D3.js tree diagram generated from 'flat' data

This is a simple d3.js tree diagram that is generated from 'flat' data as used as an example in the book D3 Tips and Tricks.

It is derived from the Mike Bostock Collapsible tree example but it is a VERY cut down version without the ability to update (collapse).

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8">

<title>Collapsible Tree Example</title>

<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>

</head>

<body>

<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.17/d3.min.js"></script>
	
<script>

var data = [
    { "name" : "Level 2: A", "parent":"Top Level" },
    { "name" : "Top Level", "parent":"null" },
    { "name" : "Son of A", "parent":"Level 2: A" },
    { "name" : "Daughter of A", "parent":"Level 2: A" },
    { "name" : "Level 2: B", "parent":"Top Level" }
    ];

// *********** Convert flat data into a nice tree ***************
// create a name: node map
var dataMap = data.reduce(function(map, node) {
	map[node.name] = node;
	return map;
}, {});

// create the tree array
var treeData = [];
data.forEach(function(node) {
	// add to parent
	var parent = dataMap[node.parent];
	if (parent) {
		// create child array if it doesn't exist
		(parent.children || (parent.children = []))
			// add node to child array
			.push(node);
	} else {
		// parent is null or missing
		treeData.push(node);
	}
});

// ************** Generate the tree diagram	 *****************
var margin = {top: 20, right: 120, bottom: 20, left: 120},
	width = 960 - margin.right - margin.left,
	height = 500 - margin.top - margin.bottom;
	
var i = 0;

var tree = d3.layout.tree()
	.size([height, width]);

var diagonal = d3.svg.diagonal()
	.projection(function(d) { return [d.y, d.x]; });

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 + ")");

root = treeData[0];
  
update(root);

function update(source) {

// Compute the new tree layout.
  var nodes = tree.nodes(root).reverse(),
	  links = tree.links(nodes);

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

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

// Enter the nodes.
  var nodeEnter = node.enter().append("g")
	  .attr("class", "node")
	  .attr("transform", function(d) { 
		  return "translate(" + d.y + "," + d.x + ")"; });

nodeEnter.append("circle")
	  .attr("r", 10)
	  .style("fill", "#fff");

nodeEnter.append("text")
	  .attr("x", function(d) { 
		  return d.children || d._children ? -13 : 13; })
	  .attr("dy", ".35em")
	  .attr("text-anchor", function(d) { 
		  return d.children || d._children ? "end" : "start"; })
	  .text(function(d) { return d.name; })
	  .style("fill-opacity", 1);

// Declare the links…
  var link = svg.selectAll("path.link")
	  .data(links, function(d) { return d.target.id; });

// Enter the links.
  link.enter().insert("path", "g")
	  .attr("class", "link")
	  .attr("d", diagonal);

}

</script>
	
  </body>
</html>

https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.17/d3.min.js