JSTS Integration

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// this example uses JSTS for which we don't have an externs file.
var source = new ol.source.Vector();
fetch('https://openlayers.org/en/v4.6.4/examples/data/geojson/roads-seoul.geojson').then(function(response) {
  return response.json();
}).then(function(json) {
  var format = new ol.format.GeoJSON();
  var features = format.readFeatures(json, {featureProjection: 'EPSG:3857'});

var parser = new jsts.io.OL3Parser();

for (var i = 0; i < features.length; i++) {
    var feature = features[i];
    // convert the OpenLayers geometry to a JSTS geometry
    var jstsGeom = parser.read(feature.getGeometry());

// create a buffer of 40 meters around each line
    var buffered = jstsGeom.buffer(40);

// convert back from JSTS and replace the geometry on the feature
    feature.setGeometry(parser.write(buffered));
  }

source.addFeatures(features);
});
var vectorLayer = new ol.layer.Vector({
  source: source
});

var rasterLayer = new ol.layer.Tile({
  source: new ol.source.OSM()
});

var map = new ol.Map({
  layers: [rasterLayer, vectorLayer],
  target: document.getElementById('map'),
  view: new ol.View({
    center: ol.proj.fromLonLat([126.979293, 37.528787]),
    zoom: 15
  })
});

<!DOCTYPE html>
<html>
  <head>
    <title>JSTS Integration</title>
    <link rel="stylesheet" href="https://openlayers.org/en/v4.6.4/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/v4.6.4/build/ol.js"></script>
    <script src="https://cdn.rawgit.com/bjornharrtell/jsts/gh-pages/1.4.0/jsts.min.js"></script>
  </head>
  <body>
    <div id="map" class="map"></div>
    <script>
      // this example uses JSTS for which we don't have an externs file.
      var source = new ol.source.Vector();
      fetch('https://openlayers.org/en/v4.6.4/examples/data/geojson/roads-seoul.geojson').then(function(response) {
        return response.json();
      }).then(function(json) {
        var format = new ol.format.GeoJSON();
        var features = format.readFeatures(json, {featureProjection: 'EPSG:3857'});

        var parser = new jsts.io.OL3Parser();

        for (var i = 0; i < features.length; i++) {
          var feature = features[i];
          // convert the OpenLayers geometry to a JSTS geometry
          var jstsGeom = parser.read(feature.getGeometry());

          // create a buffer of 40 meters around each line
          var buffered = jstsGeom.buffer(40);

          // convert back from JSTS and replace the geometry on the feature
          feature.setGeometry(parser.write(buffered));
        }

        source.addFeatures(features);
      });
      var vectorLayer = new ol.layer.Vector({
        source: source
      });

      var rasterLayer = new ol.layer.Tile({
        source: new ol.source.OSM()
      });

      var map = new ol.Map({
        layers: [rasterLayer, vectorLayer],
        target: document.getElementById('map'),
        view: new ol.View({
          center: ol.proj.fromLonLat([126.979293, 37.528787]),
          zoom: 15
        })
      });
    </script>
  </body>
</html>