First attempt.  Was going to use .bind(null, functionTarget), but that didn't seem to work and in a hurry to get this done.

A pox upon JavaScript and the need to check all the types and properties as there could be a null/undefined in there and also the adoption of TypeScript is "costs too much time" - Fucking organization constraints

applyFnToFeature(feature, callbackFn, options) {
 if (feature instanceof Leaflet.Polygon) {
	 if (feature.options.hasOwnProperty("feature") &&
	 feature.options.feature.hasOwnProperty("properties") &&
	 feature.options.feature.properties.hasOwnProperty("id")) { callbackFn(feature, options); }
	}
 },

I think this would be better if it was structured as a continuation chain as so

applyFn(z).doOperation(z, options);