Google Maps API - how to get latitude and longitude from Autocomplete without showing the map?

You can use the code below.

<script src="http://maps.googleapis.com/maps/api/js?libraries=places" type="text/javascript"></script>

<script type="text/javascript">
    function initialize() {
        var input = document.getElementById('searchTextField');
        var autocomplete = new google.maps.places.Autocomplete(input);
        google.maps.event.addListener(autocomplete, 'place_changed', function () {
            var place = autocomplete.getPlace();
            document.getElementById('city2').value = place.name;
            document.getElementById('cityLat').value = place.geometry.location.lat();
            document.getElementById('cityLng').value = place.geometry.location.lng();
            //alert("This function is working!");
            //alert(place.name);
           // alert(place.address_components[0].long_name);

        });
    }
    google.maps.event.addDomListener(window, 'load', initialize); 
</script>

and this part is inside your form:

<input id="searchTextField" type="text" size="50" placeholder="Enter a location" autocomplete="on" runat="server" />  
<input type="hidden" id="city2" name="city2" />
<input type="hidden" id="cityLat" name="cityLat" />
<input type="hidden" id="cityLng" name="cityLng" />  

I hope it helps.

Only need:

var place = autocomplete.getPlace();
// get lat
var lat = place.geometry.location.lat();
// get lng
var lng = place.geometry.location.lng();

You can't get latitude/longitude from autocomplete API since the data generated by google from that API doesn't contain latitude and logitude field. Example:-

{
   "predictions" : [
      {
         "description" : "IIT Mumbai, Mumbai, Maharashtra, India",
         "id" : "ac3235cda973818a89b5fe21ad0f5261ac9b6723",
         "matched_substrings" : [
            {
               "length" : 10,
               "offset" : 0
            }
         ],
         "reference" : "CkQ0AAAAHWg-RSngiYHHdz_yqyFKmfSoBwT-_PW0k8qQDhsbiVrk7BlPHCyJb58OthledMUTGYS5Vhec1Zj2L7w9Rq0zDxIQrbn05RX1QgWHkSXsmTk1TRoU45APW2BBRIUsA3t6XBy_8s0BPPA",
         "terms" : [
            {
               "offset" : 0,
               "value" : "IIT Mumbai"
            },
            {
               "offset" : 12,
               "value" : "Mumbai"
            },
            {
               "offset" : 20,
               "value" : "Maharashtra"
            },
            {
               "offset" : 33,
               "value" : "India"
            }
         ],
         "types" : [ "establishment", "geocode" ]
      }
   ],
   "status" : "OK"
}

You can use the Google API to get the Longitude and Latitude from your address. As you already said, you should implement a hidden field where the result should be inserted. Then you can save the location together with the coordinates.

e.g https://maps.googleapis.com/maps/api/geocode/json?address=IIT%20Area,%20Mumbai,%20Maharashtra,%20India&sensor=false

I recently implemented this function in one of my projects:

function getLatLngFromAddress(city, country){

  var address = city +", "+ country;
  var geocoder = new google.maps.Geocoder();

  geocoder.geocode( { 'address': address}, function(results, status) {

    if (status == google.maps.GeocoderStatus.OK) {
      $('#latitude').val(results[0].geometry.location.Pa);
      $('#longitude').val(results[0].geometry.location.Qa);

    } else {
      console.log("Geocode was not successful for the following reason: " + status);
    }
  });
}

Yes you can:

var place = autocomplete.getPlace();

document.getElementById('lat').value = place.geometry.location.lat();
document.getElementById('lon').value = place.geometry.location.lng();

You can get lat, lng from the place object i.e.

var place = autocomplete.getPlace();

var latitude = place.geometry.location.lat();
var longitude = place.geometry.location.lng();