Node.js Cannot set headers after they are sent to the client

Question

I'm following a course about node.js on Udemy which is kinda outdated and came across these errors which I'm unable to find a solution for.

What I tried:

using next();
adding return res inside all if statements

Can someone help me fix these? I would greatly appreciate it!

Thanks in advance!

Username exists error: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client at ServerResponse.setHeader (_http_outgoing.js:526:11) at ServerResponse.header (C:\Users\Documents\projects\chat-app\chat-app-backend\node_modules\express\lib\response.js:771:10) at ServerResponse.send (C:\Users\Documents\projects\chat-app\chat-app-backend\node_modules\express\lib\response.js:170:12)
at ServerResponse.json (C:\Users\Documents\projects\chat-app\chat-app-backend\node_modules\express\lib\response.js:267:15)
at C:\Users\Documents\projects\chat-app\chat-app-backend\controllers\auth.js:38:56 at processTicksAndRejections (internal/process/task_queues.js:97:5) { code: 'ERR_HTTP_HEADERS_SENT' }

module.exports = {
    CreateUser(req, res) {

        const schema = Joi.object().keys({
            username: Joi.string().min(5).max(15).required(),
            email: Joi.string().email().required(),
            password: Joi.string().min(5).required()
        });

        const {error, value} = Joi.validate(req.body, schema);

        if (error && error.details) {
            return res.status(HttpStatus.BAD_REQUEST).json({message: error.details});
        } 

        async function EmailExists() {
            return await User.findOne({email: Helpers.lowerCase(req.body.email)}) != undefined;
        }

        async function UsernameExists() {
            return await User.findOne({username: Helpers.firstLetterUppercase(req.body.username)});
        }

        EmailExists().then(exists => {
            if (exists) {
                return res.status(HttpStatus.CONFLICT).json({message: 'Email already exists'});
            }
        }).catch((err) => console.log('Email exists error: ', err));

        UsernameExists().then(exists => {
            if (exists) {
                return res.status(HttpStatus.CONFLICT).json({message: 'Username already exists'}) != undefined;
            }
        }).catch((err) => console.log('Username exists error: ', err));


        return BCrypt.hash(value.password, 10, (error, hash) => {
            if (error) {
                return res.status(HttpStatus.BAD_REQUEST).json({message: 'Error hashing password'});
            }

            const body = {
                username: Helpers.firstLetterUppercase(value.username),
                email: Helpers.lowerCase(value.email),
                password: hash
            };

            User.create(body).then((user) => {
                res.status(HttpStatus.CREATED).json({message: 'User created successfully'});
            }).catch((err) => {
                res.status(HttpStatus.INTERNAL_SERVER_ERROR).json({message: 'Something went wrong. Could not save user'});
            });
        });
    }
}

you are trying to send response twice, thats the reason you are facing this error — Narendra Chouhan

BENARD Patrick BENARD Patrick · Accepted Answer · 2020-05-19T10:50:07

You're executing some promise and don't wait for the answer before executing the next code....

There are many ways to handle this, this next code is just one way...

    const hash = () => BCrypt.hash(value.password, 10, (error, hash) => {
        if (error) {
            return res.status(HttpStatus.BAD_REQUEST).json({message: 'Error hashing password'});
        }

        const body = {
            username: Helpers.firstLetterUppercase(value.username),
            email: Helpers.lowerCase(value.email),
            password: hash
        };

        User.create(body).then((user) => {
            res.status(HttpStatus.CREATED).json({message: 'User created successfully'});
        }).catch((err) => {
            res.status(HttpStatus.INTERNAL_SERVER_ERROR).json({message: 'Something went wrong. Could not save user'});
        });
    });


    EmailExists().then(exists => {
        if (exists) {
            return res.status(HttpStatus.CONFLICT).json({message: 'Email already exists'});
        }

           UsernameExists().then(exists => {
                if (exists) {
                   return res.status(HttpStatus.CONFLICT).json({message: 'Username already exists'}) != undefined;
                }

                return hash();
           }).catch((err) => console.log('Username exists error: ', err));          


    }).catch((err) => console.log('Email exists error: ', err));

Node.js Cannot set headers after they are sent to the client

1 Answers