score:1

Accepted answer

try

if (d3.select(geography).datum().blogentries > 0) {
    // ....
}

Edit

You have to put the map-fill-data in a separate variable and use the geography.id to get the value for blogentries

var mapData = {
  'FIN': {
    fillKey: 'VISITED',
    blogentries: 1
  },
  'AUT': {
    fillKey: 'VISITED',
    blogentries: 1
  },
};
var map = new Datamap({
  element: document.getElementById('worldmap'),
  responsive: true,
  geographyConfig: {
    highlightOnHover: false,
    popupOnHover: false
  },
  fills: {
    'VISITED': '#13304F',
    defaultFill: '#d6e5f5'
  },
  data: mapData,
  done: function(datamap) {
    datamap.svg.selectAll('.datamaps-subunit').on('click', function(geography) {
      if (mapData[geography.id] && mapData[geography.id].blogentries > 0) {
        window.location = "https://www.test.com/" + (geography.properties.name);
      }
    });
  }
});

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