index.html
<!DOCTYPE html>
<html>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Map - Fit to extent</title>
  <style type="text/css">
    html,body{margin:0px;height:100%;width:100%}
    .container{width:100%;height:100%}
    .pane{background:#34495e;line-height:28px;color:#fff;z-index:10;position:absolute;top:20px;right:20px}
    .pane a{display:block;color:#fff;text-align:left;padding:0 10px;min-width:28px;min-height:28px;float:left}
  </style>
  <link rel="stylesheet" href="https://unpkg.com/maptalks/dist/maptalks.css">
  <script type="text/javascript" src="https://unpkg.com/maptalks/dist/maptalks.min.js"></script>
  <body>

    <div id="map" class="container"></div>
    <div class="pane"><a href="javascript:fitExtent();">Fit to extent</a></div>

    <script>
      var map = new maptalks.Map('map', {
        center: [-0.113049,51.498568],
        zoom: 14,
        baseLayer: new maptalks.TileLayer('base', {
          urlTemplate: 'https://{s}.basemaps.cartocdn.com/light_all/{z}/{x}/{y}.png',
          subdomains: ["a","b","c","d"],
          attribution: '&copy; <a href="http://osm.org">OpenStreetMap</a> contributors, &copy; <a href="https://carto.com/">CARTO</a>'
        }),
        layers : [
          new maptalks.VectorLayer('v')
        ]
      });

      var center = map.getCenter();
      var polygon = new maptalks.Polygon([
        center.add(-0.005, 0.005),
        center.add(0.005, 0.005),
        center.add(0.005, -0.005),
        center.add(-0.005, -0.005)
      ], {
        symbol : {
          polygonFill : '#fff',
          polygonOpacity : 0.5
        }
      });
      map.getLayer('v').addGeometry(polygon);

      function fitExtent() {
        // fit map's extent to polygon's
        // 0 is the zoom offset
        map.fitExtent(polygon.getExtent(), 0);
      }

    </script>
  </body>
</html>
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