After refresh "Cannot set headers after they are sent to the client" - node.js

Question

I am using express js, mongoose and ejs as templates.
When I refresh my page, in second or third try, I get this error.
Here is my app.js :
{consts..}
var index = require('./routes/index.js');
var users = require('./routes/users.js');
app.set("port", ("2401"));
app.set("views", __dirname + "/views");
app.set("view engine", "ejs");
app.use(express.static(path.join(__dirname, 'public')));
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));
app.use('/', index);
app.use('/users', users);
var port = '2401';
app.set('port', port);
var server = http.createServer(app);
server.listen(port);
server.on('error', onError);
server.on('listening', onListening);
function onError(error) {
if (error.syscall !== 'listen') {
throw error;
}
var bind = typeof port === 'string'
? 'Pipe ' + port
: 'Port ' + port;
switch (error.code) {
case 'EACCES':
console.error(bind + ' requires elevated privileges');
process.exit(1);
break;
case 'EADDRINUSE':
console.error(bind + ' is already in use');
process.exit(1);
break;
default:
throw error;
}
}
function onListening() {
var addr = server.address();
var bind = typeof addr === 'string'
? 'pipe ' + addr
: 'port ' + addr.port;
console.log('server started on port : ' + app.get('port'));
}
users router :
{consts..}
router.get('/login', function (req, res) {
users.getLogin(req, res);
});
router.post('/login', [
check('email')
.isEmail().withMessage('Please enter a valid email address')
.trim()
.normalizeEmail(),
check('terms', 'Please accept our terms and conditions').equals('yes'),
],
function (req, res) {
users.postLogin(req, res);
});
module.exports = router;
and controller :
usersController.getLogin = function (req, res) {
const connector = mongoose.connect(conStr, { useNewUrlParser: true, useUnifiedTopology: true });
mongoose.connection.on('open', function (ref) {
categories.find(function (err, cats) {
if (err) {
console.log("Error:", err);
}
else {
return res.render("../views/login.ejs", {
_: _,
categories: cats
});
}
});
});
}
index router :
var express = require('express');
var router = express.Router();
var index = require("../controllers/indexController.js");
router.get('/', function (req, res) {
index.list(req, res);
});
module.exports = router;
index controller:
var mongoose = require("mongoose");
var categories = require("../models/categories.js");
var _ = require("underscore");
var indexController = {};
indexController.list = function (req, res) {
const connector = mongoose.connect(conStr, { useNewUrlParser: true, useUnifiedTopology: true });
mongoose.connection.on('open', function (ref) {
categories.find(function (err, cats) {
if (err) {
console.log("Error:", err);
}
else {
console.log("indexe geldi");
return res.render("../views/index.ejs", {
_: _,
categories: cats,
sess: req.session
});
}
})
})
};
module.exports = indexController;
I tried almost everything but still no progress. Can it be about usage of "next()" ?
I also tried the hole router get / post functions with next, mongoose find functions with exec().

Answer 1

The issue is that you're opening a connection to mongo in every route and:
mongoose.connection.on('open' is firing for past requests, requests that are already finished, that's why you're getting Cannot set headers after they are sent to the client.
Move the mongoose.connect & the listener outside of each route.
usersController.getLogin = function (req, res) {
categories.find(function (err, cats) {
if (err) {
console.log("Error:", err);
return res.status(500).send('error');
}
else {
return res.render("../views/login.ejs", {
_: _,
categories: cats
});
}
});
}