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?.

Solution

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)

Comments

[Marcus Castanho]

I was taking a look at the documentation and I still don't understand: does the res.end() method only signals that the message is complete or does it actually ends the request? is there any other way of using it in the above example as you would with a "return" at the end of a if condition to ignore the other cases?

[Lajos Arpad]

@MarcusCastanho kindly read nodejs.org/api/… basically you end writing into the writable. In the code above you could remove all the res.end() lines and put them after the if-else if-else blocks, since it is called in all cases. return assigns a value to a function and stops its execution. res.end signifies the end of the writing. You could do return res.end(); if that helps.