Edit

Example of exporting a map as a PNG image.

Example of exporting a map as a PNG image.

main.js
import 'ol/ol.css';
import GeoJSON from 'ol/format/GeoJSON';
import Map from 'ol/Map';
import View from 'ol/View';
import {OSM, Vector as VectorSource} from 'ol/source';
import {Tile as TileLayer, Vector as VectorLayer} from 'ol/layer';

var map = new Map({
  layers: [
    new TileLayer({
      source: new OSM(),
    }),
    new VectorLayer({
      source: new VectorSource({
        url: 'data/geojson/countries.geojson',
        format: new GeoJSON(),
      }),
      opacity: 0.5,
    }) ],
  target: 'map',
  view: new View({
    center: [0, 0],
    zoom: 2,
  }),
});

document.getElementById('export-png').addEventListener('click', function () {
  map.once('rendercomplete', function () {
    var mapCanvas = document.createElement('canvas');
    var size = map.getSize();
    mapCanvas.width = size[0];
    mapCanvas.height = size[1];
    var mapContext = mapCanvas.getContext('2d');
    Array.prototype.forEach.call(
      document.querySelectorAll('.ol-layer canvas'),
      function (canvas) {
        if (canvas.width > 0) {
          var opacity = canvas.parentNode.style.opacity;
          mapContext.globalAlpha = opacity === '' ? 1 : Number(opacity);
          var transform = canvas.style.transform;
          // Get the transform parameters from the style's transform matrix
          var matrix = transform
            .match(/^matrix\(([^\(]*)\)$/)[1]
            .split(',')
            .map(Number);
          // Apply the transform to the export map context
          CanvasRenderingContext2D.prototype.setTransform.apply(
            mapContext,
            matrix
          );
          mapContext.drawImage(canvas, 0, 0);
        }
      }
    );
    if (navigator.msSaveBlob) {
      // link download attribuute does not work on MS browsers
      navigator.msSaveBlob(mapCanvas.msToBlob(), 'map.png');
    } else {
      var link = document.getElementById('image-download');
      link.href = mapCanvas.toDataURL();
      link.click();
    }
  });
  map.renderSync();
});
index.html
<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Map Export</title>
    <!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer -->
    <script src="https://unpkg.com/elm-pep"></script>
    <style>
      .map {
        width: 100%;
        height:400px;
      }
    </style>
  </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>
    <a id="image-download" download="map.png"></a>
    <script src="main.js"></script>
  </body>
</html>
package.json
{
  "name": "export-map",
  "dependencies": {
    "ol": "6.4.3"
  },
  "devDependencies": {
    "parcel": "1.11.0"
  },
  "scripts": {
    "start": "parcel index.html",
    "build": "parcel build --experimental-scope-hoisting --public-url . index.html"
  }
}