Sending different POST response in NodeJS - node.js

Question

I would like to get some help with the following problem. I'm writing my bsc thesis, and this small part of code would be responsible for registering a user. (I'm new at nodejs actually). I'm using express and mongoose for this too.
I would like to process the request data, and check for some errors, first I would like to check if all fields exist, secondly if someone already registered with this e-mail address.
Based on the errors (or on success), I would like to send different responses. If a field is missing, then a 400 Bad request, if a user exists, then 409 Conflict, and 200 OK, if everything is ok. But I would only like to do the callback if there are no errors, but I'm kinda stuck here... I get the error Can't set headers after they are sent, which is obvious actually, because JS continues processing the code even if a response is set.
app.post('/register', function (req, res) {
var user = new User(req.body);
checkErrors(req, res, user, registerUser);
});
var registerUser = function(req, res, user){
user.save(function(err, user){
if (err) return console.log(err);
});
res.sendStatus(200);
};
var checkErrors = function(req, res, user, callback){
var properties = [ 'firstName', 'lastName', 'email', 'password', 'dateOfBirth' ];
for(var i = 0; i < properties.length; i++){
if(!req.body.hasOwnProperty(properties[i])){
res.status(400).send('field ' + properties[i] + ' not found');
}
}
var criteria = {
email: req.body.email
};
User.find(criteria).exec(function(err, user){
if(user.length > 0){
res.status(409).send('user already exists');
}
});
callback(req, res, user);
};

Answer 1

I think the problem is in the for loop in checkErrors. Since you call res.status(400).send() within the loop, you can end up calling it multiple times, which will trigger an error after the first call since a response will already have been sent back to the client.
Inside the loop, you can instead add missing fields to an array, then check the length of the array to see if you should respond with a 400 or continue. That way, you will only call res.status(400).send() one time.
For example:
...
var missingFields = [];
for(var i = 0; i < properties.length; i++){
if(!req.body.hasOwnProperty(properties[i])){
missingFields.push(properties[i]);
}
}
if(missingFields.length > 0) {
return res.status(400).send({"missingFields" : missingFields});
}
...
In general, I advise that you put return in front of each res.send() call, to ensure that no others are accidentally called later on.
An example of this is:
User.find(criteria).exec(function(err, user){
// We put return here in case you later add conditionals that are not
// mutually exclusive, since execution will continue past the
// res.status() call without return
if(user.length > 0){
return res.status(409).send('user already exists');
}
// Note that we also put this function call within the block of the
// User.find() callback, since it should not execute until
// User.find() completes and we can check for existing users.
return callback(req, res, user);
});
You probably noticed that I moved callback(req, res, user). If we leave callback(req, res, user) outside the body of the User.find() callback, it is possible that it will be executed before User.find() is completed. This is one of the gotchas of asynchronous programming with Node.js. Callback functions signal when a task is completed, so execution can be done "out of order" in relation to your source code if you don't wrap operations that you want to be sequential within callbacks.
On a side note, in the function registerUser, if user.save fails then the client will never know, since the function sends a 200 status code for any request. This happens for the same reason I mentioned above: because res.sendStatus(200) is not wrapped inside the user.save callback function, it may run before the save operation has completed. If an error occurs during a save, you should tell the client, probably with a 500 status code. For example:
var registerUser = function(req, res, user){
user.save(function(err, user){
if (err) {
console.error(err);
return res.status(500).send(err);
}
return res.sendStatus(201);
});
};

Answer 2

Your call to registerUser() is defined after the route and would be undefined since it's not a hoisted function.
Your use of scope in the closures isn't correct. For your specific error, it's because you're running res.send() in a loop when it's only supposed to be called once per request (hence already sent headers a.k.a. response already sent). You should be returning from the function directly after calling res.send() as well.