class MyClass {
    private p: number = 123;
    protected pp: number = 123;
    public ppp: number = 123;
}

interface MyClass {
    extension1(p: number): void;
    extension2(p: number): void;
    extension3(p: number): void;
}

MyClass.prototype.extension1 = function (this: MyClass, p: number) { 
    this.p = p;
    this.pp = p;
    this.ppp = p;
}

MyClass.prototype.extension2 = function<T extends MyClass> (this: T, p: number) { 
    this.p = p;
    this.pp = p;
    this.ppp = p;
}

function extension3<T extends MyClass> (this: T, p: number) { 
    this.p = p;
    this.pp = p;
    this.ppp = p;
}

MyClass.prototype.extension3 = extension3;