Error: Can't set headers after they are sent.

I don't get it, where the error is. How do I find out in which line the error is?

I get the following error message in the console:

throw new Error('Can\'t set headers after they are sent.');                 
^                                                                           

Error: Can't set headers after they are sent.
at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:344:11)
at Server.<anonymous> (/home/treehouse/workspace/app.js:14:12)
at emitTwo (events.js:87:13)
at Server.emit (events.js:172:7)
at HTTPParser.parserOnIncoming [as onIncoming] (_http_server.js:528:12)
at HTTPParser.parserOnHeadersComplete (_http_common.js:103:23)

and here is the code:

const http = require('http');

const hostname = '127.0.0.1'; const port = 3000;

const server = http.createServer((request, response) => { response.statusCode = 200; response.write("This is before the end"); setInterval(function(){ response.write(new Date() + "/n"); }, 1000); response.setHeader('Content-Type', 'text/plain'); response.end('Hello World\n'); });

server.listen(3000), console.log(Server running at http://${hostname}:${port}/);