main.js

#!/usr/bin/env node

const http = require('http'),
      path = require("path"),
      fs = require("fs");

// Let's define a port we want to listen on
const DEFAULT_PORT = 80;
var port = DEFAULT_PORT;

// same with webpage title
var DEFAULT_TITLE = "Merry christmas!";
var title = DEFAULT_TITLE;

const html = `
<header>
  <title>TITLE</title>
  <style>
    body {
        background-image: url("snow.gif");
        background-repeat: repeat;
    }
  </style>
</header>
`;

const rootdir = path.resolve(__dirname);
var CONFIG_DIR = process.env.SNAP_DATA;
if (!CONFIG_DIR) {
    CONFIG_DIR = '.'
}
const CONFIG_FILE = path.join(CONFIG_DIR, 'config');

function handleRequest(request, response){
    if (request.url === "/snow.gif") {
        fs.readFile(path.join(rootdir, 'snow.gif'), 'binary', function(err, file) {
            if (err) {
                response.writeHead(404, {"Content-Type": "text/plain"});
                response.write("404 Not Found\n");
                response.end();
                return;
            }
            response.writeHeader(200, {"Content-Type": "image/gif"});
            response.write(file, "binary");
            response.end();
        });
        return;
    }
    response.writeHeader(200, {"Content-Type": "text/html"});
    response.end(html.replace("TITLE", title));
}

var server = http.createServer(handleRequest);

server.activeconnections = {}
server.on('connection', function(conn) {
    var key = conn.remoteAddress + ':' + conn.remotePort;
    server.activeconnections[key] = conn;
    conn.on('close', function() {
        delete server.activeconnections[key];
    });
});
// we want to immediatetly close all connections on server restart (for port change)
server.force_close = function(cb) {
    server.close(cb);
    for (var key in server.activeconnections)
        server.activeconnections[key].destroy();
};


fs.watchFile(CONFIG_FILE, (filename, prev) => {
    // file doesn't exist (even on startup) or exists and changed from default port
    loadconfig_and_start_server();
});
// if file exists on startup: load config and start server here
if (fs.existsSync(CONFIG_FILE)) {
    loadconfig_and_start_server();
}

function loadconfig_and_start_server() {
    fs.readFile(CONFIG_FILE, (err, data) => {

        new_port = DEFAULT_PORT;
        new_title = DEFAULT_TITLE;

        if (err) {
            if (port != DEFAULT_PORT) {
                console.log("Error reading config file, reverting to defaut port:" + err);
            }
        } else {
            console.log("Load port configuration");
            fs.readFileSync(CONFIG_FILE).toString().split('\n').forEach(function (line) {
                var data = line.split("=");

                if (data[1] == undefined) {
                    return
                }

                switch(data[0]) {
                    case 'port':
                        port_candidate = parseInt(data[1]);
                        if (isNaN(port_candidate)) {
                            console.log(`Port defined "${data[1]}" is not a valid port. Ignoring.`)
                        } else {
                            new_port = port_candidate;
                        }
                        break;
                    case 'title':
                        new_title = data[1];
                        break;
                }
            });
        }

        title = new_title

        // reload server if needed
        if (new_port != port || !server._handle) {
            server.force_close(() => {
                port = new_port;
                server.listen(port, function(){
                    console.log("Server listening on: http://localhost:%s", port);
                });
            });
        }
    });
}