This is a simple implementation of a tracking pixel using Express.js

ExpressTrackingPixel.js

import express from 'express';

const app = express();

const pixel = Buffer.from('R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7', 'base64');

app.get('/pixel.gif', (req, res) => {
  res.writeHead(200, {
    'Content-Type': 'image/gif',
    'Content-Length': pixel.length,
  });
  res.end(pixel);
});

app.listen(3000);