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 = 600 - margin.left - margin.right,
    height = 300 - 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(!isTransitioning(d3.selectAll("circle"))) {
  if (d.children) {
   d._children = d.children;
   d.children = null;
    } else {
   d.children = d._children;
   d._children = null;
    }
  update(d);
 }
  }
}
function isTransitioning(selection) {
 var transitioning = false;
 selection.each(function() { if(d3.active(this)) { transitioning = true; } })
 return transitioning;
}
.node circle {
  fill: #fff;
  stroke: steelblue;
  stroke-width: 3px;
}
.node text {
  font: 12px sans-serif;
}
.link {
  fill: none;
  stroke: #ccc;
  stroke-width: 2px;
}
<script src="https://d3js.org/d3.v4.min.js"></script>