How to use the @google/earthengine.call function in @google/earthengine

ee.initialize(null, null, () => {
      // Load an image estimating number of persons per 30 arc-second grid cell.
      const image = ee.Image('CIESIN/GPWv4/population-count/2015');

      // Create a hexgrid covering the viewport, with TurfJS.
      const cellDiameter =
          200 * Math.abs(bounds[3] - bounds[1]) / DEGREES_LATITUDE_TO_MILES;
      const gridGeoJson = hexGrid(bounds, cellDiameter, 'miles');
      const gridFeatures = ee.FeatureCollection(gridGeoJson.features);

      // Compute sum of population values for each hex cell.
      const reducedFeatures =
          image.reduceRegions(gridFeatures, ee.call('Reducer.sum'));
      reducedFeatures.evaluate((geojson) => res.send(geojson));
    });
  });