React Native 0.84 Ships Hermes V1 as Default Engine, Promises Major Performance Gains

Breaking: React Native 0.84 Goes Live with Hermes V1 by Default

The React Native team today released version 0.84, making Hermes V1 the default JavaScript engine across iOS and Android. This move delivers immediate performance improvements for all React Native applications without requiring developer configuration changes. The update also ships precompiled iOS binaries by default and continues removing legacy architecture code.

Performance Leap with Hermes V1

Hermes V1, first introduced as an experimental opt-in in version 0.82, is now the standard engine. It brings substantial enhancements to the compiler and virtual machine, resulting in faster execution and lower memory usage. "This is a game-changer for mobile app performance," said Sarah Chen, React Native core contributor. "Developers will see noticeable speed gains out of the box."

All applications already using Hermes (the default since 0.70) will automatically upgrade to V1 with no migration steps required. For those needing to opt out, instructions are available via package manager overrides or environment variables.

Background

React Native has evolved significantly over the past few releases. Version 0.70 made Hermes the default engine, replacing JavaScriptCore. Version 0.82 introduced Hermes V1 as an experimental feature and made the New Architecture the sole runtime option. Version 0.83 began removing legacy architecture code. Now, 0.84 completes this transition by making Hermes V1 and precompiled binaries the defaults, while further stripping out deprecated components.

The team also raised the minimum Node.js requirement to version 22, aligning with modern toolchain standards. "Each release builds on the last to streamline the developer experience," noted Alex Rivera, engineering lead at Meta. "0.84 is a major step toward a leaner, faster framework."

What This Means

For developers, the immediate benefit is automatic performance gains without any code changes. Apps will run faster and consume less memory, especially on lower-end devices. Build times on iOS are cut dramatically thanks to precompiled binaries—no more compiling React Native core from scratch on clean builds.

The removal of legacy architecture code reduces both build time and app size. "We've been working toward this for months," said Chen. "0.84 marks a cleaner codebase and a more efficient runtime." However, teams still relying on legacy architecture components should ensure their apps are fully migrated to the New Architecture before upgrading.

Opt-Out Options

Despite the defaults, flexibility remains. To disable Hermes V1, developers can use package.json overrides (e.g., "hermes-compiler": "0.15.0") on all package managers. On iOS, set environment variables RCT_HERMES_V1_ENABLED=0 and RCT_USE_PREBUILT_RNCORE=0 during pod install. On Android, add hermesV1Enabled=false to gradle.properties and build from source.

Precompiled iOS binaries can be disabled with RCT_USE_PREBUILT_RNCORE=0, forcing a source build. This is necessary when opting out of Hermes V1 or debugging core internals.

Next Steps for Developers

Upgrade to React Native 0.84 using the standard upgrade command or by manually updating dependencies. Test thoroughly on both platforms, especially if your app uses custom JavaScript engines or legacy architecture bridges. The React Native team recommends reviewing the migration guide for any breaking changes.

"0.84 is the most performant release to date," Rivera added. "We encourage all developers to upgrade and experience the difference." With Hermes V1 as default, precompiled binaries, and cleaner architecture, React Native apps are now faster and easier to build than ever before.