Using rejected promises to handle bad HTTP calls? - node.js

Question

So, I have the following code:
function SignUp(req, res, next){
const userCreds = {
email: req.body.email,
password: req.body.password
}
//Username and password must exist
if(!userCreds.email || !userCreds.password){
res.status(422).send({ error: 'Email and Password required'});
throw new Error(('Email and Password Required'));
}
//See if email is already being used
Users.findOne({ email: userCreds.email })
.then(function(user){
//If user does exist, return Error
if(user){
res.status(422).send({ error: 'Email is in use'});
throw new Error(('Email and Password Required'));
}
//Else if email is true, create and save user error
const newUser = new Users(userCreds);
//Save the user
return newUser.save(); //Return promise
})
.then(function(doc){
//Respond saying all OK
res.json({
success: true,
email: doc.email
});
})
.catch(function(err){
if(err)
return next(err);
});
}
The function above is passed to an Express route, like this
app.get('/signup', SignUp);
In this code, there are two different 'errors' that can occur and I need to handle. One kind of error is that the user request cannot be processed (Trying to create an account without supplying both Email and Password, or using an Email that already is being used). The second kind of error is one that I have less control in: rejected promises from the Mongoose package.
Let's say that I have received a bad request, an error of type 1. I want to handle it by setting the header of the response to 422, and sending that response with a message detailing why it could not be processed. At that point the execution would end.
If I get an error of type 2, I want to call next(error) and stop execution at that point.
The problem is, by chaining .then() functions, I cannot return from a block of code without jumping into the following .next().
One way to get around this is by throwing an error via throw new Error() when I get an error of either type 1 or 2, and handle the case in .catch(), but I am unsure how much of a good or bad practice this would be.
How can I make it so that I can handle the error in a .then() block and then stop execution? And would that be the best way to do it?
Is there a better way to handle these kind of situations in Express? Am I missing anything?
Thank you!

Answer 1

A solution would be to create an Error subclass (for instance, using error-subclass) and throw instances of those in case you want to signal a processing error.
Subsequently, in the .catch() handler you'd check if the error is an instance of that custom error class, and if so, return a 422 response. If not, pass it to next instead:
const ErrorSubclass = require('error-subclass').default;
class ProcessingError extends ErrorSubclass {}
Users.findOne({ email: userCreds.email })
.then(function(user){
if (user) {
throw new ProcessingError('Email and Password Required');
}
...
}).catch(function(err) {
if (err instanceof ProcessingError) {
return res.status(422).send({ error: err.message });
}
next(err);
});