Creating makeWatchedObject function with overloading

type PropEventSource<T> = {
    on<K extends string & keyof T>(eventName: `${K}Changed`, callback: (newValue: T[K]) => void): void;
};

type ReturnType<T1> = T1 & PropEventSource<T1>;

function makeWatchedObject<T1>(obj1: T1): ReturnType<T1>
function makeWatchedObject<T1, T2>(obj1: T1, obj2: T2): ReturnType<T1 & T2>
function makeWatchedObject<T1, T2, T3>(obj1: T1, obj2: T2, obj3: T3): ReturnType<T1 & T2 & T3>
function makeWatchedObject<T1, T2, T3>(obj1: T1, obj2?: T2, obj3?: T3): any {

    const events = new Map<string, (a: any) => void>();

    const on_: PropEventSource<T1 & T2 & T3> = {
        on(eventName: string, callback: (newValue: any) => void): void {

            events.set(eventName, callback);
        }
    }
    const obj = { ...obj1, ...obj2, ...obj3, ...on_ };

    const proxyObj = new Proxy(obj, {
        set: function (target, property, value, receiver) {

            const event = events.get(`${property as string}Changed`);

            if (event) {
                event(value);
                return true;
            }

            return Reflect.set(target, property, value, receiver);
        }
    });

    return proxyObj;
}

Usage

const personInfo = { id: 1, firstName: 'kenan', lastName: 'hancer', age: 36 };
const personAddress = { country: 'UK', city: 'London' };
const personContact = { email: 'kh@kh.com', phone: '077111-33-33' };

let person = makeWatchedObject(personInfo, personAddress, personContact);


// works! 'newName' is typed as 'string'
person.on("firstNameChanged", (newName: string) => {

    console.log(`new name is ${newName.toUpperCase()}`);
});


// works! 'newAge' is typed as 'number'
person.on("ageChanged", (newAge: number) => {
    if (newAge < 0) {
        console.log("warning! negative age");
    }
});

person.age = 34;