React Native: How do you implement a custom React Native module?

1import type { TurboModule } from "react-native";
2import { TurboModuleRegistry } from "react-native";
3
4export interface Spec extends TurboModule {
5  getConstants(): Promise<{ PI: number }>;
6  multiply(a: number, b: number): number;
7}
8
9export default TurboModuleRegistry.getEnforcing<Spec>("MyModule");

1public class MyModule: NSObject, MyModuleSpec {
2  public func multiply(_ a: Double, b: Double) -> Double {
3    return a * b
4  }
5}

1import MyModule from "./MyModule";
2const result = MyModule.multiply(3, 4); // 12