How to get features from cluster source for current extent?

How to get features from cluster source for current extent? I need to collect all features from cluster for current extent and zoom state.

I tried this:

  this.mapService.map.getView().on('change:resolution', (evt) => {
   const a =  (this.clusterVector.getSource() as VectorSource).getFeaturesByExtent(this.clusterVector.getSource().getExtent());
   console.log(a);
  });

It does not work for me correctly, it returns more features when user zoom in instead less.

I have made some research and now there is:

    this.mapService.map.getView().on('change:resolution', (evt) => {
            const extent = this.mapService.map
              .getView()
              .calculateExtent(this.mapService.map.getSize());
            const features = 

this.clusterSource.getFeaturesInExtent(extent);

Is it proper way?