problematic-old-school-hit-counter.js

/**
 * This gist is not recommended to be used for anything, other than
 * an example for a story about old school hit counters, and the
 * complexity of "dynamic" web pages vs static web pages.
 */
var http = require('http'),
    fs   = require('fs');

http.createServer(onHTTPReq).listen(8000);

function startCount(next) {
    write(0, function onStartWrite(err) {
        if (err) {
            next(err);
            return;
        }
        next(null, 0);
    });
}

function read(next) {
    fs.readFile('./count', 'utf-8', function callbackNumber(err, val) {
        if (!err) {
            next(null, +val);
            return;
        }
        if (err.code === 'ENOENT') {
            startCount(next);
        } else {
            next(err);
        }
    });
}

function write(val, next) {
    fs.writeFile('./count', val, next);
}

function increment(next) {
    read(function onIncRead(err, val) {
        if (err) {
            next(err);
            return;
        }
        val = +val || 0;
        val += 1;
        write(val, function (err) {
            if (err) {
                next(err);
                return;
            }
            next(null, +val);
        });
    });
}

function serverError(req, res, err) {
    res.writeHead(500, {'Content-Type': 'text/plain'});
    res.end('Fatal Error ' + err.message);
}

function notFound(req, res) {
    res.writeHead(404, {'Content-Type': 'text/plain'});
    res.end(['Sorry', req.url, 'Not Found.'].join(' '));
}

function onHTTPReq(req, res) {
    if (req.url !== '/') {
        notFound(req, res);
        return;
    }
    increment(function onIncrement(err, val) {
        if (err) {
            serverError(req, res, err);
            return;
        }
        res.writeHead(200, {'Content-Type': 'text/plain'});
        res.end('Visits: ' + val);
    });
}