Map Export

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<!DOCTYPE html>
<html>
  <head>
    <title>Map Export</title>
    <link rel="stylesheet" href="https://openlayers.org/en/v4.2.0/css/ol.css" type="text/css">
    <!-- The line below is only needed for old environments like Internet Explorer and Android 4.x -->
    <script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=requestAnimationFrame,Element.prototype.classList,URL"></script>
    <script src="https://openlayers.org/en/v4.2.0/build/ol.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/FileSaver.js/1.3.3/FileSaver.min.js"></script>
  </head>
  <body>
    <div id="map" class="map"></div>
    <a id="export-png" class="btn btn-default"><i class="fa fa-download"></i> Download PNG</a>
    <script>
      // this example uses FileSaver.js for which we don't have an externs file.
      var map = new ol.Map({
        layers: [
          new ol.layer.Tile({
            source: new ol.source.OSM()
          }),
          new ol.layer.Vector({
            source: new ol.source.Vector({
              url: 'https://openlayers.org/en/v4.2.0/examples/data/geojson/countries.geojson',
              format: new ol.format.GeoJSON()
            })
          })
        ],
        target: 'map',
        controls: ol.control.defaults({
          attributionOptions: /** @type {olx.control.AttributionOptions} */ ({
            collapsible: false
          })
        }),
        view: new ol.View({
          center: [0, 0],
          zoom: 2
        })
      });

      document.getElementById('export-png').addEventListener('click', function() {
        map.once('postcompose', function(event) {
          var canvas = event.context.canvas;
          if (navigator.msSaveBlob) {
            navigator.msSaveBlob(canvas.msToBlob(), 'map.png');
          } else {
            canvas.toBlob(function(blob) {
              saveAs(blob, 'map.png');
            });
          }
        });
        map.renderSync();
      });
    </script>
  </body>
</html>