Express error Can't set headers after they are sent.

Question

I am sometimes getting error with this code:

// Check if sent user properties are valid
user.validate((err) => {
  if (err) {
    res.status(400).json({
      error: 'User data not valid.',
    });
  }
});

// Check if sent password properties are valid
password.validate((err) => {
  if (err || !password.arePasswordsEqual(req.body.password, req.body.passwordConfirm)) {
    res.status(400).json({
      error: 'Password data not valid.',
    });
  }
});

Any help appreciated.

Answer 1

The error you are talking about is occur when you send response multiple time back .

If this is your whole block then see you are sending two response back one is inside user.validate function and other in password.validate .

// Check if sent user properties are valid
user.validate((err) => {
  if (err) {
    res.status(400).json({
      error: 'User data not valid.',
    });
  }
});

// Check if sent password properties are valid
password.validate((err) => {
  if (err || !password.arePasswordsEqual(req.body.password, req.body.passwordConfirm)) {
    res.status(400).json({
      error: 'Password data not valid.',
    });
  }
});

May be you can do like this

   // Check if sent user properties are valid
user.validate((err) => {
  if (err) {
    res.status(400).json({
      error: 'User data not valid.',
    });
  }else{
     // Check if sent password properties are valid
     password.validate((err) => {
       if (err || !password.arePasswordsEqual(req.body.password, req.body.passwordConfirm)) {
         res.status(400).json({
           error: 'Password data not valid.',
         });
       }
     });

  }
});

This will help you .

Answer 2

I believe res.status will already start sending response headers. Even if it doesn't, res.json definitely will. That is to say, you can't call res.status or res.json twice which will happen in the above code if both the user and password are invalid.

Instead, if you want to have multiple errors at once you can collect them in an object and build the response once:

const errors = [];
user.validate((err) => {
  if (err) {
    errors.push[err];
  }
});

// Check if sent password properties are valid
password.validate((err) => {
  if (err || !password.arePasswordsEqual(req.body.password, req.body.passwordConfirm)) {
    errors.push('Password data not valid.');
  }
});

if (errors.length) {
  res.status(400).json(errors);
} else { /* 200 case? */ }

Note that if .validate is asynchronous you will have to do something to synchronize the code above.