code: function (properties, featureCollection, mergeKey) {
  		var features = featureCollection.features;
  		var featureIndex = L.GeometryUtils.indexFeatureCollection(features, mergeKey);
  		var property;
  		var mergeValue;
  		var newFeatureCollection = {
  			type: 'FeatureCollection',
  			features: []
  		};
  
  		for (var key in properties) {
  			if (properties.hasOwnProperty(key)) {
  				property = properties[key];
  
  				mergeValue = property[mergeKey];
  
  				if (mergeValue) {
  					var feature = featureIndex[mergeValue];
  
  					for (var prop in property) {
  						feature.properties[prop] = property[prop];
  					}
  
  					newFeatureCollection.features.push(feature);
  				}
  			}
  		}
  
  		return newFeatureCollection;
  	}
docs: Merges a set of properties into the properties of each feature of a GeoJSON FeatureCollection