title: D3 error - Uncaught TypeError: Cannot read property 'children' of undefined
body: <p>I know similar questions have been answered before, and I tried those before posting here. I am trying to build a Dendrogram but getting this error. I suppose it's unable to find the "children" but I don't understand why.</p>
  
  <p>Below is my json. It's valid.</p>
  
  <pre><code>{
  "name": "flare",
  "children": [{
      "name": "Animals &amp; Pet Supplies",
      "children": [{
          "name": "Live Animals"
      }, {
          "name": "Pet Supplies",
          "children": [{
              "name": "Bird Supplies",
              "children": [{
                  "name": "Bird Cages &amp; Stands"
              }, {
                  "name": "Bird Food"
              }, {
                  "name": "Bird Ladders &amp; Perches"
              }, {
                  "name": "Bird Toys"
              }, {
                  "name": "Bird Treats"
              }]
          }, {
              "name": "Cat Supplies"
          }]
      }]
  }]
  </code></pre>
  
  <p>}</p>
  
  <p>And below is the html.</p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html&gt;
     &lt;head&gt;
        &lt;meta http-equiv="Content-Type" content="text/html;charset=utf-8"/&gt;
        &lt;link type="text/css" rel="stylesheet" href="style.css"/&gt;
        &lt;!--&lt;script type="text/javascript" src="d3/d3.js"&gt;&lt;/script&gt;
           &lt;script type="text/javascript" src="d3/d3.layout.js"&gt;&lt;/script&gt;--&gt;
        &lt;script src="http://d3js.org/d3.v3.min.js"&gt;&lt;/script&gt;
        &lt;style type="text/css"&gt;
           .node circle {
           cursor: pointer;
           fill: #fff;
           stroke: steelblue;
           stroke-width: 1.5px;
           }
           .node text {
           font-size: 11px;
           }
           path.link {
           fill: none;
           stroke: #ccc;
           stroke-width: 1.5px;
           }
        &lt;/style&gt;
     &lt;/head&gt;
     &lt;body&gt;
        &lt;div id="body"&gt;
           &lt;div id="footer"&gt;
              d3.layout.tree
              &lt;div class="hint"&gt;click or option-click to expand or collapse&lt;/div&gt;
           &lt;/div&gt;
        &lt;/div&gt;
        &lt;script type="text/javascript"&gt;
           var m = [20, 120, 20, 120],
               w = 1280 - m[1] - m[3],
               h = 800 - m[0] - m[2],
               i = 0,
               root;
  
           var tree = d3.layout.tree()
               .size([h, w]);
  
           var diagonal = d3.svg.diagonal()
               .projection(function(d) { return [d.y, d.x]; });
  
           var vis = d3.select("#body").append("svg:svg")
               .attr("width", w + m[1] + m[3])
               .attr("height", h + m[0] + m[2])
             .append("svg:g")
               .attr("transform", "translate(" + m[3] + "," + m[0] + ")");
  
           d3.json("flare_orig_reduced.json", function(json) {
             root = json;
             root.x0 = h / 2;
             root.y0 = 0;
  
             function toggleAll(d) {
               if (d.children) {
                 d.children.forEach(toggleAll);
                 toggle(d);
               }
             }
  
             // Initialize the display to show a few nodes.
             root.children.forEach(toggleAll);
             toggle(root.children[1]);
             toggle(root.children[1].children[2]);
             toggle(root.children[9]);
             toggle(root.children[9].children[0]);
  
             update(root);
           });
  
           function update(source) {
             var duration = d3.event &amp;&amp; d3.event.altKey ? 5000 : 500;
  
             // Compute the new tree layout.
             var nodes = tree.nodes(root).reverse();
  
             // Normalize for fixed-depth.
             nodes.forEach(function(d) { d.y = d.depth * 180; });
  
             // Update the nodes…
             var node = vis.selectAll("g.node")
                 .data(nodes, function(d) { return d.id || (d.id = ++i); });
  
             // Enter any new nodes at the parent's previous position.
             var nodeEnter = node.enter().append("svg:g")
                 .attr("class", "node")
                 .attr("transform", function(d) { return "translate(" + source.y0 + "," + source.x0 + ")"; })
                 .on("click", function(d) { toggle(d); update(d); });
  
             nodeEnter.append("svg:circle")
                 .attr("r", 1e-6)
                 .style("fill", function(d) { return d._children ? "lightsteelblue" : "#fff"; });
  
             nodeEnter.append("svg:text")
                 .attr("x", function(d) { return d.children || d._children ? -10 : 10; })
                 .attr("dy", ".35em")
                 .attr("text-anchor", function(d) { return d.children || d._children ? "end" : "start"; })
                 .text(function(d) { return d.name; })
                 .style("fill-opacity", 1e-6);
  
             // Transition nodes to their new position.
             var nodeUpdate = node.transition()
                 .duration(duration)
                 .attr("transform", function(d) { return "translate(" + d.y + "," + d.x + ")"; });
  
             nodeUpdate.select("circle")
                 .attr("r", 4.5)
                 .style("fill", function(d) { return d._children ? "lightsteelblue" : "#fff"; });
  
             nodeUpdate.select("text")
                 .style("fill-opacity", 1);
  
             // Transition exiting nodes to the parent's new position.
             var nodeExit = node.exit().transition()
                 .duration(duration)
                 .attr("transform", function(d) { return "translate(" + source.y + "," + source.x + ")"; })
                 .remove();
  
             nodeExit.select("circle")
                 .attr("r", 1e-6);
  
             nodeExit.select("text")
                 .style("fill-opacity", 1e-6);
  
             // Update the links…
             var link = vis.selectAll("path.link")
                 .data(tree.links(nodes), function(d) { return d.target.id; });
  
             // Enter any new links at the parent's previous position.
             link.enter().insert("svg:path", "g")
                 .attr("class", "link")
                 .attr("d", function(d) {
                   var o = {x: source.x0, y: source.y0};
                   return diagonal({source: o, target: o});
                 })
               .transition()
                 .duration(duration)
                 .attr("d", diagonal);
  
             // Transition links to their new position.
             link.transition()
                 .duration(duration)
                 .attr("d", diagonal);
  
             // Transition exiting nodes to the parent's new position.
             link.exit().transition()
                 .duration(duration)
                 .attr("d", function(d) {
                   var o = {x: source.x, y: source.y};
                   return diagonal({source: o, target: o});
                 })
                 .remove();
  
             // Stash the old positions for transition.
             nodes.forEach(function(d) {
               d.x0 = d.x;
               d.y0 = d.y;
             });
           }
  
           // Toggle children.
           function toggle (d) {
             if (d.children) {
               d._children = d.children;
               d.children = null;
             } else {
               d.children = d._children;
               d._children = null;
             }
           }
  
  
        &lt;/script&gt;
     &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>Any help greatly appreciated.</p>