How to fix the problem with return values in NodeJS (Express) and Eslint?

const checkAuthorization = function(request, response, next) {
  const token = request.headers.authorization;
  if (!token) {
    return response.status(401).json({ message: 'Invalid or missing token' });
  }

  const accessToken = token.split(' ')[1];

  jwt.verify(accessToken, 'access', (error, user) => {
    if (error) {
      response.status(401).json({ message: 'User not authenticated' });
    } else {
      request.user = user;
      next();
    }
  });

  // Return null at the end of the function to satisfy Eslint
  return null;
};

// Handling error responses in a meaningful way
const handleErrors = function(response) {
  return response.status(503).json({ message: 'Service unavailable' });
}

// How to deal with Cannot set headers after they are sent to the client error
// One suggestion is to make sure that only one response is being sent

An issue with error handling and header setting has been encountered. Suggestions on managing this would be greatly appreciated.

Answer №1

When using a middleware function like this, the return value is not examined, so it does not need to return anything. In order to comply with eslint rules, using return null; is acceptable.

The issue with including a "meaningful" return statement is that it immediately sends a response to the client without waiting for the outcome of the jwt.verify statement. This verification process occurs asynchronously in the (error, user) => ... callback function. This means that first, a 503 response is sent with a "service unavailable" message, and then either:

  • a 401 response is sent with a "user not authenticated" message (if an error occurs in the callback), or
  • the next middleware is invoked to attempt to send a response.

In both scenarios, the headers of the second response are set after the first response has already been sent, resulting in an error. Sending two responses consecutively does not serve any purpose, as expected.

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