foo1.js

var Foo = exports.Foo = function (cb) {
  this.foo = 0;
  if(!!cb) cb(this);
}

Foo.prototype.toString = function () {
  return "[object Foo foo:"+this.foo+"]";
}

foo2.js

exports.Foo = Foo;

function Foo (cb) {
  this.foo = 0;
  if(!!cb) cb(this);
}

Foo.prototype.toString = function () {
  return "[object Foo foo:"+this.foo+"]";
}

test.js

var sys = require('sys')
  , Foo = require('./foo').Foo;

sys.puts(new Foo());

sys.puts(new Foo(function (e) {
  e.foo = 1;
  sys.puts(e);
  e.foo = 2;
}));