Headers cannot be reset after being sent while attempting to log in a user

Question

Headers cannot be reset after being sent while attempting to log in a user

I'm very puzzled at the moment, as I'm utilizing bcrypt to authenticate a user login and encountering an unexpected error:

Error: Can't set headers after they are sent.

Let me share with you the code snippet for that particular route:

router.post('/login', (req, res, next) => {
  User.find({ email: req.body.email })
    .exec()
    .then(user => {
      if (user.length < 1) {
        res.status(401).json({
          message: 'Auth failed'
        });
      }
      bcrypt.compare(req.body.password, user[0].password, (err, result) => {
        if (err) {
          res.status(401).json({
            message: 'Auth failed'
          });
        }
        if (result) {
          res.json(200).json({
            message: 'Auth successful'
          });
        }
        return res.status(401).json({
          message: 'Auth failed 3'
        });
      });
    })
    .catch(err => {
      res.status(500).json({
        error: err
      });
    });
});

I initially suspected that the issue might have stemmed from the if else clauses and inadvertently setting the header twice, but I believe I'm clearing them before proceeding to the next condition. Could it be that I'm misinterpreting something here?

javascript express bcrypt

Answer 1

Answer №1

Encountering the error you are seeing is a result of your code attempting to send multiple responses for a single request. This issue typically arises from mishandling asynchronous operations within your code. Upon reviewing the code you provided, I identified several mistakes that could lead to this error:

When user.length < 1, you use res.status(401).json(...), but then proceed to send additional responses.
If an error occurs in bcrypt.compare(), you send an error response and still allow for the execution of other responses.
If bcrypt.compare() is successful, you use res.json(200).json(...) incorrectly. The correct syntax should be res.status(200).json(...).
Upon a successful bcrypt.compare() and having a result, you send two responses simultaneously.

Based on your code, it seems you believe that once you use res.json(), the function will terminate and no further code will execute. However, this is not accurate. Until a return is encountered, the function will continue to execute the remaining code.

To rectify these issues, you can make adjustments such as introducing return statements and an else clause:

router.post('/login', (req, res, next) => {
    User.find({ email: req.body.email }).exec().then(user => {
        if (user.length < 1) {
          res.status(401).json({message: 'Auth failed'});
          return;
        }
        bcrypt.compare(req.body.password, user[0].password, (err, result) => {
          if (err) {
            res.status(401).json({message: 'Auth failed'});
            return;
          }
          if (result) {
            res.json({message: 'Auth successful'});
          } else {
            res.status(401).json({message: 'Auth failed 3'});
          }
        });
    }).catch(err => {
        res.status(500).json({error: err});
    });
});

Alternatively, for a cleaner implementation with consolidated responses and better flow control using promises, consider the following approach:

const util = require('util');
bcyrpt.compareAsync = util.promisify(bcrypt.compare);

router.post('/login', (req, res, next) => {
    User.find({ email: req.body.email }).exec().then(user => {
        if (user.length < 1) {
            throw new Error('No user match');
        }
        return bcrypt.compareAsync(req.body.password, user[0].password).then(result => 
            if (!result) {
                throw new Error('Auth failed 2');
            }
            res.json({message: 'Auth successful'});
          }
        }).catch(err => {
            res.json(401).json({message: err.message});
        });
    }).catch(err => {
        res.status(500).json({error: err});
    });
});

Answer 2

Encountering the error you are seeing is a result of your code attempting to send multiple responses for a single request. This issue typically arises from mishandling asynchronous operations within your code. Upon reviewing the code you provided, I identified several mistakes that could lead to this error:

When user.length < 1, you use res.status(401).json(...), but then proceed to send additional responses.
If an error occurs in bcrypt.compare(), you send an error response and still allow for the execution of other responses.
If bcrypt.compare() is successful, you use res.json(200).json(...) incorrectly. The correct syntax should be res.status(200).json(...).
Upon a successful bcrypt.compare() and having a result, you send two responses simultaneously.

Based on your code, it seems you believe that once you use res.json(), the function will terminate and no further code will execute. However, this is not accurate. Until a return is encountered, the function will continue to execute the remaining code.

To rectify these issues, you can make adjustments such as introducing return statements and an else clause:

router.post('/login', (req, res, next) => {
    User.find({ email: req.body.email }).exec().then(user => {
        if (user.length < 1) {
          res.status(401).json({message: 'Auth failed'});
          return;
        }
        bcrypt.compare(req.body.password, user[0].password, (err, result) => {
          if (err) {
            res.status(401).json({message: 'Auth failed'});
            return;
          }
          if (result) {
            res.json({message: 'Auth successful'});
          } else {
            res.status(401).json({message: 'Auth failed 3'});
          }
        });
    }).catch(err => {
        res.status(500).json({error: err});
    });
});

Alternatively, for a cleaner implementation with consolidated responses and better flow control using promises, consider the following approach:

const util = require('util');
bcyrpt.compareAsync = util.promisify(bcrypt.compare);

router.post('/login', (req, res, next) => {
    User.find({ email: req.body.email }).exec().then(user => {
        if (user.length < 1) {
            throw new Error('No user match');
        }
        return bcrypt.compareAsync(req.body.password, user[0].password).then(result => 
            if (!result) {
                throw new Error('Auth failed 2');
            }
            res.json({message: 'Auth successful'});
          }
        }).catch(err => {
            res.json(401).json({message: err.message});
        });
    }).catch(err => {
        res.status(500).json({error: err});
    });
});

Headers cannot be reset after being sent while attempting to log in a user

Answer №1

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