Hermes
Also called Hermes engine
- Part of
- React Native Glossary
- Also known as
- Hermes engine
- Mentioned in
- JavaScript BundleJavaScript RuntimeJSI
- Updated
What is Hermes in React Native?
Hermes is the JavaScript engine Meta built for React Native. Default since version 0.70, it precompiles code to bytecode for faster startup and lower memory use.
Hermes is an open-source JavaScript engine Meta built specifically for running React Native on mobile. It has been the default engine since React Native 0.70, replacing JavaScriptCore.
It is optimised for constraints a browser engine does not have:
- Ahead-of-time bytecode. Instead of parsing JavaScript on every launch, Hermes compiles the bundle to bytecode at build time. That removes parse cost from startup, which is the single biggest win on low-end Android devices.
- Smaller memory footprint, from a garbage collector tuned for mobile.
- No JIT. Hermes trades peak throughput for start-up speed and predictability — the right trade for UI code, less so for heavy computation.
Two practical notes. Debugging targets React Native DevTools rather than a Chrome tab, because Hermes is not Chrome's V8. And engine gaps are real: an Intl or regex feature that works in a browser may be absent, so test on device rather than trusting a web preview.
See Hermes.