JSONP middleware callback.

api-callback.js

module.exports = function(opts){
  return function(req, res, next){
    if ('callback' in req.query){
      var callback = req.query.callback.replace(/[^A-Z0-9\.a-z_\$]/g, '');
      if (callback.length == 0){ 
        res.setHeader('X-Callback', 'Invalid');
        next();
        return;
      }
      var origHead = res.writeHead,
          origEnd  = res.end,
          is_json  = false;
      res.writeHead = function(){
        origHead.apply(this, arguments);
        if ('content-type' in res._headers && 
            res._headers['content-type'].indexOf('application/json') !== -1){
          res.write(callback + '(');
          is_json = true;
        }
      }
      res.end = function(){
        if (is_json){
          res.write(');');
        }
        origEnd.apply(this, arguments);
      }
    }
    next();
  };
};