Change Value Of A Number Object

I'm hacking on a LISP interpreter in JS [1], and running into trouble with numbers and Number objects.

This is what I have:

var a = new Number(1);
var b = a;
document.writeln(a == b); // true
b = 2;
document.writeln(a == b); // false
document.writeln(a); // 1

Instead of replacing the *Number object* b with the *number* 2, I'd like to assign 2 to the value of the Number object, which should still be referenced by b *and* a. In other words, the two last lines should output "true" and "2".