js/leaflet.markercluster.js
function (layer, callback) {

    var showMarker = function () {
      if ((layer._icon || layer.__parent._icon) && !this._inZoomAnimation) {
        this._map.off('moveend', showMarker, this);
        this.off('animationend', showMarker, this);

        if (layer._icon) {
          callback();
        } else if (layer.__parent._icon) {
          var afterSpiderfy = function () {
            this.off('spiderfied', afterSpiderfy, this);
            callback();
          };

          this.on('spiderfied', afterSpiderfy, this);
          layer.__parent.spiderfy();
        }
      }
    };

    if (layer._icon && this._map.getBounds().contains(layer.getLatLng())) {
      //Layer is visible ond on screen, immediate return
      callback();
    } else if (layer.__parent._zoom < this._map.getZoom()) {
      //Layer should be visible at this zoom level. It must not be on screen so just pan over to it
      this._map.on('moveend', showMarker, this);
      this._map.panTo(layer.getLatLng());
    } else {
      var moveStart = function () {
        this._map.off('movestart', moveStart, this);
        moveStart = null;
      };

      this._map.on('movestart', moveStart, this);
      this._map.on('moveend', showMarker, this);
      this.on('animationend', showMarker, this);
      layer.__parent.zoomToBounds();

      if (moveStart) {
        //Never started moving, must already be there, probably need clustering however
        showMarker.call(this);
      }
    }
  }
function (layer, callback) {

    var showMarker = function () {
      if ((layer._icon || layer.__parent._icon) && !this._inZoomAnimation) {
        this._map.off('moveend', showMarker, this);
        this.off('animationend', showMarker, this);

        if (layer._icon) {
          callback();
        } else if (layer.__parent._icon) {
          var afterSpiderfy = function () {
            this.off('spiderfied', afterSpiderfy, this);
            callback();
          };

          this.on('spiderfied', afterSpiderfy, this);
          layer.__parent.spiderfy();
        }
      }
    };

    if (layer._icon && this._map.getBounds().contains(layer.getLatLng())) {
      //Layer is visible ond on screen, immediate return
      callback();
    } else if (layer.__parent._zoom < this._map.getZoom()) {
      //Layer should be visible at this zoom level. It must not be on screen so just pan over to it
      this._map.on('moveend', showMarker, this);
      this._map.panTo(layer.getLatLng());
    } else {
      var moveStart = function () {
        this._map.off('movestart', moveStart, this);
        moveStart = null;
      };

      this._map.on('movestart', moveStart, this);
      this._map.on('moveend', showMarker, this);
      this.on('animationend', showMarker, this);
      layer.__parent.zoomToBounds();

      if (moveStart) {
        //Never started moving, must already be there, probably need clustering however
        showMarker.call(this);
      }
    }
  }
Zoom down to show the given layer (spiderfying if necessary) then calls the callback