Call the ViewAddress on TextBox change event.

<script type="text/javascript" src="https://maps.googleapis.com/maps/api/js?key=API_Key"></script>
<script type="text/javascript">
    function ViewAddress() {
        navigator.geolocation.getCurrentPosition(
            function (position) {
                var LatLng = new google.maps.LatLng(position.coords.latitude, position.coords.longitude);
                var geocoder = geocoder = new google.maps.Geocoder();
                geocoder.geocode({ 'latLng': LatLng }, function (results, status) {
                    if (status == google.maps.GeocoderStatus.OK) {
                        document.getElementById('txtaddress').value = results[0].formatted_address;
                    } elese {
                        document.getElementById('txtaddress').value = "address is not valid";
                    }
                });
            }, function (error) {
                alert(error.code + ": " + error.message);
            }, {
            enableHighAccuracy: true,
            maximumAge: 10000,
            timeout: 5000
        });
    }
</script>
<input type="text" id="txtaddress" onchange="ViewAddress()" />