Add a GeoJSON layer with a popup showing feature attributes with the Google Maps API

google-geojson-popup.js

/* This example demonstrates how to create a popup, or an "infowindow", from
 * GeoJSON data (USGS earthquake feed). "place" and "mag" are the names of properties in the GeoJSON file. 
 * This code can be placed before or after other code for adding layers.
 */
map.data.loadGeoJson('http://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/2.5_day.geojson');

// Create an infowindow object to use later
var infowindow = new google.maps.InfoWindow();

/* Create a "listener" that will wait for the user to click an earthquake point,
 * and then display the infowindow with details about that earthquake.
 */
map.data.addListener('click', function(event) {
  // in the geojson feature that was clicked, get the "place" and "mag" attributes
  let place = event.feature.getProperty("place");
  let magnitude = event.feature.getProperty("mag");
  let html = magnitude + ' magnitude, ' + place; // combine place and magnitude, inserting additional text between them
  infowindow.setContent(html); // show the html variable in the infowindow
  infowindow.setPosition(event.feature.getGeometry().get()); // anchor the infowindow at the marker
  infowindow.setOptions({pixelOffset: new google.maps.Size(0,-30)}); // move the infowindow up slightly to the top of the marker icon
  infowindow.open(map);
});

not working