022) and is what 95%+ of production apps run today. Mention its three big wins: bytecode-precompiled bundles (faster startup), smaller memory footprint, and built-in sampling profiler. JSC still ships as the fallback on iOS for very old projects.