title: How to expand D3js tree chart onclick a button
body: <p>I have D3js Tree chart,every thing is working fine but as per my requirement I need to expand whole chart onclick 'Expand' button and collapse again onclick 'collapse' button.It should be same like onclick of 'Top Level' is happening in the chart.Here is code below.Thanks in advance for help.</p>
  
  <h2>HTML</h2>
  
  <pre><code>&lt;style&gt;
      .node {
          cursor: pointer;
      }
  
      .node circle {
        fill: #fff;
        stroke: steelblue;
        stroke-width: 3px;
      }
  
      .node text {
        font: 12px sans-serif;
      }
  
      .link {
        fill: none;
        stroke: #ccc;
        stroke-width: 2px;
      }
  
      &lt;/style&gt;
  &lt;script src="http://d3js.org/d3.v3.min.js"&gt;&lt;/script&gt;
  &lt;div id="test"&gt;&lt;/div&gt;
  &lt;div id="expand"&gt;Expand&lt;/div&gt;&lt;br&gt;&lt;div id="collapse"&gt;Collapse&lt;/div&gt;
  </code></pre>
  
  <h2>Javascript</h2>
  
  <pre><code>var treeData = [
    {
      "name": "Top Level",
      "parent": "null",
      "children": [
        {
          "name": "Level 2: A",
          "parent": "Top Level",
          "children": [
            {
              "name": "Son of A",
              "parent": "Level 2: A"
            },
            {
              "name": "Daughter of A",
              "parent": "Level 2: A"
            }
          ]
        },
        {
          "name": "Level 2: B",
          "parent": "Top Level"
        }
      ]
    }
  ];
  
  
  // ************** 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,
      duration = 750,
      root;
  
  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("#test").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];
  root.x0 = height / 2;
  root.y0 = 0;
  
  update(root);
  
  d3.select(self.frameElement).style("height", "500px");
  
  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; });
  
    // Update the nodes…
    var node = svg.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("g")
        .attr("class", "node")
        .attr("transform", function(d) { return "translate(" + source.y0 + "," + source.x0 + ")"; })
        .on("click", click);
  
    nodeEnter.append("circle")
        .attr("r", 1e-6)
        .style("fill", function(d) { return d._children ? "lightsteelblue" : "#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", 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", 10)
        .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 = svg.selectAll("path.link")
        .data(links, function(d) { return d.target.id; });
  
    // Enter any new links at the parent's previous position.
    link.enter().insert("path", "g")
        .attr("class", "link")
        .attr("d", function(d) {
          var o = {x: source.x0, y: source.y0};
          return diagonal({source: o, target: o});
        });
  
    // 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 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);
  }
  </code></pre>