var doc = app.activeDocument;
var layerRef = doc.layers.getByName("Modular Grid");
layerRef.positionLocked = true;

if (layerRef.fillOpacity >= 100) {
    layerRef.fillOpacity = 0;
} else if (layerRef.fillOpacity <= 0) {
    layerRef.fillOpacity = 40;
} else {
    layerRef.fillOpacity = 100;
}