app.js

map.on("zoomend", function(e){
  //Event docs: http://leafletjs.com/reference.html#event/
  var themap = e.target;
  
  // get the zoom of the map noww that we are zoomed out http://leafletjs.com/reference.html#map-getzoom
  if(themap.getZoom() > 10){
    // zoomed in more then 10
  } else {
    // zoomed out above 10
  }
});