Encountering the "Cannot set headers after they are sent to the client" error within an Express.js application

In my recent project, I created a middleware to authenticate users and verify if they are verified or not. Initially, when I access protected routes for the first time, everything works fine and I receive success messages along with verification of the JWT token. However, upon repeating the same request, I encounter an error.

Here is the code snippet:

const Anime = require("../models/admin_model");
const jwt = require("jsonwebtoken");
const checkUserAuthentication = async (req, res, next) => {
  const token = req.headers.token;
  if (token) {
    jwt.verify(token.toString(), "secret", async (err, token_decode) => {
      if (err) {
        return res.json({
          status: 0,
          err: err,
          msg: "Authorization failed",
        });
      }
      console.log(token_decode);
      res.json({ data: token_decode });
      next();
      return;
    });
  }
  return res.json({
    status: 0,
    msg: "Authorization failed",
  });
};

module.exports = checkUserAuthentication;

Answer №1

 res.json({ payload: decodedToken });
 next();

After sending JSON data in your response, you are passing control to the next middleware or route endpoint. This can cause issues as the subsequent handler won't be able to send its own response due to the prior response being already sent.

Answer №2

I made a few adjustments to my code.

Implemented well-structured if-else statements.

const Anime = require("../models/admin_model");
const jwt = require("jsonwebtoken");
const checkUserAuthentication = async (req, res, next) => {
  const token = req.headers.token;
  if (token) {
    jwt.verify(token.toString(), "thisissecs", async (err, token_decode) => {
      if (!err) {
        console.log(token_decode);
        res.json({ data: token_decode });
        next();
      } else {
        return res.json({
          status: 0,
          msg: "Authorization failed",
        });
      }
    });
  }
  else{
    return res.json({
      status: 0,
      msg: "Authorization failed",
    });
}
};

module.exports = checkUserAuthentication;

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