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

Question

So I saw this great post Error: Can't set headers after they are sent to the client , but still do not understand, what is wrong with my Headers section, because when I changed it from my commented out wrong type into two separated sections:
app.use(doesNotModifyBody); app.use(doesModifyBody); (like in answer above) it does not work normally.

And for now, instead of receiving the right database answer, I always resive the string from the app.use(doesModifyBody); - res.write("<p>Hello World</p>");.

So the question, how to solve the error with the Headers in the question title and make right database answer receving, instead of res.write("<p>Hello World</p>"); P.S. deleting the res.write("<p>Hello World</p>"); is does not solve the problem.

My server code:

'use strict'
const express = require('express');
const mongoose = require('mongoose');
const bodyParser = require('body-parser');
const app = express();
const router = express.Router();
const EmployersSchemaDB = require('./SchemaDB/EmployersSchemaDB');

mongoose.connect('mongodb://myDB');

app.use(bodyParser.urlencoded({ extended: true }));
app.use(bodyParser.json());

// app.use((req, res, next) => {
//     res.setHeader('Access-Control-Allow-Origin', '*');
//     res.setHeader('Access-Control-Allow-Credentials', 'true');
//     res.setHeader('Access-Control-Allow-Methods', 'GET,POST,PUT,DELETE');
//     res.setHeader('Access-Control-Allow-Headers', 'Access-Control-Allow-Headers, Origin,Accept, X-Requested-With, Content-Type, Access-Control-Request-Method, Access-Control-Request-Headers');
//     res.setHeader('Cache-Control', 'no-cache');
//     next();
// }); I change this on:

let doesNotModifyBody = (req, res, next) => {
    res.setHeader('Access-Control-Allow-Origin', '*');
    res.setHeader('Access-Control-Allow-Credentials', 'true');
    res.setHeader('Access-Control-Allow-Methods', 'GET,POST,PUT,DELETE');
    res.setHeader('Access-Control-Allow-Headers', 'Access-Control-Allow-Headers, Origin,Accept, X-Requested-With, Content-Type, Access-Control-Request-Method, Access-Control-Request-Headers');
    res.setHeader('Cache-Control', 'no-cache');
    next();
  }; // this 
let doesModifyBody = (req, res, next) => {
    res.setHeader("Content-Type", "text/html");
    res.write("<p>Hello World</p>");
    res.end();
  }; // and this

app.use(doesNotModifyBody);
app.use(doesModifyBody);

router.get('/', (req, res) => {
    res.json({ message: 'Server is Initialized!'});
});

router.route('/employers')
    .get((req, res) => {
        EmployersSchemaDB.find((err, employers) => {
            if (err) { res.send(err) }

            res.json(employers);
        });
    })
    .post((req, res) => {
        let employer = new EmployersSchemaDB();

        employer.first_name = req.body.first_name;
        employer.last_name = req.body.last_name;
        employer.birth_date = req.body.birth_date;
        employer.salary = req.body.salary;

        employer.save((err) => {
            if (err) { res.send(err) }

            res.json({ message: 'Comment successfully added!', employer });
            console.log('---===--- \n Employer added: \n', employer + '\n ---===---');
        });  
    });

router.route('/employers/:employer_id')
    .get((req, res) => {
        EmployersSchemaDB.findById(req.params.employer_id, (err, employer) => {
            if (err) { res.send(err) }

            res.json({ message: 'Employer has been getted', employer: employer });
        });
    })
    .put((req, res) => {
        EmployersSchemaDB.findById(req.params.employer_id, (err, employer) => {
            if (err) { res.send(err) }

            (req.body.id) ? employer.id = req.body.id : null;
            (req.body.first_name) ? employer.first_name = req.body.first_name : null;
            (req.body.last_name) ? employer.last_name = req.body.last_name : null;
            (req.body.birth_date) ? employer.birth_date = req.body.birth_date : null;
            (req.body.salary) ? employer.salary = req.body.salary : null;

            employer.save((err) => {
                if (err) { res.send(err) }

                res.json({ message: 'Employer has been updated', employer: employer  });
            });
        });
    })
    .delete((req, res) => {
        EmployersSchemaDB.remove({ _id: req.params.employer_id }, (err, employer) => {
            if (err) { res.send(err) }

            res.json({ message: 'Comment has been deleted' })
        })
    });

app.use('/', router);

const port = process.env.API_PORT || 3016;
app.listen(port, () => {
    console.log(`Server running on port ${port}`);
});

Answer 1

Th issue is that you are calling

res.write("<p>Hello World</p>");
res.end();

Which sends the response back to the client. Once you do this you cannot send any further data. You try to do that by calling res.json() in your middleware, which is why you want get the error that you get;

Remove BOTH the res.end and res.write lines and it should work.