CommonJS: Module instances of global objects share prototypes?

commonjs_app.js

var myString1 = require('commonjs_mod').myString;
var myString2 = "app";

myString1.foo(); // no Fail. This string is an instance of a String whose prototype has a `foo` method

myString2.foo(); // Fail. This string is an instance of the global String native

commonjs_mod.js

String.prototype.foo = function(){ return "foo'd"; };
exports.myString = "mod";