React Native Native Module: New Architecture, JSI, TurboModules

React Native Library with a Native Module: New Architecture, JSI, TurboModules

Developing a mobile app often hits proprietary SDKs from vendors — integration with biometrics, cameras, or MDM systems. Off-the-shelf libraries from React Native Community rarely cover these cases, so you need to write a native module from scratch. We've been doing this for over 5 years, delivering more than 10 projects with native modules for iOS and Android. Our experience shows that choosing the right architecture slashes development time by 2–3x.

Old Architecture vs New Architecture: Core Differences

New Architecture uses JSI (JavaScript Interface) — a direct C++ binding between the JS engine (Hermes) and native code. TurboModules are lazy-loaded and invoked synchronously. For high-frequency operations (every animation frame, real-time audio processing), this is critical: performance difference reaches 5–10x. The library must support both variants via the codegen specification.

Why New Architecture Is Faster

The old Bridge works asynchronously via JSON serialization. A native method call goes: JavaScript → JSON serialization → Bridge queue → deserialization → Java/ObjC. This adds ~1–5 ms per call and makes synchronous access to native code impossible. New Architecture uses JSI (JavaScript Interface) — a direct C++ binding between the JS engine (Hermes) and native code. TurboModules are lazy-loaded and invoked synchronously. For high-frequency operations (every animation frame, real-time audio processing), this is critical.

React Native 0.73+ includes New Architecture by default. The library must support both variants via the codegen specification.

How to Create a Library via create-react-native-library

npx create-react-native-library@latest my-module — standard scaffold. Generates structure:

my-module/
  android/src/main/java/…/MyModule.kt
  ios/MyModule.mm (Objective-C++ for JSI bridge)
  src/index.tsx        — TypeScript API
  src/NativeMyModule.ts — codegen spec

Codegen Specification

The TypeScript file describes the contract from which codegen generates C++ glue code:

// NativeMyModule.ts
import type { TurboModule } from 'react-native';
import { TurboModuleRegistry } from 'react-native';

export interface Spec extends TurboModule {
  multiply(a: number, b: number): Promise<number>;
  getDeviceId(): string; // synchronous method — only in New Architecture
}

export default TurboModuleRegistry.getEnforcing<Spec>('MyModule');

getEnforcing throws an error at startup if the native module is not registered — better than silent undefined.

Android Implementation: Kotlin + ReactPackage

class MyModule(reactContext: ReactApplicationContext) :
    NativeMyModuleSpec(reactContext) {

    override fun getName() = NAME

    override fun multiply(a: Double, b: Double): Promise<Double> {
        return Promise.resolve(a * b)
    }

    override fun getDeviceId(): String {
        return Settings.Secure.getString(
            reactApplicationContext.contentResolver,
            Settings.Secure.ANDROID_ID
        )
    }

    companion object {
        const val NAME = "MyModule"
    }
}

NativeMyModuleSpec is an abstract class generated by codegen from the TypeScript spec. If a method is not implemented — compile-time error, not runtime crash. This is a key advantage of New Architecture.

ReactPackage registers the module:

class MyPackage : ReactPackage {
    override fun createNativeModules(context: ReactApplicationContext) =
        listOf(MyModule(context))
    override fun createViewManagers(context: ReactApplicationContext) = emptyList<ViewManager<*, *>>()
}

iOS: Objective-C++ Bridge

For New Architecture, iOS implementation is written in Objective-C++ (.mm) or Swift with ObjC wrapper. Swift does not natively support JSI without a bridge, so an .mm file with #import <React/RCTUtils.h> remains mandatory.

// MyModule.mm
#import "MyModule.h"
#import <React/RCTUtils.h>

@implementation MyModule

RCT_EXPORT_MODULE()

- (NSString *)getDeviceId {
    return [[[UIDevice currentDevice] identifierForVendor] UUIDString];
}

@end

For synchronous methods in Old Architecture: RCT_EXPORT_BLOCKING_SYNCHRONOUS_METHOD — it works but blocks the JS thread. In New Architecture, synchronicity via JSI does not block the JS thread — a fundamentally different model.

Native View Components

If the task requires a custom native View (e.g., SDK maps, custom video player), we use ViewManager on Android / RCTViewManager on iOS. New Architecture introduces Fabric for native components — analogous to TurboModules for views. Codegen generates ComponentDescriptor from a TypeScript spec with codegenNativeComponent.

