MarkerCluster and MarkerClusterGroup should be classes, to be consistent with definitions of similar layers in Leaflet.ts (TileLayer, GridLayer, etc...). Using interface definitions also breaks inheritance chain.

types/leaflet.markercluster/index.d.ts

@@ -7,7 +7,7 @@
 import * as L from 'leaflet';
 
 declare module 'leaflet' {
-    interface MarkerCluster extends Marker {
+    class MarkerCluster extends Marker {
         /*
         * Recursively retrieve all child markers of this cluster.
         */
@@ -117,7 +117,7 @@ declare module 'leaflet' {
         chunkDelay?: number;
     }
 
-    interface MarkerClusterGroup extends FeatureGroup {
+    class MarkerClusterGroup extends FeatureGroup {
         /*
         * Bulk methods for adding and removing markers and should be favoured over the
         * single versions when doing bulk addition/removal of markers.

types/leaflet.markercluster/leaflet.markercluster-tests.ts

@@ -81,3 +81,34 @@ markerClusterGroup.zoomToShowLayer(marker, () => {});
 let hasLayer: boolean;
 hasLayer = markerClusterGroup.hasLayer(layer);
 hasLayer = markerClusterGroup.hasLayer(marker);
+
+// inheritance
+const Subclass1 = L.MarkerClusterGroup.extend({
+    myFunction() {}
+});
+class Subclass2 extends L.MarkerClusterGroup {
+    myFunction() {}
+}
+const Subclass3 = L.MarkerCluster.extend({
+    myFunction() {}
+});
+class Subclass4 extends L.MarkerCluster {
+    myFunction() {}
+}
+
+const s1 = new Subclass1(); // any
+const s2 = new Subclass2();
+const s3 = new Subclass3(); // any
+const s4 = new Subclass4([1, 2]);
+
+// call subclass function
+s1.myFunction();
+s2.myFunction();
+s3.myFunction();
+s4.myFunction();
+
+// call base class function
+s1.refreshClusters();
+s2.refreshClusters();
+s3.getAllChildMarkers();
+s4.getAllChildMarkers();