app.js

module.exports = function(app, io) {

  // import modules
  var createError = require('http-errors');
  var path = require('path');
  var cookieParser = require('cookie-parser');
  var logger = require('morgan');
  let passport = require('passport');
  let session = require('express-session');
  let flash = require('connect-flash'); 
  let express = require('express');
  require('./passport_setup')(passport);
  var indexRouter = require('./routes/index')(io);
  var usersRouter = require('./routes/users');



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

  app.use(logger('dev'));
  app.use(express.json());
  app.use(express.urlencoded({ extended: false }));
  app.use(cookieParser());
  app.use(express.static(path.join(__dirname, 'public')));

  /******************* use Session **************************/
  // take note that we have to write the following codes in order
  // bcus nodejs read codes line by line
  app.use(session({ secret: 'our little secret' }));
  app.use(passport.initialize());   // we want to set up passport in a separate file from app.js. for that, we add in the line at line 12
  app.use(passport.session());

  /** if user is not logged in, direct to usersRouter */
  // app.all('/secret', isLoggedIn)
  app.use(flash());
  app.use('/', indexRouter);
  app.use('/users', usersRouter);

  // catch 404 and forward to error handler
  app.use(function(req, res, next) {
    next(createError(404));
  });

  // 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');
  });

  return app;
}