var min_zoom =  3; // how far OUT can they zoom?
var max_zoom = 18; // how far IN can they zoom?


function checkzoom() {
   var newzoom = map.getZoom();
   if (newzoom > max_zoom) return map.setZoom(max_zoom);
   if (newzoom < min_zoom) return map.setZoom(min_zoom);
   return newzoom;
}