Hermes Roadmap

Hermes aims to support more modern JavaScript features while avoiding performance regressions. The guest says features are generally launched only after they are shown not to slow performance or to improve it. Promise scheduling is another priority because React Native currently polyfills Promises with setImmediate, which schedules them like timers rather than microtasks, and improving this is needed for React Concurrent Mode.

The team is working on garbage collector improvements to keep pauses short, which would help React Native’s New Architecture as it adds synchronous calls into JavaScript. Hermes Value32 would use 32-bit values with pointer compression instead of 64-bit tagged values. Small array optimization targets tuple-like arrays common in React Hooks, such as values returned by useState and useReducer, to avoid separate storage allocation.

Hermes work also covers debugging, profiling, and source maps. An unstable Metro transform profile can already remove about half of Babel’s transforms to speed builds; the team is working to remove more transforms and stabilize the profile. The guest says React Native’s needs help focus Hermes development, alongside requests from the open-source community.

The guest describes native Intl support as an important condition for broader Hermes adoption in React Native. Microsoft contributed Android support, while React Native Windows is adapting its implementation to use sanctioned Windows globalization APIs instead of ICU. macOS and iOS also need platform-specific approaches because ICU access is restricted; the guest could not confirm the current macOS effort after moving to Teams.

The guest says Proxy became the default in React Native 0.64, addressing a community request even though Facebook did not use the feature internally. Android Intl support was close to completion, with a plan to release it in React Native 0.65. Facebook uses FBT internally for internationalization, while the Intl API is being added for the wider community.