Q:

Add Tooltips to Interactive D3 Diagram in D3

I am new to D3 and trying to combine two examples from bl.ocks.org to add tooltips to the nodes of an interactive tree diagram.  You can see the original code here: http://bl.ocks.org/d3noob/8375092 & http://bl.ocks.org/d3noob/a22c42db65eb00d4e369 .
I believe my issue stems from not knowing how to attach the .on("mouseover") portion to the nodes div.  The current code does not render any output.  Any help is greatly appreciated.
<!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;
}

div.tooltip {   
    position: absolute;         
    text-align: center;         
    width: 60px;                    
    height: 28px;                   
    padding: 2px;               
    font: 12px sans-serif;      
    background: lightsteelblue; 
    border: 0px;        
    border-radius: 8px;         
    pointer-events: none;           
}

</style>

<body>

<!-- load the d3.js library --> 
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>

var treeData =
  {
    "name": "Total: 300",
    "children": [
      { 
        "name": "ABC: 150",
        "children": [
          { "name": "BCD: 150" },
          { "name": "CDE: 50" }
        ]
      },
      { "name": "DEF: 25" }
    ]
  };

// 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 + ")");

// Define the div for the tooltip
var div = d3.select("body").append("div")   
    .attr("class", "tooltip")               
    .style("opacity", 0);

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); });
      .on("mouseover", function(d) {        
        div.transition()        
            .duration(200)      
            .style("opacity", .9);      
        div .html("1" + "<br/>"  + "2") 
            .style("left", (d3.event.pageX) + "px")     
            .style("top", (d3.event.pageY - 28) + "px");    
        })                  
    .on("mouseout", function(d) {       
        div.transition()        
            .duration(500)      
            .style("opacity", 0);

  // 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>

A:

Two issues! one is misplaced ; and one is missing }); Following should work
<!DOCTYPE html>
<html>
<head>
    <style>

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

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

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

        div.tooltip {
            position: absolute;
            text-align: center;
            width: 60px;
            height: 28px;
            padding: 2px;
            font: 12px sans-serif;
            background: lightsteelblue;
            border: 0px;
            border-radius: 8px;
            pointer-events: none;
        }
    </style>
</head>
<body>

    <script src="https://d3js.org/d3.v4.min.js"></script>
    <script>
        var treeData =
        {
            "name": "Total: 300",
            "children": [
                {
                    "name": "ABC: 150",
                    "children": [
                        { "name": "BCD: 150" },
                        { "name": "CDE: 50" }
                    ]
                },
                { "name": "DEF: 25" }
            ]
        };

        // 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 + ")");

        // Define the div for the tooltip
        var div = d3.select("body").append("div")
            .attr("class", "tooltip")
            .style("opacity", 0);

        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); })
                .on("mouseover", function (d) {
                    div.transition()
                        .duration(200)
                        .style("opacity", .9);
                    div.html("1" + "<br/>" + "2")
                        .style("left", (d3.event.pageX) + "px")
                        .style("top", (d3.event.pageY - 28) + "px");
                })
                .on("mouseout", function (d) {
                    div.transition()
                        .duration(500)
                        .style("opacity", 0);
                });

            // 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>
</html>

