ol3 example vectorlayer geojson export

HTML

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/css/bootstrap.min.css">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/js/bootstrap.min.js"></script>
<script src="http://openlayers.org/en/v3.10.1/build/ol.js"></script>
<link rel="stylesheet" href="http://openlayers.org/en/v3.10.1/css/ol.css">
<div class="row-fluid">
  <div class="span12">
    <div id="map" class="map"></div>
    <div id="no-download" class="alert alert-danger" style="display: none">
      This example requires a browser that supports the
      <a href="http://caniuse.com/#feat=download">link download</a> attribute.
    </div>
    <a id="export-png" class="btn btn-default" download="map.png"><i class="fa fa-download"></i> Export PNG</a>
  </div>
</div>

CSS

html,body, .span12, .row-fluid
{
    
    width:100%;
    height:100%;
    margin: 0px !important;
}
#map
{
    width:100%;
    height:100%;
    margin: 0px !important;
    
}

JavaScript

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: 'http://openlayers.org/en/master/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
  })
});

var exportPNGElement = document.getElementById('export-png');

if ('download' in exportPNGElement) {
  exportPNGElement.addEventListener('click', function(e) {
    map.once('postcompose', function(event) {
      var canvas = event.context.canvas;
      exportPNGElement.href = canvas.toDataURL('image/png');
    });
    map.renderSync();
  }, false);
} else {
  var info = document.getElementById('no-download');
  /**
   * display error message
   */
  info.style.display = '';
}