Error [ERR_STREAM_WRITE_AFTER_END]: write after end

Question

Code explanation: I am returning specific HTML file when user visit specific url:

const http = require('http');
const fs = require('fs');

fs.readFile('./funkcionalnosti-streznika.html', function(err1, html1) {
    fs.readFile('./posebnosti.html', function(err2, html2) {
        if (err1 || err2) {
            throw new Error();
        }

        http.createServer(function(req, res) {
            if (req.url == '/funkcionalnosti-streznika') {
                res.write(html1);
                res.end();
            }
            if (req.url == '/posebnosti') {
                res.write(html2)
                res.end();
            } else {
                res.write('random');
                res.end();
            }
        }).listen(8080)
    })
});

on terminal I've got this error when I visit localhost:8080/funkcionalnosti-streznika:

events.js:288
      throw er; // Unhandled 'error' event
      ^

Error [ERR_STREAM_WRITE_AFTER_END]: write after end
    at write_ (_http_outgoing.js:637:17)
    at ServerResponse.write (_http_outgoing.js:629:15)
    at Server.<anonymous> (/*filelocation*/:19:21)
    at Server.emit (events.js:311:20)
    at parserOnIncoming (_http_server.js:784:12)
    at HTTPParser.parserOnHeadersComplete (_http_common.js:119:17)
Emitted 'error' event on ServerResponse instance at:
    at writeAfterEndNT (_http_outgoing.js:692:7)
    at processTicksAndRejections (internal/process/task_queues.js:85:21) {
  code: 'ERR_STREAM_WRITE_AFTER_END'

I think there is I problem when I close the response too early. How should I change this to be async?.

Answer 1

You have already realized what the problem is. Let's take a look at this code:

    http.createServer(function(req, res) {
        if (req.url == '/funkcionalnosti-streznika') {
            res.write(html1);
            res.end();
        }
        if (req.url == '/posebnosti') {
            res.write(html2)
            res.end();
        } else {
            res.write('random');
            res.end();
        }
    }).listen(8080)

Let's suppose that req.url is '/funkcionalnosti-streznika'. What happens? It enters the first if, writes html1 and ends res. Then it is checked against '/posebnosti', but it's different, because the first if was true. This means that the else branch will be executed, so res.write('random'); is called, but res was already closed in the first if. Suggestion:

http.createServer(function(req, res) {
    if (req.url == '/funkcionalnosti-streznika') {
        res.write(html1);
        res.end();
    }
    else if (req.url == '/posebnosti') {
        res.write(html2)
        res.end();
    } else {
        res.write('random');
        res.end();
    }
}).listen(8080)