Nodejs error when making a POST request [ERR_HTTP_HEADERS

I am new to NODEJS / express , and I was following a tutorial to build a small MERN app. In the initial steps, while setting up a POST route, when sending a POST request with Postman, I am getting this Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client I´ve reading about it, and I somehow understand is because I am calling res.json two times in the same response.

My router is this :

router.post('/login',
    async (req, res) => {

      try {
          const user = await userModel.findOne({email:req.body.email});
          !user && res.status(404).send("user not found");

          const correctPassword = await bcrypt.compare(req.body.password, user.password);
          !correctPassword && res.status(400).json('incorrect password') 
          
           res.status(200).json(user); // if not invalid  return user

          } catch (err) {
            console.log(err)
          }
      
     })

I tried different solutions i found here (IF & if Else statements, using return ...) without any success. This very same code (with different variable names) is working flawless in mentioned tutorial.

Any idea how could I solve it? Thanks in advance.

EDIT : I add the resto of my user Route for completion ´´´

import express from 'express';
   const router = express.Router(); 
   import bcrypt from 'bcrypt';
   import userModel from '../models/userModel.js'

router.post('/register',
    async (req, res) => {
      
      try {

        // hash password
        const salt = await bcrypt.genSalt(10);
        const hashedPassword = await bcrypt.hash(req.body.password, salt)

        // Create New User
        const newUser = new userModel({
          userName: req.body.userName,
          email: req.body.email,
          password: hashedPassword,
        });

        // Save New User and return response
        const user = await newUser.save();
        res.status(200).json(user);
      } catch(err){
        console.log(err)
      }  
    });

CodePudding user response：

Likely user is falsy or correctPassword evaluates to false, which will send a response of "user not found" or "incorrect password".

However, it continues to execute the rest of your function and eventually executes this line

res.status(200).json(user);

This line will cause the error you've mentioned, as you have already sent a response to the client previously and cannot send another response in the same request-response-cycle.

This should fix the issue:

router.post("/login", async (req, res) => {
  try {
    const user = await userModel.findOne({ email: req.body.email });
    if (!user) {
        return res.status(404).send("user not found");
    }

    const correctPassword = await bcrypt.compare(req.body.password, user.password);
    if (!correctPassword) {
        return res.status(400).json("incorrect password");
    }

    return res.status(200).json(user); // if not invalid  return user
  } catch (err) {
    console.log(err);
    return res.status(500).send(err.message);
  }
});

I'd recommend using ifs here, as it makes the code easier to read. I'd also recommend using return anytime you send a response as it will avoid the issue you ran into.