This example uses OpenLayers v10.9.0. The latest is v.
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#### Layer Opacity

opacity 1

#####

Layer opacity

Adjust layer opacity based on user input

The `layer.setOpacity()` method can be called to adjust the opacity of a layer.

##### main.js

    import Map from 'ol/Map.js';
    import View from 'ol/View.js';
    import TileLayer from 'ol/layer/WebGLTile.js';
    import ImageTile from 'ol/source/ImageTile.js';
    import OSM from 'ol/source/OSM.js';

    const key = 'Get your own API key at https://www.maptiler.com/cloud/';

    const imagery = new TileLayer({
      className: 'ol-layer-imagery',
      source: new ImageTile({
        attributions:
          '<a href="https://www.maptiler.com/copyright/" target="_blank">&copy; MapTiler</a> ',
        url: 'https://api.maptiler.com/maps/satellite/{z}/{x}/{y}.jpg?key=' + key,
        tileSize: 512,
        maxZoom: 20,
      }),
    });

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

    const map = new Map({
      layers: [imagery, osm],
      target: 'map',
      view: new View({
        center: [0, 0],
        zoom: 2,
      }),
    });

    const opacityInput = document.getElementById('opacity-input');
    const opacityOutput = document.getElementById('opacity-output');
    function update() {
      const opacity = parseFloat(opacityInput.value);
      osm.setOpacity(opacity);
      opacityOutput.innerText = opacity.toFixed(2);
    }
    opacityInput.addEventListener('input', update);
    update();

##### index.html

    <!DOCTYPE html>
    <html lang="en">
      <head>
        <meta charset="UTF-8">
        <title>Layer Opacity</title>
        <link rel="stylesheet" href="node_modules/ol/ol.css">
        <style>
          .map {
            width: 100%;
            height: 400px;
          }
        </style>
      </head>
      <body>
        <div id="map" class="map"></div>
        <label>
          Layer opacity
          <input id="opacity-input" type="range" min="0" max="1" step="0.01" value="1" />
          <span id="opacity-output"></span>
        </label>

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

##### package.json

    {
      "name": "layer-opacity",
      "dependencies": {
        "ol": "10.9.0"
      },
      "devDependencies": {
        "vite": "^3.2.3"
      },
      "scripts": {
        "start": "vite",
        "build": "vite build"
      }
    }