Property event change

Generic.ts

type PropEventSource<Type> = {
    on<Key extends string & keyof Type>
        (eventName: `${Key}Changed`, callback: (newValue: Type[Key]) => void): void;
};
 
declare function makeWatchedObject<Type>(obj: Type): Type & PropEventSource<Type>;
 
const person = makeWatchedObject({
  firstName: "lynsei",
  lastName: "asynyn",
  age: 32
});
 
person.on("firstNameChanged", newName => {
                                

    console.log(`new name is ${newName.toUpperCase()}`);
});
 
person.on("ageChanged", newAge => {
                          

    if (newAge < 0) {
        console.warn("warning! negative age");
    }
})