Response.write doesn't display variable string

So, I'm learning node, tried to write simple server by my self. When it comes to write data to response, it doesn't work properly. I converted data to String, i tried to display data to the console and it had benn displayed normally, but when I pass it to response.write() nothing changes at my page. I created file called 'lorem' in poems/lorem directory, I write this path to my link in browser. As you can see, I was writting some debugging stuff, but I deleted most of it.

var http = require('http');
var fs   = require('fs');
var path = require('path');

var port = process.argv[2];

var readFileContent = (filePath) => 
{
    var readData = '';

    fs.readFile(path.normalize(process.cwd() + filePath), 'utf-8', (err, data) => 
    {
        let invalid = false;
        if(err || data === undefined)
        {
            invalid = true;
        }
        if(!invalid) readData = data;
        console.log(readData); // logs data from a file
    });

    return readData;
}

var serverHandler = (request, response) =>
{   
        let responseData = readFileContent(request.url);

        if(responseData != undefined)
        {
            response.writeHead(200, {'Content-Type':'text/html'});
            response.write(''+ responseData.toString()); // there is a problem: writes data only when I pass some exact string (e.g. 'LOREM')
        }
        else{
            response.writeHead(404);
            response.write('Error occured');
        }
        response.end(responseData.toString());
    }


http.createServer(serverHandler).listen(+ port);


console.log('Server is lisntening on port ' + port);

There some console output for 'C:/folderpath/ node server.js 8080' cmd:

Server is lisntening on port 8080

lorem

lorem

lorem

Content of 'poems/lorem' file == 'lorem'