By using this function when we will click on Marker, its location will be shown on tha map. Here we have a function , first we will create "infowindow" object outside the function (global scope), then we will make infowindow objec constructor , within updateMarker function. Watch below-

 var geocoder = new google.maps.Geocoder(), infowindow;
     function geocodePosition(pos) {
   geocoder.geocode({
    latLng: pos
  }, function(responses) {
    if (responses && responses.length > 0) {
      updateMarkerAddress(responses[0].formatted_address);
    } else {
      updateMarkerAddress('Cannot determine address at this location.');
    }
  });
}

function updateMarkerStatus(str) {
  document.getElementById('markerStatus').innerHTML = str;
}

function updateMarkerPosition(latLng) {
  document.getElementById('info').innerHTML = [
    latLng.lat(),
    latLng.lng()
  ].join(', ');
}


function updateMarkerAddress(str) {
  infowindow = new google.maps.InfoWindow({
      content:  str
  });
  document.getElementById('address').innerHTML = str;
}

function initialize() {
  var latLng = new google.maps.LatLng(-34.397, 150.644);
  var map = new google.maps.Map(document.getElementById('mapCanvas'), {
    zoom: 8,
    center: latLng,
    mapTypeId: google.maps.MapTypeId.ROADMAP
  });
  var marker = new google.maps.Marker({
    position: latLng,
    title: 'Point A',
    map: map,
    draggable: true
  });
  var geocoder = new google.maps.Geocoder(), infowindow;
     function geocodePosition(pos) {
   geocoder.geocode({
    latLng: pos
  }, function(responses) {
    if (responses && responses.length > 0) {
      updateMarkerAddress(responses[0].formatted_address);
    } else {
      updateMarkerAddress('Cannot determine address at this location.');
    }
  });
}
  google.maps.event.addListener(marker, 'click', function() {
    infowindow.open(map, marker);
  });

  // Update current position info.
  updateMarkerPosition(latLng);
  geocodePosition(latLng);

  // Add dragging event listeners.
  google.maps.event.addListener(marker, 'dragstart', function() {
    updateMarkerAddress('Dragging...');
  });

  google.maps.event.addListener(marker, 'drag', function() {
    updateMarkerStatus('Dragging...');
    updateMarkerPosition(marker.getPosition());
  });

  google.maps.event.addListener(marker, 'dragend', function() {
    updateMarkerStatus('Drag ended');
    geocodePosition(marker.getPosition());
  });
}

 getLocation();
  var x=document.getElementById("demo");
  function getLocation() {
    if (navigator.geolocation) {
      navigator.geolocation.getCurrentPosition(showPosition,showError);
    } else{
      x.innerHTML="Geolocation is not supported by this browser.";
    }
  }
  function showPosition(position) {
    lat=position.coords.latitude;
    lon=position.coords.longitude;
    latlon=new google.maps.LatLng(lat, lon)
    mapholder=document.getElementById('mapholder')
    mapholder.style.height='500px';
    mapholder.style.width='1000px';
    var myOptions={
      center:latlon,
      zoom:14,
      mapTypeId:google.maps.MapTypeId.ROADMAP,
      mapTypeControl:false,
      navigationControlOptions:{
        style:google.maps.NavigationControlStyle.SMALL
      }
    };
    var map=new google.maps.Map(document.getElementById("mapholder"),myOptions);
    var marker=new google.maps.Marker(
      {position:latlon,map:map,title:"You are here!"}
    );
  }