`UnhandledPromiseRejectionWarning: Unhandled promise rejection` in my node application - node.js

Question

For learning Node.js I follow a course where they use a async/await like this:
exports.signup = async (req, res) => {
const userExists = await userExists.findOne({ email: req.body.email });
if (userExists) {
return res.status(403).json({
error: "Email is taken!"
});
} else {
const user = await new User(req.body);
await user.save();
return res.status(200).json({ user });
}
};
But it gives me a UnhandledPromiseRejectionWarning with crashing the application.
(node:10780) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise
which was not handled with .catch(). (rejection id: 1)
(node:10780) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero
exit code.
It seems I didn't handle the error part but I did it with my else block, isn't it working like this?
Any help will be appreciated.

Answer 1

You need to catch any rejected promise from await statements by surrounding it with try/catch.
exports.signup = async (req, res) => {
try {
const userExists = await User.findOne({ email: req.body.email });
if (userExists) {
return res.status(403).json({
error: "Email is taken!"
});
} else {
const user = new User(req.body);
await user.save();
return res.status(200).json({ user });
}
} catch(e) {
// some sort of internal error (probably database issue)
console.log(e);
res.sendStatus(500);
}
};
This also removes the await in await new User(req.body) as await only does something useful when you await a promise and new User() is not asynchronous and does not return a promise so there's no reason to use await with it.
Note, that to avoid a race condition where two separate requests might both find that the user doesn't exist and both requests may try to create one, you need to make sure that your user email is configured in the database as a unique key so you can never get duplicate users for the same email. This is a subtlety in server programming that is important to understand to avoid race conditions.

Answer 2

As #jfriend00 said You need to catch any rejected promise from await statements by surrounding it with try/catch. You get ReferenceError because of using userExists before it creates. I am pretty sure that it should be User as you named your Schema User const user = new User(req.body); Let me know if you have any issue after change the code:
exports.signup = async (req, res) => {
try {
const userExists = await User.findOne({ email: req.body.email }); //I changed it to the Schema name
if (userExists) {
return res.status(403).json({
error: "Email is taken!"
});
} else {
const user = new User(req.body);
await user.save();
return res.status(200).json({ user });
}
} catch(e) {
// some sort of internal error (probably database issue)
console.log(e);
res.sendStatus(500);
}
};