Expo Modules API

The Expo Modules API is Expo’s framework for writing native code that JavaScript can call. You describe a module in a declarative Swift or Kotlin DSL — its name, functions, constants, events and views — and Expo generates the binding to JavaScript over JSI. Every package in the Expo SDK is built with it.

import ExpoModulesCore

public class GreeterModule: Module {
  public func definition() -> ModuleDefinition {
    Name("Greeter")
    Function("greet") { (name: String) -> String in
      return "Hello, \(name)"
    }
  }
}

From JavaScript you call requireNativeModule('Greeter').greet('Ada'). The Kotlin version mirrors the Swift one almost line for line.

Why React Native developers choose it over a hand-written TurboModule:

• Modern languages only. Swift and Kotlin, with no Objective-C++ or Java glue to maintain.
• Type conversion handled for you, including records, enums and optional arguments, with argument validation.
• Async functions, events and native views declared in the same definition.
• Lifecycle hooks for app delegate and activity events, without editing AppDelegate or MainActivity.

Getting started is npx create-expo-module for a publishable library, or npx create-expo-module --local to add one under modules/ in your app. Pair it with a config plugin when the module also needs Info.plist or manifest changes.

The common misconception is that it only works in Expo-managed apps. It runs in any React Native project that has the expo package installed, which npx install-expo-modules sets up in a bare app. It does not work in Expo Go, though: custom native code always needs a development build.

It is not a TurboModule under the hood and does not use React Native’s Codegen; it reaches the same goal — fast, typed JSI calls — through its own runtime. Both approaches are supported by the New Architecture. See the Expo Modules API docs.