LineString Arrows

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import Map from 'ol/Map.js';
import View from 'ol/View.js';
import Point from 'ol/geom/Point.js';
import Draw from 'ol/interaction/Draw.js';
import {Tile as TileLayer, Vector as VectorLayer} from 'ol/layer.js';
import {OSM, Vector as VectorSource} from 'ol/source.js';
import {Icon, Stroke, Style} from 'ol/style.js';

var raster = new TileLayer({
  source: new OSM()
});

var source = new VectorSource();

var styleFunction = function(feature) {
  var geometry = feature.getGeometry();
  var styles = [
    // linestring
    new Style({
      stroke: new Stroke({
        color: '#ffcc33',
        width: 2
      })
    })
  ];

geometry.forEachSegment(function(start, end) {
    var dx = end[0] - start[0];
    var dy = end[1] - start[1];
    var rotation = Math.atan2(dy, dx);
    // arrows
    styles.push(new Style({
      geometry: new Point(end),
      image: new Icon({
        src: 'data/arrow.png',
        anchor: [0.75, 0.5],
        rotateWithView: true,
        rotation: -rotation
      })
    }));
  });

return styles;
};
var vector = new VectorLayer({
  source: source,
  style: styleFunction
});

var map = new Map({
  layers: [raster, vector],
  target: 'map',
  view: new View({
    center: [-11000000, 4600000],
    zoom: 4
  })
});

map.addInteraction(new Draw({
  source: source,
  type: 'LineString'
}));

<!DOCTYPE html>
<html>
  <head>
    <title>LineString Arrows</title>
    <link rel="stylesheet" href="https://openlayers.org/en/v5.3.0/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>

  </head>
  <body>
    <div id="map" class="map"></div>
    <script>
      import Map from 'ol/Map.js';
      import View from 'ol/View.js';
      import Point from 'ol/geom/Point.js';
      import Draw from 'ol/interaction/Draw.js';
      import {Tile as TileLayer, Vector as VectorLayer} from 'ol/layer.js';
      import {OSM, Vector as VectorSource} from 'ol/source.js';
      import {Icon, Stroke, Style} from 'ol/style.js';

      var raster = new TileLayer({
        source: new OSM()
      });

      var source = new VectorSource();

      var styleFunction = function(feature) {
        var geometry = feature.getGeometry();
        var styles = [
          // linestring
          new Style({
            stroke: new Stroke({
              color: '#ffcc33',
              width: 2
            })
          })
        ];

        geometry.forEachSegment(function(start, end) {
          var dx = end[0] - start[0];
          var dy = end[1] - start[1];
          var rotation = Math.atan2(dy, dx);
          // arrows
          styles.push(new Style({
            geometry: new Point(end),
            image: new Icon({
              src: 'data/arrow.png',
              anchor: [0.75, 0.5],
              rotateWithView: true,
              rotation: -rotation
            })
          }));
        });

        return styles;
      };
      var vector = new VectorLayer({
        source: source,
        style: styleFunction
      });

      var map = new Map({
        layers: [raster, vector],
        target: 'map',
        view: new View({
          center: [-11000000, 4600000],
          zoom: 4
        })
      });

      map.addInteraction(new Draw({
        source: source,
        type: 'LineString'
      }));
    </script>
  </body>
</html>