How to fix error: Cannot set headers after they are sent? - node.js

Question

I am using PostgreSQL for the first time with an express server and am running into an error. On my register user route I am trying to check if the username or email already exists, because they need to be unique. What keeps happening is, say I pass in a username that is already in the database then the first return will run and return that the username is already in use. But what is happening is it is returning the username is already in use and it still running the rest of the code so it trying to return multiple json responses.
module.exports.register = async (req, res, next) => {
try {
const { username, email, password } = req.body;
postgres
.query("SELECT * FROM users WHERE username = $1", [username])
.then((data) => {
if (data.rows.length > 0) {
return res.status(409).json({
msg: "Username is already in use",
status: false,
});
}
})
.catch((err) => {
console.log(err);
});
postgres
.query("SELECT * FROM users WHERE email = $1", [email])
.then((data) => {
if (data.rows.length > 0) {
return res.status(409).json({
msg: "Email is already in use",
status: false,
});
}
})
.catch((err) => {
console.log(err);
});
const hashedPassword = await bcrypt.hash(password, 10);
postgres.query(
"INSERT INTO users (username, email, password) VALUES ($1,$2,$3) RETURNING *",
[username, email, hashedPassword],
(err, data) => {
if (err) {
console.log(err.stack);
} else {
return res.json({ user: data.rows, status: true });
}
}
);
} catch (err) {
next(err);
}
};
I can't figure out why the rest of the code is running even though I am returning res.json. If anybody has any suggestions/solutions I would really appreciate it!

Answer 1

The return in front of the res.status(409) is returning you out of the then of the postgres.query function instead of the full register function. As a result it jumps out of the then and runs the rest of the code from there, so it's still hashing the password and attempting an insert into the users table (which hopefully fails on a unique index).
In order to fix this you can either 1) Define a variable before the function, change it if something was found and then do a return outside of the then statement if the variable was changed 2) perform all the rest of the code in the then statement (since you're returning out of that it will not be run) or 3) use awaits instead and throw/next+return/res.json+return an the HTTP 409 error.
Option 3 will take the most effort but you should definitely learn to use this route as soon as possible as it makes writing async code a lot easier (plus you'll avoid getting a bunch of nasty nested then statement). You could try out using option 1 and 2 just to get a feel for how the flow of the express code works.