在 node.js express 中返回响应的正确方法是什么？

Question

I am having trouble understanding what is wrong with my code. I am creating simple node.js REST api and I'm getting error no matter what I do:

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at ServerResponse.setHeader (_http_outgoing.js:535:11)
    at ServerResponse.header (D:\04. MOJE\08. STUDIA\6 semestr\Wybrane technologie webowe\Projekt\silicon store api\node_modules\express\lib\response.js:771:10)
    at ServerResponse.send (D:\04. MOJE\08. STUDIA\6 semestr\Wybrane technologie webowe\Projekt\silicon store api\node_modules\express\lib\response.js:170:12)
    at ServerResponse.json (D:\04. MOJE\08. STUDIA\6 semestr\Wybrane technologie webowe\Projekt\silicon store api\node_modules\express\lib\response.js:267:15)
    at Query.<anonymous> (D:\04. MOJE\08. STUDIA\6 semestr\Wybrane technologie webowe\Projekt\silicon store api\controllers\users.js:38:32)
    at Query.<anonymous> (D:\04. MOJE\08. STUDIA\6 semestr\Wybrane technologie webowe\Projekt\silicon store api\node_modules\mysql\lib\Connection.js:526:10)
    at Query._callback (D:\04. MOJE\08. STUDIA\6 semestr\Wybrane technologie webowe\Projekt\silicon store api\node_modules\mysql\lib\Connection.js:488:16)
    at Query.Sequence.end (D:\04. MOJE\08. STUDIA\6 semestr\Wybrane technologie webowe\Projekt\silicon store api\node_modules\mysql\lib\protocol\sequences\Sequence.js:83:24)
    at Query._handleFinalResultPacket (D:\04. MOJE\08. STUDIA\6 semestr\Wybrane technologie webowe\Projekt\silicon store api\node_modules\mysql\lib\protocol\sequences\Query.js:149:8)
    at Query.EofPacket (D:\04. MOJE\08. STUDIA\6 semestr\Wybrane technologie webowe\Projekt\silicon store api\node_modules\mysql\lib\protocol\sequences\Query.js:133:8) {
  code: 'ERR_HTTP_HEADERS_SENT'

The code that is causing the error is as follows:

exports.register = function (req, res) {
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(422).json({ errors: errors.array() });
  }

  var userId;
  // check whether email is taken
  mysql.query(
    'SELECT email FROM users where email = ?',
    [req.body.email],
    (err, rows, fields) => {
      if (err) {
        return res.sendStatus(500);
      }
      if (rows.length > 0) {
        return res.status(400).json({
          message: 'Mail taken',
        });
      }
    }
  );
  // check whether name is taken
  mysql.query(
    'SELECT name FROM users where name = ?',
    [req.body.name],
    (err, rows, fields) => {
      if (err) {
        return res.status(500);
      }
      if (rows.length > 0) {
        return res.status(400).json({
          message: 'Name taken',
        });
      }
    }
  );

where line 38 is:

return res.status(400).json({ message: 'Name taken' });

However the response is correct and I'm getting error 400 with error set to Mail taken

From my understanding this has something to do with trying to send the response multiple times. Register function doesn't end after checking whether the name or email are taken. There is more code there, but the principle will be the same.

I don't know nor can find any information on the web what is the correct way to handle responses.

Answer 1

You are returning the response twice, which causes the cannot set headers error. You would need to run the second mysql.query within the callback function of the first. Alternatively, you can run each of this queries using async await .

exports.register = function (req, res) {
    const errors = validationResult(req);
    if (!errors.isEmpty()) {
        return res.status(422).json({ errors: errors.array() });
    }

    var userId;
    // check whether email is taken
    mysql.query(
        'SELECT email FROM users where email = ?',
        [req.body.email],
        (err, rows, fields) => {
            if (err) {
                return res.sendStatus(500);
            }
            if (rows.length > 0) {
                return res.status(400).json({
                    message: 'Mail taken',
                });
            }
            // check whether name is taken
            mysql.query(
                'SELECT name FROM users where name = ?',
                [req.body.name],
                (nameErr, nameRows, fields) => {
                    if (nameErr) {
                        return res.status(500);
                    }
                    if (nameRows.length > 0) {
                        return res.status(400).json({
                            message: 'Name taken',
                        });
                    }
                    // if no errors, run the rest of your code here
                }
            );
        }
    );
}

Answer 2

To avoid callback hell you could use promises and manage your state outside the asynchronous parts. Maybe, whatever your mysql is, it already provides a promise based interface.

I'll provide you some abstract code:

If you can use async/await:

// "promisified" email check
function checkEmail(email) {
    return new Promise((resolve, reject) => {
        mysql.query(
            'SELECT email FROM users where email = ?',
            email,
            (err, rows, fields) => {
                if (err) {
                    reject({status: 500});
                }
                if (rows.length > 0) {
                    reject({
                        status: 400,
                        message: 'Mail taken'
                    })
                }
                resolve()
            })
    })
}


// ... same for name or genralized function

exports.register = async function(req, res) {
    // ...
    const emailOk = checkEmail(req.body.email)
    const nameOk = checkName(req.body.name)

    // ...
    await Promise.all([
        emailOk,
        nameOk
    ])
    .then(() => console.log('both ok'))
    .catch(e => res.status(e.status).json(...))
}