Leaflet .js reference layers as variables

Question

I am advised to reference my layers as variables rather than as functions, so I can order the layers using .bringtofront.

I have tried several combinations of var skull = L.geoJSON(fatalities)etc.) but everything I have tried simply causes all the layers to vanish from the map.

Here's the map: https://pdxcyclesafetymap.neocities.org/

And here's the .js for one of the layers, as it works now:

var skull = L.icon({
    iconUrl: 'images/blackSkull.svg',
    iconSize: [38, 95],
    popupAnchor: [-10, -30],
    }); 

function fatalitiesLayer (feature, 
layer) {
    layer.bindPopup("<h1>Cyclist Fatality</h1><p> "+feature.properties.name+
    "</p>"+feature.properties.description+
    "</p>Date: "+feature.properties.date+ 
    "</p>Location: "+feature.properties.location+ 
    "</p>Cause: "+feature.properties.cause+ 
    "</p>");
    layer.setIcon(skull); 
    }


L.geoJson(fatalities,{
    onEachFeature: fatalitiesLayer
    }).addTo(map);