Facebook has built a JavaScript engine, called Hermes, that improves the performance of React Native applications on Android devices. While the open-source engine is optimized for React Native today, it could be used to improve the performance of JavaScript-based mobile applications generally. 

Specifically, Hermes is geared to fast startup, leveraging compact bytecode and ahead-of-time static optimization. In many cases, enabling Hermes will improve startup time, decrease memory usage, and result in smaller app size.

Hermes currently is an opt-in feature in React Native. Most of Facebook’s React Native products on Android are already using Hermes, including Crisis Response and the Oculus companion app.

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