How can an object be unmodifiable?

I have a mongodb function call that return a user object

User.findOne(condition, function(err, user) {
    console.log(user)   //  {username : 'blah', picture : 'blah', _id : 'blah'}
    delete user.picture;
});

somehow user object cannot be modified, delete does nothing. If I deep copy a brand new user object

var new_user = {};
for (var key in user)
    new_user[key] = user[key]

and do the deletion there, it works. Is there any situation where in Javascript when a object is not modifiable? Or did I miss something?