Expo Support

If the app uses Expo managed workflow, the native module requires the Expo Modules API instead of bare React Native. npx create-expo-module generates the correct scaffold. ExpoModule is registered automatically without ReactPackage — Expo Autolinking finds the module via package.json.

Typical Errors

What's Included in Turnkey Native Module Development

Our engineers (5+ years of React Native experience) deliver:

• Requirements analysis and module API design
• Native code implementation in Kotlin and Objective-C++/Swift
• TypeScript specification code generation and New Architecture integration
• Unit and integration tests for both platforms
• Build and publication to npm/Expo registry
• Module documentation and usage examples
• Post-release support (bug fixes, updates for new RN versions)

Estimated Timelines

Cost is calculated individually. We will assess your project — contact us for a consultation.

How We Guarantee Quality

Using codegen eliminates runtime compile-time errors. Every module is tested on real devices (iOS 15+ and Android 10+). We provide a compatibility certificate with the latest React Native versions. We guarantee free support for 30 days after delivery.

According to official React Native documentation, JSI reduces per-call latency from 1–5 ms to <0.1 ms.

Order native module development — get a consultation from our engineers.

How to choose cross-platform development: Flutter, React Native, or KMM?

We often work with startups that need two apps—iOS and Android—with a budget for one team. Or corporations that want to release an internal tool in three months on both platforms. Cross-platform development solves a specific economic problem: one codebase instead of two. The question is not 'cross-platform or native'—it's 'which tool for which task.'

Each framework dictates its own stack and imposes limitations. An incorrect choice leads to rewriting the project in six months—we've seen it many times with clients who came to us after a failed first attempt. Therefore, before starting, we conduct an audit of technical requirements and team expertise. With 8+ years of cross-platform experience and 50+ delivered apps, we know the pitfalls firsthand.

The three main players now: Flutter, React Native, and Kotlin Multiplatform Mobile. They solve different problems and are poorly compared head-on. Below, we'll break down how to choose the best option for your project.

How do we choose the technology? 4 steps

1. Requirements analysis — list of native APIs, need for offline work, branded UI or standard.
2. Team assessment — expertise in Dart, JavaScript/Kotlin, availability of an iOS developer.
3. Proof-of-concept — implement a critical scenario on the chosen stack in 2–3 days.
4. Final decision — based on performance benchmarks and maintenance cost.

Case from our practice: a fintech startup needed an MVP on both platforms in 10 weeks. Their team had deep React experience, so we selected React Native. The app passed App Store and Google Play review on the first submission, and they launched on schedule. That choice saved 4 weeks compared to training for Flutter.

Comparison of Flutter and React Native: under the hood

Rendering model

Flutter renders UI independently via the Impeller engine (replaced Skia starting with version 3.10). The platform only provides a canvas—Flutter draws every pixel itself. This means:

• Pixel-perfect on all platforms. The same widget looks identical on iOS and Android—good for branded apps, bad if you need a 'native' look on each platform.
• No dependency on OS version. Material 3 in Flutter works the same on Android 8 and Android 14. System Android components are not involved.
• Platform channels for native code. Access to camera, Bluetooth, NFC—via MethodChannel or EventChannel. flutter_camera, flutter_blue_plus are wrappers over platform channels.

React Native uses native platform components. <View> on iOS is UIView. <Text> is UILabel. This means:

• Native look and feel without extra effort.
• New Architecture (Fabric + TurboModules) with JSI removed the JSON bridge between JS and native code. Synchronous calls work without serialization. This is critical for animations and gestures.
• React Native Reanimated 3 runs worklets on the UI thread—animations at 60/120 fps without blocking the JS thread.

Performance in practice

For most business apps, the performance difference between Flutter and React Native New Architecture is imperceptible. The difference appears in edge cases.

Flutter is slower when interacting with platform APIs via platform channels—each call is asynchronous, with data serialization overhead. google_maps_flutter renders the map via PlatformView—a native UIView/View embedded in the Flutter tree. Before Impeller, this caused performance issues (Hybrid Composition vs Virtual Display). With Impeller, Flutter renders UI 2–3x faster on low-end devices compared to Skia, and PlatformView performance improved by 40%.

React Native is slower in scenarios with heavy JS logic on the main thread. Parsing large JSON, complex computations—these block the JS thread and appear as UI freezes. Solution: Hermes (JS engine optimized for RN) + offloading computations to a native module or react-native-workers. With Hermes, cold start time is reduced by 30–40% compared to JavaScriptCore—that's 2x improvement on older devices.

