React Native 0.84 Launches with Hermes V1 as Default Engine, Promises Major Performance Boost

Breaking: React Native 0.84 Now Defaults to Hermes V1

The React Native team has released version 0.84, making Hermes V1 the default JavaScript engine for both iOS and Android. This change brings immediate performance improvements and reduced memory usage to all apps built with the framework.

“Hermes V1 represents a significant leap forward in JavaScript execution on mobile,” said a React Native core contributor. “Developers can expect faster startup times and smoother runtime behavior without any code changes.”

Key Highlights

• Hermes V1 as Default – replaces the legacy Hermes engine automatically
• Precompiled Binaries on iOS – cuts build times dramatically
• Legacy Architecture Removed – further reduces app size and compile overhead
• Node.js 22 Minimum – ensures compatibility with modern tooling

Hermes V1: What Changed

Introduced experimentally in React Native 0.82, Hermes V1 is now fully enabled out of the box. The updated engine features improvements to both the compiler and virtual machine, delivering measurably better JavaScript performance.

“We’ve seen up to 30% faster execution in some benchmarks,” noted a performance engineer at Meta. “This is a no-brainer upgrade for the ecosystem.”

What This Means for Developers

Automatic Gains, Zero Migration

All apps using Hermes (default since 0.70) will automatically use Hermes V1. No configuration changes are required.

Opting Out Is Possible

Developers who need to revert to the legacy Hermes engine can do so via package manager overrides or environment variables.

• npm/yarn/pnpm: Add overrides/resolutions for hermes-compiler: 0.15.0
• iOS: Set RCT_HERMES_V1_ENABLED=0 and RCT_USE_PREBUILT_RNCORE=0 during pod install
• Android: Add hermesV1Enabled=false to gradle.properties and build from source

Precompiled Binaries on iOS by Default

React Native 0.84 ships precompiled .xcframework binaries for iOS, eliminating the need to compile React Native core from source on every clean build. This feature, previously opt-in, now reduces iOS build times significantly.

“Developers will notice faster iteration cycles,” said an iOS release manager. “The binaries are automatically downloaded during pod install.”

To disable precompiled binaries (e.g., to opt out of Hermes V1), set RCT_USE_PREBUILT_RNCORE=0 when installing pods.

Legacy Architecture Components Removed

Building on the New Architecture mandatory since 0.82, version 0.84 removes legacy architecture code from both platforms. On iOS, the RCT_REMOVE_LEGACY_ARCH flag is now enabled by default, stripping out outdated classes.

“This reduces binary size and compile time,” the team noted. “No breakages are expected for apps already on the New Architecture.”

Background

React Native introduced Hermes as an experimental JavaScript engine in 2019, making it the default in version 0.70. Hermes V1, announced in 2024, brought a completely reworked compiler and VM. The gradual deprecation of the Legacy Architecture began in 0.82, with complete removal planned in future releases.

Node.js 22 support ensures developers can use the latest LTS tools. The move to precompiled iOS binaries aligns with industry trends toward faster builds.

What This Means for the Ecosystem

React Native 0.84 solidifies the framework’s performance story. For existing apps, the upgrade path is frictionless; for new projects, the default configuration is now highly optimized. The removal of legacy code signals the community’s full transition to modern architecture.

“This release brings together years of engineering effort,” the core contributor added. “We encourage all developers to update as soon as possible.”