For BLW espressox

counter.client.js

window.addEventListener('load', function() {
  fetch('/hit-counter')
    .then(response => response.text())
    .then(response => {
      window.TOTAL_HITS = repsonse << 0;
    })
    .catch(() => {
      window.TOTAL_HITS = 0;
    });
});

/*
  
  if (window.TOTAL_HITS) {
    // do something
  }

*/

counter.js

"use strict";

const fs         = require('fs').promises;
const metrohash  = require('metrohash').metrohash64;

class Counter {
  constructor   (divisor)        {
    this.datapath = __dirname + '/counter.data';
    this.divisor  = divisor << 0 || 10;
    this.tracker  = {
      look(fingerprint) {
        if (!(fingerprint in this.collection)) {
          this.collection[fingerprint] = 0;
        }
        
        return (this.collection[fingerprint] += 1);
      },
      collection: {},
    };

    void this.load();
  }
  exists        (location)       {
    return new Promise(function(resolve) {
      void fs
        .access(location)
        .then(()  => resolve(true))
        .catch(() => resolve(false));
    });
  }
  hash          (string)         {
    return metrohash(string, 31);
  }
  hit           (ip, useragent)  {
    if (ip && useragent) {
      const fingerprint = this.hash(String(ip) + String(useragent));

      if (this.tracker.look(fingerprint) % this.divisor === 0) {
        return 1;
      }
    }

    return 0;
  }
  async dump    ()               {
    await this.ensure(this.datapath);

    const prints = Object.keys(this.tracker.collection);
    const blob   = [];

    for (const print of prints) {
      void blob.push(print + ':' + this.tracker.collection[print]);
    }

    await fs.writeFile(this.datapath, blob.join('|'));

    return;
  }
  async ensure  (location)       {
    const exists = await this.exists(location);

    if (!exists) {
      await fs.mkdir(location, {recursive: true});
    }

    return;
  }
  async load    ()               {
    const hasdata = await this.exists(this.datapath);

    if (hasdata) {
      const data = await fs.readFile(this.datapath, 'utf8');
      const format = data.split('|');

      for (const entry of format) {
        const [fingerprint, hits] = entry.split(':');

        if (fingerprint && hits) {
          this.tracker.collection[fingerprint] = hits << 0;
        }
      }
    }

    this.loaded = true;

    return;
  }
}

counter.server.js

"use strict";

/*
  
  Reverse proxy nginx desired route to port 8000;
  
*/


const http    = require('http');
const Counter = require('./counter');
const counter = new Counter(10); // divisor;


void http.createServer(function(request, response) {
  const ip  = request.headers['x-forwarded-for'];
  const ua  = request.headers['user-agent'];

  return void response.end(counter.hit(ip, ua));
}).listen(8000);