For serving 1x1px transparent gif image with nodejs.
$ npm install --save blankgif
var blankgif = require('blankgif');
var express = require('express');
// option 1
app.get('/track.gif', blankgif.sendBlankGif);
// option 2
app.use(blankgif.middleware())
app.get('/track.gif', function(req, res) {
process.nextTick(function() { /* do tracking stuff */ });
res.set('Cache-Control', 'public, max-age=0');
res.status(200).sendBlankGif();
});