js/leaflet.markercluster.js
function (boundsToApplyTo, zoomLevelToStart, zoomLevelToStop, runAtEveryLevel, runAtBottomLevel) {
    var childClusters = this._childClusters,
        zoom = this._zoom,
      i, c;

    if (zoomLevelToStart > zoom) { //Still going down to required depth, just recurse to child clusters
      for (i = childClusters.length - 1; i >= 0; i--) {
        c = childClusters[i];
        if (boundsToApplyTo.intersects(c._bounds)) {
          c._recursively(boundsToApplyTo, zoomLevelToStart, zoomLevelToStop, runAtEveryLevel, runAtBottomLevel);
        }
      }
    } else { //In required depth

      if (runAtEveryLevel) {
        runAtEveryLevel(this);
      }
      if (runAtBottomLevel && this._zoom === zoomLevelToStop) {
        runAtBottomLevel(this);
      }

      //TODO: This loop is almost the same as above
      if (zoomLevelToStop > zoom) {
        for (i = childClusters.length - 1; i >= 0; i--) {
          c = childClusters[i];
          if (boundsToApplyTo.intersects(c._bounds)) {
            c._recursively(boundsToApplyTo, zoomLevelToStart, zoomLevelToStop, runAtEveryLevel, runAtBottomLevel);
          }
        }
      }
    }
  }
function (boundsToApplyTo, zoomLevelToStart, zoomLevelToStop, runAtEveryLevel, runAtBottomLevel) {
    var childClusters = this._childClusters,
        zoom = this._zoom,
      i, c;

    if (zoomLevelToStart > zoom) { //Still going down to required depth, just recurse to child clusters
      for (i = childClusters.length - 1; i >= 0; i--) {
        c = childClusters[i];
        if (boundsToApplyTo.intersects(c._bounds)) {
          c._recursively(boundsToApplyTo, zoomLevelToStart, zoomLevelToStop, runAtEveryLevel, runAtBottomLevel);
        }
      }
    } else { //In required depth

      if (runAtEveryLevel) {
        runAtEveryLevel(this);
      }
      if (runAtBottomLevel && this._zoom === zoomLevelToStop) {
        runAtBottomLevel(this);
      }

      //TODO: This loop is almost the same as above
      if (zoomLevelToStop > zoom) {
        for (i = childClusters.length - 1; i >= 0; i--) {
          c = childClusters[i];
          if (boundsToApplyTo.intersects(c._bounds)) {
            c._recursively(boundsToApplyTo, zoomLevelToStart, zoomLevelToStop, runAtEveryLevel, runAtBottomLevel);
          }
        }
      }
    }
  }
Run the given functions recursively to this and child clusters boundsToApplyTo: a L.LatLngBounds representing the bounds of what clusters to recurse in to zoomLevelToStart: zoom level to start running functions (inclusive) zoomLevelToStop: zoom level to stop running functions (inclusive) runAtEveryLevel: function that takes an L.MarkerCluster as an argument that should be applied on every level runAtBottomLevel: function that takes an L.MarkerCluster as an argument that should be applied at only the bottom level