Vector Clipping Layer

main.js

import GeoJSON from 'ol/format/GeoJSON.js';
import Map from 'ol/Map.js';
import View from 'ol/View.js';
import {Fill, Style} from 'ol/style.js';
import {OSM, StadiaMaps, Vector as VectorSource} from 'ol/source.js';
import {Tile as TileLayer, Vector as VectorLayer} from 'ol/layer.js';
import {fromLonLat} from 'ol/proj.js';
import {getVectorContext} from 'ol/render.js';

//A distinct className is required to use another canvas for the background
const background = new TileLayer({
  className: 'toner',
  source: new StadiaMaps({
    layer: 'stamen_toner',
  }),
});

const base = new TileLayer({
  source: new OSM(),
});

const clipLayer = new VectorLayer({
  style: null,
  source: new VectorSource({
    url: './data/geojson/switzerland.geojson',
    format: new GeoJSON(),
  }),
});

//Giving the clipped layer an extent is necessary to avoid rendering when the feature is outside the viewport
clipLayer.getSource().on('addfeature', function () {
  base.setExtent(clipLayer.getSource().getExtent());
});

const style = new Style({
  fill: new Fill({
    color: 'black',
  }),
});

base.on('postrender', function (e) {
  const vectorContext = getVectorContext(e);
  e.context.globalCompositeOperation = 'destination-in';
  clipLayer.getSource().forEachFeature(function (feature) {
    vectorContext.drawFeature(feature, style);
  });
  e.context.globalCompositeOperation = 'source-over';
});

const map = new Map({
  layers: [background, base, clipLayer],
  target: 'map',
  view: new View({
    center: fromLonLat([8.23, 46.86]),
    zoom: 7,
  }),
});

index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Vector Clipping Layer</title>
    <link rel="stylesheet" href="node_modules/ol/ol.css">
    <style>
      .map {
        width: 100%;
        height: 400px;
      }
      #map {
        background: transparent;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>

    <script type="module" src="main.js"></script>
  </body>
</html>

package.json

{
  "name": "layer-clipping-vector",
  "dependencies": {
    "ol": "9.1.0"
  },
  "devDependencies": {
    "vite": "^3.2.3"
  },
  "scripts": {
    "start": "vite",
    "build": "vite build"
  }
}