fs library : Error when use it - node.js

Question

I use fs library for reading file from system. I don't know I use this library and meet error.
I'm using PhpStorm. at line : fs.readFile() : there's a line under that, that notice me : unresolved function or method readFile(). It means IDE doesn't figure out where is this function. Nevertheless, I have checked fs.js and I see no problem.
I receive this error when running:
events.js:72
throw er; // Unhandled 'error' event
^ Error: listen EADDRINUSE
at errnoException (net.js:901:11)
at Server._listen2 (net.js:1039:14)
at listen (net.js:1061:10)
at Server.listen (net.js:1127:5)
at Object. (/home/hqt/PhpstormProjects/NodeApp/sample/AsynchronouslyReading.js:21:4)
at Module._compile (module.js:456:26)
at Object.Module._extensions..js (module.js:474:10)
at Module.load (module.js:356:32)
at Function.Module._load (module.js:312:12)
at Function.Module.runMain (module.js:497:10)
Here is my code :
var http = require('http');
var fs = require('fs');
// create http server
http.createServer(function(req, res) {
fs.readFile('helloworld.js', 'utf-8', function(err, data) {
res.writeHead(200, {'content-type' : 'text/plain'});
if (err) res.write('could not find or open file');
else res.write(data);
// ending
res.end();
});
}).listen(8124, function() {console.log('server is running at 8124');});
console.log('server is runnint at 8124 port');
Please help me figure out why. I'm using Ubuntu machine for development.
Thanks :)

Answer 1

That's because something else is already listening on port 8124. On Linux, you can use something like netstat -tanp | grep LISTEN | grep 8124 to see what.