leaflet - Map with D3.js outline

[Gist] https://gist.github.com/Hirosaji/5640cb9123673372da2b8ed826cb5c37

<!DOCTYPE html>
<head>
	<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/leaflet/0.7.7/leaflet.css"/>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/leaflet/0.7.7/leaflet-src.js"></script>
  <script src="https://d3js.org/d3.v3.min.js"></script>
  <style>
    #mapid { height: 500px; }
  </style>
</head>
<body>
  <div id="mapid"></div>
	<script>
    d3.json("ken.geojson", function(json) {
      mapdraw(json);
    });

function mapdraw(json){

//地形データ取得
      if (json.type === "Topology"){
        //読み込みファイルがtopojsonの場合(json.objects.△△)　「△△」は変換時のgeojsonファイル名に合わせる
        var geojson = topojson.feature(json, json.objects.ken)
      } else {
        var geojson = json;
      }

//Leaflet初期設定
      var map = L.map('mapid').setView([38.2, 137.2], 5);
      var mapLink = '<span class="removed_link" title="https://portal.cyberjapan.jp/help/termsofuse.html">国土地理院 地理院地図 標準地図</span>';

//Tile Map Serviceの指定
      L.tileLayer(
          'https://cyberjapandata.gsi.go.jp/xyz/std/{z}/{x}/{y}.png', {
          attribution: '© ' + mapLink + ' Contributors',
          maxZoom: 18,
        }).addTo(map);

//Leafletに用意されたsvgを使う
      map._initPathRoot();

// svg要素を選択
      var svg = d3.select("#mapid").select("svg");
      var g = svg.append("g");

//緯度経度->パスジェネレーター関数作成
      var transform = d3.geo.transform({point: projectPoint});
      var path = d3.geo.path().projection(transform);

featureElement = g.selectAll("path")
        .data(geojson.features)
        .enter()
        .append("path")
        .attr({
          "stroke": "red",
          "fill": "green",
          "fill-opacity": 0.4
        });

map.on("viewreset", update);

update();

//pathのd属性を更新
      function update() {
        featureElement.attr("d", path);
      }

//位置→座標変換
      function projectPoint(x, y) {
        var point = map.latLngToLayerPoint(new L.LatLng(y, x));
        this.stream.point(point.x, point.y);
      }

}
  </script>
</body>

https://cdnjs.cloudflare.com/ajax/libs/leaflet/0.7.7/leaflet-src.js

https://d3js.org/d3.v3.min.js