Tile layers and basemap switching

• Provide subdomains whenever the URL template contains {s}, and set maxZoom to the real limit.
• Pass all basemaps to L.control.layers so only one is visible and attribution stays in sync.
• Use tms: true when the service counts rows from south to north.

var osm = L.tileLayer('https://tile.openstreetmap.org/{z}/{x}/{y}.png', {
  maxZoom: 19,
  attribution: '© OpenStreetMap contributors'
});

var cartoLight = L.tileLayer(
  'https://{s}.basemaps.cartocdn.com/light_all/{z}/{x}/{y}{r}.png', {
    subdomains: 'abcd',
    maxZoom: 20,
    attribution: '© OpenStreetMap contributors © CARTO'
  }
);

var map = L.map('map', {
  center: [39.907359, 116.391263],
  zoom: 11,
  layers: [osm]
});

L.control.layers({
  'OpenStreetMap': osm,
  'Carto 浅色': cartoLight
}, null, { position: 'topright' }).addTo(map);

var failed = 0;
tiles.on('tileerror', function (e) {
  failed++;
  if (failed === 3) {
    console.warn('瓦片连续失败，检查网络或服务地址', e.tile.src);
  }
});

map.on('baselayerchange', function (e) {
  failed = 0;
  console.log('当前底图', e.name);
});

• A hand-written switcher that only adds layers leaves the old basemap requesting tiles underneath.
• WGS84 points over GCJ-02 tiles from Gaode or Tencent stay offset no matter the URL.