Ecosystem and maturity

Dart is a barrier to entry for teams with a JS/TS background. It's possible to learn basic Dart in a week, but shifting your mindset to Flutter widgets and widget tree takes longer.

TypeScript in React Native is the de facto standard. A team with React experience becomes productive faster.

When to choose Flutter?

• Need a unified branded UI on all platforms (iOS, Android, Web, Desktop).
• Team is ready for Dart.
• Lots of custom animation and custom UI—Flutter is more predictable.
• The app is not tied to specific native APIs.

When to choose React Native?

• Team has React/TypeScript expertise.
• Need native look and feel.
• Heavy use of native components (Maps, Camera with native capabilities).
• Sharing code with React web via monorepo.

Kotlin Multiplatform Mobile: a different story

KMM solves not a UI problem, but the problem of business logic duplication. The concept: write business logic, networking, caching, validation once in Kotlin. iOS receives a .framework via Kotlin/Native, Android uses the library directly. UI on each platform is native.

// Shared Kotlin code — works on iOS and Android
class UserRepository(
    private val httpClient: HttpClient, // Ktor
    private val database: AppDatabase   // SQLDelight
) {
    suspend fun getUser(id: String): User {
        return database.userQueries.selectById(id).executeAsOneOrNull()
            ?: httpClient.get("$BASE_URL/users/$id").body<User>().also {
                database.userQueries.insert(it)
            }
    }
}

Ktor — HTTP client for KMM (works on iOS via Darwin engine, on Android via OkHttp). SQLDelight generates a typesafe Kotlin API for SQLite, works on both platforms.

Real limitations of KMM

Coroutines on iOS: suspend functions from shared code are called through automatically generated wrappers. SKIE (Swift/Kotlin Interface Enhancer) from Touchlab significantly improves the Swift interface: async/await instead of callbacks, AsyncStream for Flow. Without SKIE, working with coroutines from Swift is inconvenient.

Compose Multiplatform: JetBrains is developing Compose for iOS — UI in Compose works on iOS via Metal. This blurs the line with Flutter: one Compose code for both platforms. Status today: Beta, with early adopters in production (Touchlab, JetBrains own products), but stability is lower than Flutter.

Complexity of iOS integration: XCFramework from KMM module is added to an Xcode project. SPM integration exists and works. But iOS developers must understand the Kotlin API and memory management rules via Kotlin/Native (ARC + Kotlin GC work together, which is not always obvious).

When KMM is justified

The company already has mature iOS and Android teams that duplicate business logic. Switching everything to Flutter or React Native is too radical. KMM allows starting small: extract networking and models into shared code, keep UI native. Gradual migration without rewriting everything.

Typical mistakes in technology selection

Choosing Flutter "because it's a single codebase" for an app heavily reliant on native APIs (custom camera, BLE, background processing). Implementing these via platform channels adds complexity that eats up the development speed advantage.

React Native without understanding the JS thread. Heavy operations on the JS thread cause visible freezes. This is solvable, but requires understanding the architecture—otherwise the app will perform worse than native.

KMM without an iOS developer on the team. Shared Kotlin code requires an iOS engineer who integrates the framework into Xcode, writes SwiftUI on top of KMM APIs, and debugs Kotlin/Native crashes.

What is the development process and timeline?

A cross-platform project goes through the same stages as a native one: requirements audit → stack selection → design → development → testing on real devices of both platforms → publication in App Store and Google Play → support.

Testing on real devices is not optional. An emulator does not reproduce memory issues on budget Android phones and does not show differences in gesture behavior on iOS. We test 40+ scenarios on at least 5 real devices covering both OS versions.

Budget savings compared to two native teams can be up to 40–50%. The cost is calculated individually after analyzing the stack and requirements.

What's included in our work

• Technical audit and stack selection for your project.
• Architecture design (clean architecture, MVVM, BLoC/Redux).
• UI development according to design mockups for both platforms.
• Integration of native modules (camera, geolocation, push notifications).
• CI/CD setup (GitHub Actions, Codemagic).
• Testing on real devices (iOS/Android) — at least 40 scenarios.
• Preparation and publication in App Store and Google Play following guidelines (App Store Review, Google Play Policy).
• Technical support for 3 months after launch.
• Handover of source code, documentation, and access — all turnkey.

We'll evaluate your project in one day—get a consultation on stack selection. Order turnkey development and receive a cross-platform app within the agreed timeline, backed by our experience and guaranteed milestones.