Turbo Native Modules with JSI
Define a TypeScript spec file for a Turbo Native Module, implement it in Kotlin and Swift using the codegen-generated interface, and call the module synchronously via JSI.
Turbo Native Modules with JSI is a free React Native Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the React Native Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
The Problem with the Legacy Bridge
The legacy React Native bridge serializes every message between JavaScript and native into JSON, then deserializes it on the other side. This round-trip adds latency for every native call and blocks threads while serialization happens. For frequent calls like scroll events or animations, this overhead causes frame drops. JSI (JavaScript Interface) was created to eliminate this bottleneck.
What Is JSI
JSI (JavaScript Interface) is a lightweight C++ layer that lets JavaScript hold direct references to C++ host objects. Instead of sending a JSON-serialized message across the bridge, a JS call invokes a C++ function pointer directly and synchronously. This means native calls can be synchronous, zero-copy, and orders of magnitude faster than bridge-based calls.
Turbo Modules Architecture Overview
Turbo Native Modules are the new module system built on JSI. Key differences from legacy modules include: lazy loading (modules initialize only when first accessed instead of all at startup), typed interfaces (generated from a TypeScript spec so types are guaranteed on both sides), and synchronous calls possible without blocking the JS thread via JSI. The codegen tool generates the boilerplate C++ and native glue code automatically.
Writing the TypeScript Spec File
A Turbo Module starts with a TypeScript spec file that declares the module's interface. This file lives in your project and is processed by react-native-codegen to generate native interface files. The spec must import TurboModuleRegistry and define a type that extends TurboModule.
// NativeDeviceInfo.ts (spec file)
import type { TurboModule } from 'react-native';
import { TurboModuleRegistry } from 'react-native';
export interface Spec extends TurboModule {
getDeviceModel(): string;
getBatteryLevel(): Promise<number>;
addListener(eventType: string): void;
removeListeners(count: number): void;
}
export default TurboModuleRegistry.getEnforcing<Spec>('DeviceInfo');Running Codegen to Generate Native Code
After writing the TypeScript spec, you run React Native Codegen to generate the native interface files. For managed Expo projects, this happens automatically during expo prebuild. For bare React Native projects, run yarn react-native codegen. Codegen outputs Swift/Kotlin interfaces and C++ glue code in the build/generated folder that your native implementation must conform to.
# Bare React Native — run codegen manually
cd android && ./gradlew generateCodegenArtifactsFromSchema
# Then check generated files
ls android/app/build/generated/source/codegen/jni/
# NativeDeviceInfoJSI-generated.cpp
# NativeDeviceInfoJSI.hImplementing the Module in Kotlin
The Kotlin implementation must extend the codegen-generated abstract class (e.g., NativeDeviceInfoSpec) and override each method declared in the spec. Because the signatures are generated from TypeScript, there is no type mismatch between JS and native — the codegen enforces them at build time rather than at runtime.
// DeviceInfoModule.kt
package com.yourapp
import com.facebook.react.bridge.ReactApplicationContext
import com.yourapp.NativeDeviceInfoSpec
class DeviceInfoModule(context: ReactApplicationContext) :
NativeDeviceInfoSpec(context) {
companion object {
const val NAME = 'DeviceInfo'
}
override fun getName(): String = NAME
override fun getDeviceModel(): String {
return android.os.Build.MODEL
}
}Implementing the Module in Swift
The Swift implementation conforms to the codegen-generated protocol (e.g., NativeDeviceInfoSpec). Unlike legacy modules, there is no separate bridge .m file needed for Turbo Modules — the JSI layer handles the binding automatically once the module is registered. The protocol guarantees that the Swift method signatures match the TypeScript spec exactly.
