DRY?

route.js

exports.whater = function (req, res) {
    var format = req.params.format ? req.params.format : req.format;

    User.findOne({ username: req.params.username }, function (err, doc) {
        if (err) throw err;
        if (!doc) {
            if (format == 'json') {
                res.json({ error: "no such user"});
            } else {
                res.render('404');
            }
        } else {
            if (format == 'json') {
                res.json(doc);
            } else {
                res.render('user/show', { user: doc });
            }
        }
    });
};

// or split it out
exports.whater = function(req, res) {
  var fmt     = req.params.format || req.format
    , respond = (format === 'json' ? respond_json : respond_html).bind(this, req, res)

  User.findOne({username:req.params.username}, respond)
}

function respond_json(req, res, err, user) {
  res.json(err ? 
    {error:'no such user'} :
    user
  )
}

function respond_html(req, res, err, user) {
  res.render.apply(res, err ?
    ['404']
  : ['user/show', {user:user}]
  )
}