React Native 0.84: Hermes V1 Default and Performance Upgrades
React Native 0.84 is here, bringing Hermes V1 as the default JavaScript engine, along with faster iOS builds and a cleaner codebase. This release automatically improves app performance without requiring any migration. Below, we answer key questions about the update, covering everything from the new engine to optional features.
Jump to a section:
- What is the major change in React Native 0.84?
- How does Hermes V1 improve app performance?
- Do I need to migrate my existing app to take advantage of Hermes V1?
- How can I opt out of Hermes V1 if needed?
- What are the benefits of precompiled iOS binaries becoming the default?
- What legacy architecture components have been removed?
- What are the new minimum requirements for React Native 0.84?
What is the major change in React Native 0.84?
The headline change is that Hermes V1 is now the default JavaScript engine on both iOS and Android. Previously an experimental opt-in since React Native 0.82, Hermes V1 is now enabled automatically for all new and existing projects. This upgrade brings a more advanced compiler and VM, delivering measurable improvements in JavaScript execution speed and memory efficiency. Additionally, iOS apps benefit from precompiled binaries out of the box, making clean builds faster than ever. The release also continues the removal of legacy architecture components, reducing build times and bundle sizes.
How does Hermes V1 improve app performance?
Hermes V1 is the next evolution of the Hermes engine, with significant enhancements to both the compiler and virtual machine. These improvements translate into faster JavaScript execution and lower memory usage for your React Native apps. The engine is optimized for mobile devices, reducing startup times and improving runtime responsiveness. Because it's a drop-in replacement, you get these gains automatically—no code changes or configuration tweaks are required. If your app was already using Hermes (the default since version 0.70), you seamlessly upgrade to V1. The result is a smoother user experience and a smaller memory footprint, especially on lower-end devices.
Do I need to migrate my existing app to take advantage of Hermes V1?
No migration is necessary. If your project already uses Hermes (which has been the default since React Native 0.70), updating to version 0.84 automatically switches you to Hermes V1. The engine is backward-compatible, so your existing JavaScript code runs without modification. This zero-effort upgrade means you can immediately benefit from the performance improvements without any configuration changes. Simply update your React Native version and rebuild—the new engine takes over directly.
How can I opt out of Hermes V1 if needed?
While Hermes V1 is the default, you can still use the legacy Hermes compiler if absolutely necessary. There are three methods depending on your platform:
- Package manager override – Force the legacy
hermes-compilerpackage by adding an override topackage.json(usingoverridesfor npm,resolutionsfor Yarn, orpnpm.overridesfor pnpm). - iOS – When running
pod install, set the environment variablesRCT_HERMES_V1_ENABLED=0andRCT_USE_PREBUILT_RNCORE=0. - Android – Add
hermesV1Enabled=falsetoandroid/gradle.propertiesand configure the app to build React Native from source.
These options allow you to revert to the legacy engine while still using React Native 0.84.
What are the benefits of precompiled iOS binaries becoming the default?
Starting with React Native 0.84, precompiled binaries are shipped for iOS by default. This means you no longer have to compile React Native core from source during a clean build. Instead, ready-to-use .xcframework binaries are automatically downloaded and integrated during pod install. The result is a significant reduction in iOS build times, especially for first-time or CI builds. If you need to build from source (e.g., to opt out of Hermes V1), simply set RCT_USE_PREBUILT_RNCORE=0 when installing pods. This feature was previously opt-in but is now enabled for all projects, streamlining the development workflow.
What legacy architecture components have been removed?
React Native 0.84 continues the cleanup of the Legacy Architecture that began in version 0.82. In 0.83, an experimental flag RCT_REMOVE_LEGACY_ARCH was introduced to compile out legacy code. Now in 0.84, this is the default behavior on iOS: Legacy Architecture code is no longer included in builds. This reduces both build times and app size. No breakages are expected for apps already on the New Architecture, as the change only removes unused code. Android sees similar removals, continuing the trend toward a leaner, modern codebase. For a full list of removed classes, refer to the official RFC.
What are the new minimum requirements for React Native 0.84?
React Native 0.84 raises the minimum required Node.js version to 22. This ensures you have access to the latest JavaScript runtime features and security updates. Developers using older Node.js versions will need to upgrade before adopting this release. Aside from that, other platform requirements remain unchanged: Xcode 15+ for iOS and Android Studio with AGP 8+ for Android. By aligning with a more recent Node.js, React Native can leverage modern tooling and provide a more consistent development experience.