How does JavaScript define a new string type? [duplicate]

Question

First, I define a default string.

var string = new String();

But I wanted to have a more powerful string, so I added some methods to the prototype of string.

String.prototype.md5sck

That's good, but I hope he's a new type.

var string2 = new String2();
string2.prototype.substring();//Method with inherit default string
string2.prototype.md5sck();//Have additional methods
string.prototype.md5sck();//null The system default string method will not be affected.

And I have to make sure my method can't be modified by others

var string2 = new string2();
string2.prototype.md5sck = ???; // After new(), I hope this behavior will be prohibited