app.js

var distance2 = DISTANCE1.dashboarddistance( 18.5289, 73.8743, 18.558, 73.886,DISTANCE1.callbackFunction);
  var finaldistnce= DISTANCE1.dashboarddistance(18.5289, 73.8743, 18.558, 73.886, function(err, distance) {
   return distance
  });
   console.log(finaldistnce);// return null

googledistance.js

module.exports = {
  dashboarddistance: function(a, b, c, d, callback) {
    var ds = 0;
    var distance = require('google-distance');
   
    distance.get({
      index: 1,
      origin: a + ',' + b,
      destination: c + ',' + d
    }, function(err, data) {
      if (err) {
        console.log("a");
        ds = 0;
        return callback(err, ds);
      } else {
        console.log("b");
        ds = data.distance;
        return callback(null, ds);
      }
    });
  },
};