Express continues to send responses after sending the first one - node.js

Question

How do I make it so that if a response has been sent back, then no more responses should be sent? Actually, the issue is that if a response is sent back, then express (or nodejs) shouldn't continue running through the rest of the code.
I've tried doing next() but terminal throws the error of next() being undefined. res.end() doesn't seem to work either?
routing.js:
router.post('/user/create', function(req, res, next) {
user.createUser(req, res);
});
user.js createUser
user.prototype.createUser = function(req, res, next) {
var body = req.body;
checkAllInput(body, res, next);
// do some more checks then finally create user
}
user.js createUser
function checkAllInput(body, res, next) {
checkError.checkUsername(body, res, next);
checkError.checkPassword(body, res, next);
}
checkError.js
userClass.prototype.checkUsername = function(username, res) {
if (!username || username === "bob) {
res.status(406).send("username");
}
}
userClass.prototype.checkPassword = function(password, res) {
if (!password || password === "hello") {
res.status(406).send("password");
}
}
Call createUser in routing, which then calls checkAllInput which calls checkUsername but should stop if username sends a response back.

Answer 1

You need to return, so the code stops there. Otherwise it will keep on going.
userClass.prototype.checkUsername = function(username, res) {
if (!username || username === "bob) {
return res.status(406).send("username");
}
}
userClass.prototype.checkPassword = function(password, res) {
if (!password || password === "hello") {
return res.status(406).send("password");
}
}
next() isn't inherent in the code, it has to be defined somewhere, and even if it is, it still doesn't stop the code as it is asynchronous.
I'm assuming you're using Express. You might want to do this with middleware.
//middleWare.js
exports.checkUserModel = function (req, res, next) {
var body = req.body,
username = body.username,
password = body.password,
if (!username || username === "bob) {
return res.status(406).send("username required");
}
if (!password || password === "hello") {
return res.status(406).send("password required");
}
next();
}
//router.js
var middleWare = require('./middleWare');
var user = require('./controllers/users');
app.post('/user/createUser', middleWare.checkUserModel, user.create);

Answer 2

You want to use the express middleware like so :
checkUsername = function(req, res, next) {
if (checkUserNameIsValid) {
//check the password
next()
}
else{
///otherwise return
res.status(406).send("username");
}
}
checkPassword = function(req, res, next) {
if (checkIfPasswordIsValid) {
//create the user when password is valid too
next();
}
else {
//else return response
res.status(406).send("password required");
}
}
createUserIfPasswordAndUserNameIsOk = function(req, res, next){
//create the user here
}
Add in sequence you want to handle the request.
router.post('/user/create', checkUserName, checkPassword, createUserIfPasswordAndUserNameIsOk );
So what will happen the express router will first call checkUserName and if you don't call next() then it returns. So if you call next() in it it will call the next method in for the current requested resource which is checkPassword. And so on.
Take a look at Having a hard time trying to understand 'next/next()' in express.js thread.
Note that you don't have to use return. Also take a look at #Brian answer