Basic Hrrtp server, to load file for the request and server response time to get for an ajax call

baiscHttpServer.js

var port = 8001;
var serverUrl = "127.0.0.1";
var http = require("http");
var path = require("path");
var fs = require("fs");
var url = require('url');
var responseStartTime;
console.log("Web server starting at " + serverUrl + ":" + port);

var extensions = {
    ".html": "text/html",
    ".js": "application/javascript",
    ".css": "text/css",
    ".txt": "text/plain",
    ".jpg": "image/jpeg",
    ".gif": "image/gif",
    ".png": "image/png"
};

var server = http.createServer(function (request, response) {
    responseStartTime = Date.now();
    var urlName = request.url || "index.html";
    var ext = path.extname(urlName);
    var pathName = url.parse(urlName).pathname;
    var physicalPath = __dirname;

    var validExt = extensions[ext];
    if (validExt) { // loads supported extension
        physicalPath += urlName;
        addFileToResponse(physicalPath, response, validExt);
    } else if (pathName == '/serverResponseTime') { // ajax call response
        sendServerResponseTime(response)
    } else {
        console.log("not supported sorry");
    }
});

function addFileToResponse(localPath, res, mimeType) {
    fs.exists(localPath, function (exists) {
        if (exists) {
            fs.readFile(localPath, function (err, contents) {
                if (!err) {
                    res.setHeader("Content-Length", contents.length);
                    res.setHeader("Content-Type", mimeType);
                    res.statusCode = 200;
                    res.write(contents);
                    res.end();
                } else {
                    res.writeHead(500);
                    res.end();
                }
            });
        } else {
            console.log("File not found: " + urlName);
            response.writeHead(404);
            response.end();
        }
    });

}

function sendServerResponseTime(response) {
    var responseTime = {
        startTime: responseStartTime,
        endTime: Date.now()
    };

    var resTimeAsJson = JSON.stringify(responseTime);
    response.setHeader("Content-Length", resTimeAsJson.length);
    response.setHeader("Content-Type", 'application/json');
    response.statusCode = 200;
    response.write(resTimeAsJson);
    response.end();
}

server.listen(port, serverUrl);