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

ee.data.authenticateViaPrivateKey(PRIVATE_KEY, () => {
    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));
    });
  });
};

client.refreshAccessToken(function (err, tokens) {
            ee.data.authToken_ = 'Bearer ' + tokens['access_token'];
            ee.data.authClientId_ = o.cliet_id;
            ee.data.authScopes_ = [ee.data.AUTH_SCOPE_];
            ee.data.DEFAULT_API_BASE_URL_ = "https://earthengine.googleapis.com/api";
            ee.initialize(ee.data.DEFAULT_API_BASE_URL_);

            onsuccess();

            process.exit();
        });
    }