GroupTileLayer is used to display a group of tile layers. Its performance is better than add TileLayers seperately and it can prevent the warning:

"WARNING: Too many active WebGL contexts. Oldest context will be lost" on chrome

<!DOCTYPE html>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>TileLayer and Geo-Projections - Add GroupTileLayer</title>
  <style type="text/css">
  <link rel="stylesheet" href="[email protected]/dist/maptalks.css">
  <script type="text/javascript" src="[email protected]/dist/maptalks.min.js"></script>
    <div id="map" class="container"></div>

      var map = new maptalks.Map('map', {
        center:     [-0.113049,51.498568],
        zoom:  6,
        pitch : 40,
        attribution: {
          content: '&copy; <a href="">OpenStreetMap</a> contributors, &copy; <a href="">CARTO</a>, &copy; BoudlessGeo'
        // add 2 TileLayers with a GroupTileLayer
        baseLayer : new maptalks.GroupTileLayer('base', [
          new maptalks.TileLayer('tile2', {
            urlTemplate: 'http://{s}{z}/{x}/{y}.png',
            subdomains  : ['a','b','c','d']

          new maptalks.WMSTileLayer('wms', {
            'urlTemplate' : '',
            'crs' : 'EPSG:3857',
            'layers' : 'ne:ne',
            'styles' : '',
            'version' : '1.3.0',
            'format': 'image/png',
            'transparent' : true,
            'uppercase' : true