How to observe value changes in JS variables

Question

Im wondering if someone might be able to help me with something that i think it fairly straight forward:

Essentially i want to extend the prototypes of all datatypes (including intrinsic types), to allow some kind of custom functions, consider:

var x = "some string";
var y = 101;

x = "some other value";
y++;

x.onChange();
y.onChange();

This is the basic idea im after, but really what i want is to actually have the onChange (in this example) to be different so a new function for the actual variable (rather than a stardard prototype extension), ie:

x.onChange = function() {
    alert("x.onChange");
}

y.onChange = function() {
    alert("y.onChange");
}

This doesnt seem to work but i must be missing something quite simple no? I mean surely i can extend all object and types and add on new functions... no?

Any help would be greatly appreciated!