_dir.txt

8 -rw-r--r--  1 jheuser  502  126 Jan 29 14:41 main.js
8 -rw-r--r--  1 jheuser  502  212 Jan 29 14:41 testa.js
8 lrwxr-xr-x  1 jheuser  502    8 Jan 29 14:37 testb.js -> testa.js

main.js

var a = new (require('./testa'))('a');
var b = new (require('./testb'))('b');

console.log(a.varIs());
console.log(b.varIs());

testa.js

var privateVariable = 1;
var myClass = function(name) {
  this.name = name;
  privateVariable++;
};
myClass.prototype.varIs = function() {
  return this.name + ': ' + privateVariable;
}

module.exports = myClass;

zzz_output

$ node main.js 
a: 3
b: 3