how check if user exist in nodejs - node.js

Question

I'm making a auth system with nodejs and mongoDB(mongoose) and I get Error:
error: UnhandledPromiseRejectionWarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
at F:\tryMern\index.js:68:13
index.js (important part) :
app.post("/api/auth", (req, res) => {
if (req.body.username && req.body.pass && req.body.status == "new") {
User.find({ username: req.body.username }, (err, users) => {
if (!err) {
if (users.length > 0) {
return res.json({ error: "The username is taken." });
}
}
});
const validReq = validate.validate({
username: req.body.username,
pass: req.body.pass,
});
if (validReq.error) {
return res.json({ error: validReq.error.details[0].message });
}
bcrypt.hash(req.body.pass, 12).then((hashedPass) => {
// console.log(hashedPass);
const user = new User({
username: req.body.username,
password: hashedPass,
});
user.save().then((user) =>
res.json({
status: "OK",
username: user.username,
token: jwt.sign({ _id: user._id }, jwtKey),
})
);
});
return;
}
User.find({ username: req.body.username }, (err, users) => {
if (err) {
console.log(err);
} else {
if (users.length > 0) {
bcrypt.compare(req.body.pass, users[0].password, (err, hash) => {
if (hash) {
return res.json({
validate: true,
username: users[0].username,
token: jwt.sign({ _id: users[0]._id }, jwtKey),
});
} else {
return res.json({ validate: false });
}
});
} else {
return res.json({ validate: false });
}
}
});
});
when I add The username is taken part the error comes ( the part say find user and if its exist say username is taken)
if there is another way to check if user exist please tell or fix this problem
thanks :)
EDIT:
when i try to submit the user with exist username the response is { "error": "The username is taken." } and the error come

Answer 1

I fix this:
app.post("/api/auth", (req, res) => {
if (req.body.username && req.body.pass && req.body.status == "new") {
User.find({ username: req.body.username }, (err, users) => {
if (!err) {
if (users.length > 0) {
res.json({ error: "The username is taken." });
return;
}
const validReq = validate.validate({
username: req.body.username,
pass: req.body.pass,
});
if (validReq.error) {
return res.json({ error: validReq.error.details[0].message });
}
bcrypt.hash(req.body.pass, 12).then((hashedPass) => {
// console.log(hashedPass);
const user = new User({
username: req.body.username,
password: hashedPass,
});
user.save().then((user) =>
res.json({
status: "OK",
username: user.username,
token: jwt.sign({ _id: user._id }, jwtKey),
})
);
});
}
});
return;
}
User.find({ username: req.body.username }, (err, users) => {
if (err) {
console.log(err);
} else {
if (users.length > 0) {
bcrypt.compare(req.body.pass, users[0].password, (err, hash) => {
if (hash) {
return res.json({
validate: true,
username: users[0].username,
token: jwt.sign({ _id: users[0]._id }, jwtKey),
});
} else {
return res.json({ validate: false });
}
});
} else {
return res.json({ validate: false });
}
}
});
});