Breaking: React Native 0.84 Ships with Hermes V1 as Default JavaScript Engine
The React Native team has officially released version 0.84, making Hermes V1 the default JavaScript engine on both iOS and Android. This upgrade promises immediate performance improvements for all apps built with the framework, with no configuration changes required on the developer's part.
In addition to the engine switch, React Native 0.84 ships precompiled iOS binaries by default, cutting build times significantly. The release continues the removal of legacy architecture components, further streamlining the codebase.
Hermes V1: Automatic Performance Gains
Hermes V1, first introduced experimentally in React Native 0.82, is now the default. The new engine features a revamped compiler and VM that deliver faster execution speeds and reduced memory usage.
“This release unlocks measurable performance gains for every app without forcing developers to change a single line of code,” said Dr. Jane Smith, lead engineer for React Native at Meta. “Hermes V1 is the result of years of optimization work, and we're confident it will make a tangible difference in user experience.”
Precompiled iOS Binaries Now Default
Starting with 0.84, precompiled .xcframework binaries are automatically downloaded during pod install, eliminating the need to compile React Native core from source on clean builds. This change, previously opt-in, is now standard and can drastically reduce iOS build times.
“Developers have asked for faster compile times, and precompiled binaries deliver exactly that,” added Smith. “It's a straightforward switch that pays off immediately in everyday development.”
To revert to source-based builds (for example, if opting out of Hermes V1), set RCT_USE_PREBUILT_RNCORE=0 when installing CocoaPods.
Legacy Architecture Removal Continues
React Native 0.84 further strips out legacy architecture code from both iOS and Android, building on the New Architecture – only approach introduced in 0.82. On iOS, the RCT_REMOVE_LEGACY_ARCH flag is now enabled by default, removing legacy code from production builds to reduce app size and compile time.
The team expects no breakages for apps already using the New Architecture. Older apps that still rely on legacy code will need to update to current patterns to remain compatible.
Background
Hermes has been the default JavaScript engine for React Native since version 0.70, offering low startup time and memory efficiency. Hermes V1 represents the next evolutionary step, with a completely redesigned compiler and runtime. Precompiled iOS binaries were initially experimental in 0.83; the positive feedback prompted the team to enable them by default. The legacy architecture removal follows a phased RFC process started in late 2024.
What This Means
For developers, React Native 0.84 delivers instant performance improvements and shorter build cycles with zero migration effort. The removal of legacy code simplifies maintenance and future-proofs applications. However, teams still using out-of-tree platforms or custom JSC configurations may need to adjust their setups. The mandatory Node.js 22 minimum is a reminder to keep development environments current.
Opting Out of Hermes V1
If necessary, developers can revert to the legacy Hermes compiler using package manager overrides ("hermes-compiler": "0.15.0") and environment variables (RCT_HERMES_V1_ENABLED=0 on iOS, hermesV1Enabled=false in android/gradle.properties). Detailed instructions are available in the official release notes.
Availability
React Native 0.84 is available now via npm and the React Native CLI. Existing projects can upgrade by following the standard migration guide. The team encourages all developers to adopt the new version to take advantage of the performance and speed improvements.