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Adding state to controls

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JavaScript
var map;
var chicago = {lat: 41.85, lng: -87.65};

/**
 * The CenterControl adds a control to the map that recenters the map on
 * Chicago.
 * @constructor
 * @param {!Element} controlDiv
 * @param {!google.maps.Map} map
 * @param {?google.maps.LatLng} center
 */
function CenterControl(controlDiv, map, center) {
  // We set up a variable for this since we're adding event listeners later.
  var control = this;

  // Set the center property upon construction
  control.center_ = center;
  controlDiv.style.clear = 'both';

  // Set CSS for the control border
  var goCenterUI = document.createElement('div');
  goCenterUI.id = 'goCenterUI';
  goCenterUI.title = 'Click to recenter the map';
  controlDiv.appendChild(goCenterUI);

  // Set CSS for the control interior
  var goCenterText = document.createElement('div');
  goCenterText.id = 'goCenterText';
  goCenterText.innerHTML = 'Center Map';
  goCenterUI.appendChild(goCenterText);

  // Set CSS for the setCenter control border
  var setCenterUI = document.createElement('div');
  setCenterUI.id = 'setCenterUI';
  setCenterUI.title = 'Click to change the center of the map';
  controlDiv.appendChild(setCenterUI);

  // Set CSS for the control interior
  var setCenterText = document.createElement('div');
  setCenterText.id = 'setCenterText';
  setCenterText.innerHTML = 'Set Center';
  setCenterUI.appendChild(setCenterText);

  // Set up the click event listener for 'Center Map': Set the center of the map
  // to the current center of the control.
  goCenterUI.addEventListener('click', function() {
    var currentCenter = control.getCenter();
    map.setCenter(currentCenter);
  });

  // Set up the click event listener for 'Set Center': Set the center of the
  // control to the current center of the map.
  setCenterUI.addEventListener('click', function() {
    var newCenter = map.getCenter();
    control.setCenter(newCenter);
  });
}

/**
 * Define a property to hold the center state.
 * @private
 */
CenterControl.prototype.center_ = null;

/**
 * Gets the map center.
 * @return {?google.maps.LatLng}
 */
CenterControl.prototype.getCenter = function() {
  return this.center_;
};

/**
 * Sets the map center.
 * @param {?google.maps.LatLng} center
 */
CenterControl.prototype.setCenter = function(center) {
  this.center_ = center;
};

function initMap() {
  map = new google.maps.Map(document.getElementById('map'), {
    zoom: 12,
    center: chicago
  });

  // Create the DIV to hold the control and call the CenterControl() constructor
  // passing in this DIV.
  var centerControlDiv = document.createElement('div');
  var centerControl = new CenterControl(centerControlDiv, map, chicago);

  centerControlDiv.index = 1;
  centerControlDiv.style['padding-top'] = '10px';
  map.controls[google.maps.ControlPosition.TOP_CENTER].push(centerControlDiv);
}
JavaScript + HTML
<!DOCTYPE html>
<html>
  <head>
    <meta name="viewport" content="initial-scale=1.0, user-scalable=no">
    <meta charset="utf-8">
    <title>Adding state to controls</title>
    <style>
      html, body {
        height: 100%;
        margin: 0;
        padding: 0;
      }
      #map {
        height: 100%;
      }
#goCenterUI, #setCenterUI {
  background-color: #fff;
  border: 2px solid #fff;
  border-radius: 3px;
  box-shadow: 0 2px 6px rgba(0,0,0,.3);
  cursor: pointer;
  float: left;
  margin-bottom: 22px;
  text-align: center;
}

#goCenterText, #setCenterText {
  color: rgb(25,25,25);
  font-family: Roboto,Arial,sans-serif;
  font-size: 15px;
  line-height: 25px;
  padding-left: 5px;
  padding-right: 5px;
}

#setCenterUI {
  margin-left: 12px;
}

    </style>
  </head>
  <body>
    <div id="map"></div>
    <script>
var map;
var chicago = {lat: 41.85, lng: -87.65};

/**
 * The CenterControl adds a control to the map that recenters the map on
 * Chicago.
 * @constructor
 * @param {!Element} controlDiv
 * @param {!google.maps.Map} map
 * @param {?google.maps.LatLng} center
 */
function CenterControl(controlDiv, map, center) {
  // We set up a variable for this since we're adding event listeners later.
  var control = this;

  // Set the center property upon construction
  control.center_ = center;
  controlDiv.style.clear = 'both';

  // Set CSS for the control border
  var goCenterUI = document.createElement('div');
  goCenterUI.id = 'goCenterUI';
  goCenterUI.title = 'Click to recenter the map';
  controlDiv.appendChild(goCenterUI);

  // Set CSS for the control interior
  var goCenterText = document.createElement('div');
  goCenterText.id = 'goCenterText';
  goCenterText.innerHTML = 'Center Map';
  goCenterUI.appendChild(goCenterText);

  // Set CSS for the setCenter control border
  var setCenterUI = document.createElement('div');
  setCenterUI.id = 'setCenterUI';
  setCenterUI.title = 'Click to change the center of the map';
  controlDiv.appendChild(setCenterUI);

  // Set CSS for the control interior
  var setCenterText = document.createElement('div');
  setCenterText.id = 'setCenterText';
  setCenterText.innerHTML = 'Set Center';
  setCenterUI.appendChild(setCenterText);

  // Set up the click event listener for 'Center Map': Set the center of the map
  // to the current center of the control.
  goCenterUI.addEventListener('click', function() {
    var currentCenter = control.getCenter();
    map.setCenter(currentCenter);
  });

  // Set up the click event listener for 'Set Center': Set the center of the
  // control to the current center of the map.
  setCenterUI.addEventListener('click', function() {
    var newCenter = map.getCenter();
    control.setCenter(newCenter);
  });
}

/**
 * Define a property to hold the center state.
 * @private
 */
CenterControl.prototype.center_ = null;

/**
 * Gets the map center.
 * @return {?google.maps.LatLng}
 */
CenterControl.prototype.getCenter = function() {
  return this.center_;
};

/**
 * Sets the map center.
 * @param {?google.maps.LatLng} center
 */
CenterControl.prototype.setCenter = function(center) {
  this.center_ = center;
};

function initMap() {
  map = new google.maps.Map(document.getElementById('map'), {
    zoom: 12,
    center: chicago
  });

  // Create the DIV to hold the control and call the CenterControl() constructor
  // passing in this DIV.
  var centerControlDiv = document.createElement('div');
  var centerControl = new CenterControl(centerControlDiv, map, chicago);

  centerControlDiv.index = 1;
  centerControlDiv.style['padding-top'] = '10px';
  map.controls[google.maps.ControlPosition.TOP_CENTER].push(centerControlDiv);
}

    </script>
    <script src="https://maps.googleapis.com/maps/api/js?signed_in=true&callback=initMap" async defer></script>
  </body>
</html>

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