Collapsible tree diagram in v5

This is a d3.js tree diagram that incldes an interactive element as used as an example in the book D3 Tips and Tricks v5.x.

Any parent node can be clicked on to collapse the portion of the tree below it, on itself. Conversly, it can be clicked on again to regrow.

It is derived from the Mike Bostock Collapsible tree example and updated to use v5.

Kudos and thanks also go out to Soumya Ranjan for steering me in the fight direction for the diagonal solution.

<!DOCTYPE html>
<meta charset="UTF-8">
<style>

.node circle {
  fill: #fff;
  stroke: steelblue;
  stroke-width: 3px;
}

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

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

</style>

<body>

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

var treeData =
  {
    "name": "Top Level",
    "children": [
      { 
        "name": "Level 2: A",
        "children": [
          { "name": "Son of A" },
          { "name": "Daughter of A" }
        ]
      },
      { "name": "Level 2: B" }
    ]
  };

// Set the dimensions and margins of the diagram
var margin = {top: 20, right: 90, bottom: 30, left: 90},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

// 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.right + margin.left)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform", "translate("
          + margin.left + "," + margin.top + ")");

var i = 0,
    duration = 750,
    root;

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

// Assigns parent, children, height, depth
root = d3.hierarchy(treeData, function(d) { return d.children; });
root.x0 = height / 2;
root.y0 = 0;

// Collapse after the second level
root.children.forEach(collapse);

update(root);

// Collapse the node and all it's children
function collapse(d) {
  if(d.children) {
    d._children = d.children
    d._children.forEach(collapse)
    d.children = null
  }
}

function update(source) {

// Assigns the x and y position for the nodes
  var treeData = treemap(root);

// Compute the new tree layout.
  var nodes = treeData.descendants(),
      links = treeData.descendants().slice(1);

// Normalize for fixed-depth.
  nodes.forEach(function(d){ d.y = d.depth * 180});

// ****************** Nodes section ***************************

// Update the nodes...
  var node = svg.selectAll('g.node')
      .data(nodes, function(d) {return d.id || (d.id = ++i); });

// Enter any new modes at the parent's previous position.
  var nodeEnter = node.enter().append('g')
      .attr('class', 'node')
      .attr("transform", function(d) {
        return "translate(" + source.y0 + "," + source.x0 + ")";
    })
    .on('click', click);

// Add Circle for the nodes
  nodeEnter.append('circle')
      .attr('class', 'node')
      .attr('r', 1e-6)
      .style("fill", function(d) {
          return d._children ? "lightsteelblue" : "#fff";
      });

// Add labels for the nodes
  nodeEnter.append('text')
      .attr("dy", ".35em")
      .attr("x", function(d) {
          return d.children || d._children ? -13 : 13;
      })
      .attr("text-anchor", function(d) {
          return d.children || d._children ? "end" : "start";
      })
      .text(function(d) { return d.data.name; });

// UPDATE
  var nodeUpdate = nodeEnter.merge(node);

// Transition to the proper position for the node
  nodeUpdate.transition()
    .duration(duration)
    .attr("transform", function(d) { 
        return "translate(" + d.y + "," + d.x + ")";
     });

// Update the node attributes and style
  nodeUpdate.select('circle.node')
    .attr('r', 10)
    .style("fill", function(d) {
        return d._children ? "lightsteelblue" : "#fff";
    })
    .attr('cursor', 'pointer');

// Remove any exiting nodes
  var nodeExit = node.exit().transition()
      .duration(duration)
      .attr("transform", function(d) {
          return "translate(" + source.y + "," + source.x + ")";
      })
      .remove();

// On exit reduce the node circles size to 0
  nodeExit.select('circle')
    .attr('r', 1e-6);

// On exit reduce the opacity of text labels
  nodeExit.select('text')
    .style('fill-opacity', 1e-6);

// ****************** links section ***************************

// Update the links...
  var link = svg.selectAll('path.link')
      .data(links, function(d) { return d.id; });

// Enter any new links at the parent's previous position.
  var linkEnter = link.enter().insert('path', "g")
      .attr("class", "link")
      .attr('d', function(d){
        var o = {x: source.x0, y: source.y0}
        return diagonal(o, o)
      });

// UPDATE
  var linkUpdate = linkEnter.merge(link);

// Transition back to the parent element position
  linkUpdate.transition()
      .duration(duration)
      .attr('d', function(d){ return diagonal(d, d.parent) });

// Remove any exiting links
  var linkExit = link.exit().transition()
      .duration(duration)
      .attr('d', function(d) {
        var o = {x: source.x, y: source.y}
        return diagonal(o, o)
      })
      .remove();

// Store the old positions for transition.
  nodes.forEach(function(d){
    d.x0 = d.x;
    d.y0 = d.y;
  });

// Creates a curved (diagonal) path from parent to the child nodes
  function diagonal(s, d) {

path = `M ${s.y} ${s.x}
            C ${(s.y + d.y) / 2} ${s.x},
              ${(s.y + d.y) / 2} ${d.x},
              ${d.y} ${d.x}`

return path
  }

// Toggle children on click.
  function click(d) {
    if (d.children) {
        d._children = d.children;
        d.children = null;
      } else {
        d.children = d._children;
        d._children = null;
      }
    update(d);
  }
}

</script>
</body>

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