response.js

res.json = function(obj, headers, status){
  var body = JSON.stringify(obj)
    , callback = this.req.query.callback
    , jsonp = this.app.enabled('jsonp callback');

  this.charset = this.charset || 'utf-8';
  this.header('Content-Type', 'application/json');

  if (callback && jsonp) {
    this.header('Content-Type', 'text/javascript');
    body = callback.replace(/[^\w$.]/g, '') + '(' + body + ');';
  }

  return this.send(body, headers, status);
};