app.js

app.get("/api", function(req, res) {
  if (req.query.lon === "" || req.query.lat === ""
      || req.query.lon = null || req.query.lat = null) { // handle undefined as well
    res.sendStatus(404).json({ error: 'Latitude and longitude are required' });
  } else {
    console.log(req.query);
    res.json(req.query);
  }
});