[рони]

caetus,

var o = {
  email: 'js@.ru',
  username: {
    first: 'Bob',
    last: 'Fox'
   },
   dob: '12.02.2002'
}



var f = function (item) {
  var a = o[item.shift()]
  while (item.length && a) {
    a = a[item.shift()]
  }
  return a;
}

alert(f(['username','first']));
alert(f(['email']));