Swift

Swift is Apple’s programming language for iOS, macOS and its other platforms, introduced in 2014 as the successor to Objective-C. In React Native work it is the language you reach for when JavaScript cannot do the job: calling an iOS SDK, wrapping a native view or handling app lifecycle events.

Where Swift shows up in a React Native app:

• Native modules you write yourself, most comfortably with the Expo Modules API, whose iOS side is Swift-first.
• The app delegate — recent React Native and Expo templates generate it in Swift rather than Objective-C.
• Third-party SDKs, which are increasingly Swift-only.

The important nuance is how Swift reaches JavaScript. React Native’s core is Objective-C++ and C++, and JSI is a C++ API. For a classic native module or TurboModule, a Swift implementation typically needs a thin Objective-C or Objective-C++ layer that exposes it to React Native, since Swift classes must be visible to the Objective-C runtime. The Expo Modules API hides that glue, which is a large part of its appeal.

@objc(Haptics)
class Haptics: NSObject {
  @objc func tap() {
    UIImpactFeedbackGenerator(style: .light).impactOccurred()
  }
}

Two practical gotchas. UIKit work must happen on the main thread, while React Native may call your module from another queue, so dispatch to DispatchQueue.main when touching views. And mixing Swift into an Objective-C project needs a bridging header; if Xcode reports that Swift symbols are missing, that header is the first place to look.

Swift is the iOS counterpart of Kotlin: learning a little of both covers most of the native code a React Native developer ever needs to write.