Tree diagram from csv using v7 by d3noob

index.html

<!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>

<!-- load the d3.js library -->      
<script src="https://d3js.org/d3.v7.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.csv("treeCsv.csv").then(function(flatData) {

  // assign null correctly
  flatData.forEach(function(d) {
      if (d.parent == "null") { d.parent = null};
    });

  // convert the flat data into a hierarchy 
  var treeData = d3.stratify()
    .id(function(d) { return d.name; })
    .parentId(function(d) { return d.parent; })
    (flatData);

  // assign the name to each node
  treeData.each(function(d) {
      d.name = d.id;
    });

  //  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>

treeCsv.csv

name,parent
Top Level,null
Level 2: A,Top Level
Level 2: B,Top Level
Son of A,Level 2: A
Daughter of A,Level 2: A