Outputting json element to the browser fails

I am trying to add a route to my express handler. My aim is to output a JSON element to the browser. I set up a GET request at the end of the document (index.js) using the express router, in order to display a JSON on my screen when I start my application and navigate to the site. But I am getting following error message.

throw new TypeError('Router.use() requires middleware function but got    a '
+ gettype(fn));
  ^

This is the my index.js for routing (just a part of my code):

router.get('/data', function (req, res) {
   var client = new pg.Client(conString);
   client.connect();
   var query = client.query(shops);

    query.on("row", function (row, result) {
       result.addRow(row);
     });
    query.on("end", function (result) {
        res.send(result.rows[0].row_to_json);
        res.end();
     });
 });

app.js:

  var express = require('express');
  var path = require('path');
  var favicon = require('serve-favicon');
  var logger = require('morgan');
  var cookieParser = require('cookie-parser');
  var bodyParser = require('body-parser');

  var index = require('./routes/index');
  var users = require('./routes/users');

  var app = express();

  // view engine setup
  app.set('views', path.join(__dirname, 'views'));
  app.set('view engine', 'jade');

  // uncomment after placing your favicon in /public
  //app.use(favicon(path.join(__dirname, 'public', 'favicon.ico')));
  app.use(logger('dev'));
  app.use(bodyParser.json());
  app.use(bodyParser.urlencoded({ extended: false }));
  app.use(cookieParser());
  app.use(express.static(path.join(__dirname, 'public')));

  app.use('/', index);
  app.use('/users', users);

  // catch 404 and forward to error handler
  app.use(function(req, res, next) {
  var err = new Error('Not Found');
  err.status = 404;
  next(err);
  });

  // error handler
  app.use(function(err, req, res, next) {
   // set locals, only providing error in development
        res.locals.message = err.message;
        res.locals.error = req.app.get('env') === 'development' ? err :  {};

   // render the error page
   res.status(err.status || 500);
   res.render('error');
 });

  module.exports = app;

via lazzat