// DeviceInfoModule.swift
import Foundation
@objc(DeviceInfoModule)
class DeviceInfoModule: NSObject, NativeDeviceInfoSpec {
func getDeviceModel() -> String {
return UIDevice.current.model
}
func getBatteryLevel(
_ resolve: @escaping RCTPromiseResolveBlock,
reject: @escaping RCTPromiseRejectBlock
) {
UIDevice.current.isBatteryMonitoringEnabled = true
resolve(UIDevice.current.batteryLevel * 100)
}
}Synchronous Calls via JSI
One of the biggest JSI benefits is the ability to call native synchronously from JS. When a Turbo Module method returns a non-Promise type (like string or number in the spec), JSI can return the value immediately without scheduling an async callback. This unlocks patterns like reading cached values synchronously that were impossible with the legacy bridge.
// TypeScript spec — synchronous return type
export interface Spec extends TurboModule {
getDeviceModel(): string; // synchronous
getBatteryLevel(): Promise<number>; // still async
}
// Usage in JS — getDeviceModel() is synchronous
import NativeDeviceInfo from './NativeDeviceInfo';
const model = NativeDeviceInfo.getDeviceModel(); // no await needed
console.log('Model:', model);Enabling the New Architecture
Turbo Modules require the New Architecture to be enabled in your project. For Android, set newArchEnabled=true in android/gradle.properties. For iOS, set RCT_NEW_ARCH_ENABLED=1 in the Podfile and run pod install. Expo SDK 50+ enables the new architecture automatically for managed workflow projects using the latest config.
# android/gradle.properties
newArchEnabled=true
# ios/Podfile — add before use_react_native!
ENV['RCT_NEW_ARCH_ENABLED'] = '1'
# Then reinstall pods
cd ios && pod installRegistering a Turbo Module
Turbo Modules are registered via a TurboModuleManagerDelegate instead of a ReactPackage. On Android you modify MainApplicationTurboModuleManagerDelegate.kt; on iOS you modify RCTAppDelegate.mm. The codegen also generates a ModuleProvider that wires up the module automatically when you follow the standard naming conventions.
// Android: MainApplicationTurboModuleManagerDelegate.kt
override fun getModule(
reactApplicationContext: ReactApplicationContext,
name: String
): NativeModule? {
return when (name) {
DeviceInfoModule.NAME -> DeviceInfoModule(reactApplicationContext)
else -> null
}
}Comparing Legacy vs Turbo Module Performance
Benchmarks show Turbo Modules can be 2–3x faster for high-frequency calls compared to legacy bridge modules. The improvement is most noticeable for calls made in rapid succession — such as gesture handlers or animation drivers. For infrequent calls like camera setup, the difference is negligible. Migrate to Turbo Modules when performance-critical native code needs to keep up with 60fps interactions.
// Performance benchmark pattern
const ITERATIONS = 10000;
const start = Date.now();
for (let i = 0; i < ITERATIONS; i++) {
// Synchronous JSI call
const model = NativeDeviceInfo.getDeviceModel();
}
const elapsed = Date.now() - start;
console.log('10k JSI calls in', elapsed, 'ms');
// Turbo: ~8ms vs Legacy bridge: ~80msQuick Check
Test your understanding of React Native Mobile Development concepts from this lesson.
Lesson Recap
In this lesson you learned: how JSI eliminates JSON serialization overhead from the legacy bridge, how to write a TypeScript spec file that codegen uses to generate native interfaces, and how to implement the generated spec in both Kotlin and Swift. You also saw how to enable the New Architecture and the performance benefits of synchronous JSI calls. Next up we explore async callbacks, promises, and native events.
Frequently asked questions
Is the “Turbo Native Modules with JSI” lesson free?
Yes — the full text of “Turbo Native Modules with JSI” is free to read here on the web, and the React Native Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the React Native Academy course, upgrade to CoddyKit PRO.
What will I learn in “Turbo Native Modules with JSI”?
Define a TypeScript spec file for a Turbo Native Module, implement it in Kotlin and Swift using the codegen-generated interface, and call the module synchronously via JSI. You practise React Native Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start React Native Academy?
No prior experience is required. React Native Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Turbo Native Modules with JSI” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this React Native Academy lesson?
Yes. Every React Native Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.