Tree diagram loading external json with v5

This is a tree diagram that loads the data for the tree from an external JSON file. It was written with d3.js v5 and is based on the simple horizontal version here

This is designed to be used as part of documenting an update to the book D3 Tips and Tricks to version 5 of d3.js.

<!DOCTYPE html>
<meta charset="utf-8">
<style> /* set the CSS */
    
.node circle {
  fill: #fff;
  stroke: steelblue;
  stroke-width: 3px;
}

.node text { font: 12px sans-serif; }

.node--internal text {
  text-shadow: 0 1px 0 #fff, 0 -1px 0 #fff, 1px 0 0 #fff, -1px 0 0 #fff;
}

.link {
  fill: none;
  stroke: #ccc;
  stroke-width: 2px;
}

</style>

<body>

<script src="https://d3js.org/d3.v5.min.js"></script>
<script>

// set the dimensions and margins of the diagram
var margin = {top: 20, right: 90, bottom: 30, left: 90},
  width = 660 - margin.left - margin.right,
  height = 500 - margin.top - margin.bottom;

// declares a tree layout and assigns the size
var treemap = d3.tree()
  .size([height, width]);

// load the external data
d3.json("treeData.json").then(function(treeData) {

//  assigns the data to a hierarchy using parent-child relationships
  var nodes = d3.hierarchy(treeData, function(d) {
    return d.children;
    });

// maps the node data to the tree layout
  nodes = treemap(nodes);

// append the svg object to the body of the page
  // appends a 'group' element to 'svg'
  // moves the 'group' element to the top left margin
  var svg = d3.select("body").append("svg")
      .attr("width", width + margin.left + margin.right)
      .attr("height", height + margin.top + margin.bottom),
    g = svg.append("g")
      .attr("transform",
        "translate(" + margin.left + "," + margin.top + ")");

// adds the links between the nodes
  var link = g.selectAll(".link")
    .data( nodes.descendants().slice(1))
    .enter().append("path")
    .attr("class", "link")
    .attr("d", function(d) {
       return "M" + d.y + "," + d.x
       + "C" + (d.y + d.parent.y) / 2 + "," + d.x
       + " " + (d.y + d.parent.y) / 2 + "," + d.parent.x
       + " " + d.parent.y + "," + d.parent.x;
       });

// adds each node as a group
  var node = g.selectAll(".node")
    .data(nodes.descendants())
    .enter().append("g")
    .attr("class", function(d) { 
      return "node" + 
      (d.children ? " node--internal" : " node--leaf"); })
    .attr("transform", function(d) { 
      return "translate(" + d.y + "," + d.x + ")"; });

// adds the circle to the node
  node.append("circle")
    .attr("r", 10);

// adds the text to the node
  node.append("text")
    .attr("dy", ".35em")
    .attr("x", function(d) { return d.children ? -13 : 13; })
    .style("text-anchor", function(d) { 
    return d.children ? "end" : "start"; })
    .text(function(d) { return d.data.name; });

});
    
</script>
</body>

https://d3js.org/d3.v5.min.js