[Vuejs]-Vuejs variable using in function google maps matrix api problem

  calculateDistances() {

        var a1={lat:this.data.adrdep.lat,lng:this.data.adrdep.lng};
        var a2={lat:this.data.adrarr.lat,lng:this.data.adrarr.lng};
        var p1 = new google.maps.LatLng(a1.lat, a1.lng);
        var p2 = new google.maps.LatLng(a2.lat, a2.lng);
     var service = new google.maps.DistanceMatrixService();
     service.getDistanceMatrix(
           {
              origins: [p1],
              destinations: [p2],
              travelMode: google.maps.TravelMode.DRIVING,
              unitSystem: google.maps.UnitSystem.METRIC,
              avoidHighways: false,
              avoidTolls: false
           }, this.callback);

  },
  callback(response, status) {
     if (status != google.maps.DistanceMatrixStatus.OK) {
           alert('Error was: ' + status);
     } else {
           var origins = response.originAddresses;
           var totalDistance = response.rows[0].elements[0].distance.value;
           var totalTime = response.rows[0].elements[0].duration.value;

           this.data.distance=parseFloat(totalDistance*0.001).toFixed(2);
           this.e1=2;
     }
  }