How to type iconCreateFunction from MarkerClusterGroup

I have created a small test using MarkerClusterGroup from react-leaflet-cluster.

Although it works, I have a hard time making all the types work in TypeScript:

<MarkerClusterGroup
    iconCreateFunction={createCustomClusterIcon}>
</MarkerClusterGroup>

The callback function looks like this:

const createCustomClusterIcon = (cluster) => {
   const markers = cluster.getAllChildMarkers();
   // ...
}

So, the problem I have is typing the cluster property. The only types that I was able to give to cluster was any and { getChildCount: () => string }. But what should this type be?

The iconCreateFunction prop mirrors the same named option from Leaflet.markercluster plugin.

Although not explicitly described, the callback is passed the MarkerCluster instance. You can get its type in L from "leaflet" after you have also done import "leaflet.markercluster" so that it performs the side effect of Leaflet module augmentation:

import L from "leaflet";
import "leaflet.markercluster"; // Import for side effect of Leaflet module augmentation
import MarkerClusterGroup from "react-leaflet-cluster";

const createCustomClusterIcon = (cluster: L.MarkerCluster) => {
    const markers = cluster.getAllChildMarkers();
    // ...
    return L.icon({ iconUrl: "" })
}

() => (
    <MarkerClusterGroup
        iconCreateFunction={createCustomClusterIcon} // Okay
    >
        (markers...)
    </MarkerClusterGroup>
);

Playground Link