how to use callback function in angular 4?

you can create oberservable and push new values on it ,

let subject = new Subject(); 
let ovservable = subject.asObservable()

subject.next("b");

ovservable.subscribe((value) => {
  console.log("Subscription got", value); // Subscription wont get 
                                          // anything at this point
});

so create observable , expose it and when you receive data from call make use of .next() method that will do

in your code

let subject = new Subject(); 
let ovservable = subject.asObservable();
let currgeocoder = new google.maps.Geocoder();
   currgeocoder.geocode({
      'location': location
   }, function(results:any, status:any) {
      if (status == google.maps.GeocoderStatus.OK) {
        let place = results[0];
        subject.next(place);
       //this.showresult(place.formatted_address);
      } else {
          alert('Geocode was not successful for the following reason: ' + status);
      }
   });

      this.myGroup.setValue({
      searchControl: 'global'
    });

    ovservable.subscribe((value) => {
      console.log("Subscription got", value); // Subscription wont get 
                                              // anything at this point
    });