Error: Authentication Error - Headers have already been sent to the client and cannot be modified

Question

Error: Authentication Error - Headers have already been sent to the client and cannot be modified

I am currently working on handling authentication errors for my website. However, when I submit incorrect data, I encounter the following error:

node:internal/errors:478 ErrorCaptureStackTrace(err); ^

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

As a result, my server application is down.

This is the code I am using:

router.post("/login", async (req, res) => {
  try {
    const user = await User.findOne({ username: req.body.username });
    !user && res.status(400).json("Wrong data");

    const validated = await bcrypt.compare(req.body.password, user.password);
    !validated && res.status(400).json("Wrong data");

    const { password, ...others } = user._doc;
    res.status(200).json(others);
  } catch (err) {
    res.status(500).json(err);
  }
});

Can anyone suggest how I can fix this issue?

javascript express

Answer 1

Answer №1

This particular error occurs when the server attempts to send a response to the client multiple times, even though it has already been sent once.

When you encounter errors like

res.status(400).json("Wrong data");

, if you do not include a return statement, the code will continue executing. This can lead to situations where multiple response attempts are made, resulting in the error being triggered.

The solution is simple - make sure to add a return statement along with res.status.

router.post("/login", async (req, res) => {
  try {
    const user = await User.findOne({ username: req.body.username });
    if(!user) return res.status(400).json("Wrong data");

    const validated = await bcrypt.compare(req.body.password, user.password);
    if(!validated) return res.status(400).json("Wrong data");

    const { password, ...others } = user._doc;
    res.status(200).json(others);
  } catch (err) {
    res.status(500).json(err);
  }
});

Answer 2

This particular error occurs when the server attempts to send a response to the client multiple times, even though it has already been sent once.

When you encounter errors like

res.status(400).json("Wrong data");

, if you do not include a return statement, the code will continue executing. This can lead to situations where multiple response attempts are made, resulting in the error being triggered.

The solution is simple - make sure to add a return statement along with res.status.

router.post("/login", async (req, res) => {
  try {
    const user = await User.findOne({ username: req.body.username });
    if(!user) return res.status(400).json("Wrong data");

    const validated = await bcrypt.compare(req.body.password, user.password);
    if(!validated) return res.status(400).json("Wrong data");

    const { password, ...others } = user._doc;
    res.status(200).json(others);
  } catch (err) {
    res.status(500).json(err);
  }
});

Error: Authentication Error - Headers have already been sent to the client and cannot be modified

Answer №1

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