getting error after opening port: Error [ERR_STREAM_WRITE_AFTER_END]: write after end (node.js)

Question

I am learning node.js and want to open a server with a simple home, about, and error page. I (seem to have) followed the syntax exactly, but each time I load a request from the server in my browser, the page loads but I get an error in the vsCode terminal and the port closes. Here is my code:

const http = require('http')


const server = http.createServer((req, res)=>{
if(req.url ==='/'){
    res.end('Home Page')
}
if(req.url ==='/about'){
    res.end('about page')
}
res.end("error")

})

server.listen(5000, ()=>{
    console.log("server is listening at port 5000...")
})

and here is the error I am getting:

server is listening at port 5000...
events.js:353
      throw er; // Unhandled 'error' event
      ^

Error [ERR_STREAM_WRITE_AFTER_END]: write after end
    at writeAfterEnd (_http_outgoing.js:694:15)
    at ServerResponse.end (_http_outgoing.js:815:7)
    at Server.<anonymous> (/home/nick/node-tutorial/app.js:11:5)
    at Server.emit (events.js:376:20)
    at parserOnIncoming (_http_server.js:896:12)
    at HTTPParser.parserOnHeadersComplete (_http_common.js:126:17)
Emitted 'error' event on ServerResponse instance at:
    at writeAfterEndNT (_http_outgoing.js:753:7)
    at processTicksAndRejections (internal/process/task_queues.js:83:21) {
  code: 'ERR_STREAM_WRITE_AFTER_END'
}

Answer 1

You are calling res.end() twice from your request handler when you get either the / or /about requests. You can only call it once.

You end up calling res.end() more than once because when your request matches either / or /about, you call res.end() for those specific pages and then you have another res.end() at the end of the function that also executes.

To fix it, change to this:

const server = http.createServer((req, res)=>{
    if (req.url ==='/') {
        res.end('Home Page')
    } else if (req.url ==='/about') {
        res.end('about page')
    } else {
        res.statusCode = 404;
        res.end("error")
    }
    
});

Keep in mind that res.end() does not stop your function from continuing to execute. So, if you don't want to execute any more code in the function after calling res.end(), you need to either protect it with conditionals like I did above or add a return after you call res.end(...).

FYI, your specific error 'ERR_STREAM_WRITE_AFTER_END' is because res.end() writes out data to the stream and then closes the stream. So, the second time you try to call res.end(), it tries to write data to an already-closed stream which gives you the error you saw.