Working with map libraries

• Convert turf.bbox output [west, south, east, north] to Leaflet's [[south, west], [north, east]] before fitBounds.
• Create the source once inside load and update it with map.getSource('zone').setData(zone).
• Precompute area and density into properties before styling features by value.

var site = turf.point([116.391263, 39.907359]);
var zone = turf.buffer(site, 1.2, { units: 'kilometers' });

var map = L.map('map').setView(
  [site.geometry.coordinates[1], site.geometry.coordinates[0]], 14
);

L.tileLayer('https://tile.openstreetmap.org/{z}/{x}/{y}.png', {
  maxZoom: 19
}).addTo(map);

var layer = L.geoJSON(zone, {
  style: {
    color: '#17694a',
    weight: 2,
    fillOpacity: 0.2
  },
  onEachFeature: function (feature, lyr) {
    var area = turf.area(feature) / 1e6;          // 平方公里
    lyr.bindPopup('影响范围 ' + area.toFixed(2) + ' 平方公里');
  }
}).addTo(map);

var box = turf.bbox(zone);                       // [西, 南, 东, 北]
map.fitBounds([[box[1], box[0]], [box[3], box[2]]], { padding: [20, 20] });

map.on('load', function () {
  map.addSource('zone', { type: 'geojson', data: emptyFC() });
  map.addSource('origin', {
    type: 'geojson',
    data: turf.featureCollection([site])
  });

  map.addLayer({
    id: 'zone-fill',
    type: 'fill',
    source: 'zone',
    paint: { 'fill-color': '#17694a', 'fill-opacity': 0.2 }
  });

  map.addLayer({
    id: 'zone-line',
    type: 'line',
    source: 'zone',
    paint: { 'line-color': '#17694a', 'line-width': 2 }
  });

  map.addLayer({
    id: 'origin-dot',
    type: 'circle',
    source: 'origin',
    paint: { 'circle-radius': 5, 'circle-color': '#c2410c' }
  });

  redraw(1.2);
});

function redraw(radiusKm) {
  var zone = turf.buffer(site, radiusKm, { units: 'kilometers' });
  map.getSource('zone').setData(zone);

  var b = turf.bbox(zone);
  map.fitBounds([[b[0], b[1]], [b[2], b[3]]], {
    padding: 40,
    duration: 600
  });
}

function emptyFC() {
  return { type: 'FeatureCollection', features: [] };
}

• Repeating map.addSource for an existing id throws Source "zone" already exists.
• Buffering lines or nearby points yields MultiPolygon, which some rendering paths handle poorly without dissolve.