Edit

Example of using WFS with a BBOX strategy.

This example loads new features from GeoServer WFS when the view extent changes.

main.js
import 'ol/ol.css';
import GeoJSON from 'ol/format/GeoJSON';
import Map from 'ol/Map';
import VectorSource from 'ol/source/Vector';
import View from 'ol/View';
import XYZ from 'ol/source/XYZ';
import {Stroke, Style} from 'ol/style';
import {Tile as TileLayer, Vector as VectorLayer} from 'ol/layer';
import {bbox as bboxStrategy} from 'ol/loadingstrategy';

var vectorSource = new VectorSource({
  format: new GeoJSON(),
  url: function (extent) {
    return (
      'https://ahocevar.com/geoserver/wfs?service=WFS&' +
      'version=1.1.0&request=GetFeature&typename=osm:water_areas&' +
      'outputFormat=application/json&srsname=EPSG:3857&' +
      'bbox=' +
      extent.join(',') +
      ',EPSG:3857'
    );
  },
  strategy: bboxStrategy,
});

var vector = new VectorLayer({
  source: vectorSource,
  style: new Style({
    stroke: new Stroke({
      color: 'rgba(0, 0, 255, 1.0)',
      width: 2,
    }),
  }),
});

var key = 'Get your own API key at https://www.maptiler.com/cloud/';
var attributions =
  '<a href="https://www.maptiler.com/copyright/" target="_blank">&copy; MapTiler</a> ' +
  '<a href="https://www.openstreetmap.org/copyright" target="_blank">&copy; OpenStreetMap contributors</a>';

var raster = new TileLayer({
  source: new XYZ({
    attributions: attributions,
    url: 'https://api.maptiler.com/tiles/satellite/{z}/{x}/{y}.jpg?key=' + key,
    maxZoom: 20,
  }),
});

var map = new Map({
  layers: [raster, vector],
  target: document.getElementById('map'),
  view: new View({
    center: [-8908887.277395891, 5381918.072437216],
    maxZoom: 19,
    zoom: 12,
  }),
});
index.html
<!DOCTYPE html>
<html lang="en">
  <head>
    <title>WFS</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>
    <script src="main.js"></script>
  </body>
</html>
package.json
{
  "name": "vector-wfs",
  "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"
  }
}