title: d3.js only the root and its childs should be opened
body: <p>I have this script</p>
  
  <p><a href="http://bl.ocks.org/mbostock/1093130" rel="nofollow">http://bl.ocks.org/mbostock/1093130</a></p>
  
  <p>and i need all sub nodes to be collapsed, but on load, only the root and its childs should be opened, not all other sub nodes.</p>
  
  <p>this is my code:</p>
  
  <pre><code>&lt;script src="http://d3js.org/d3.v3.min.js"&gt;&lt;/script&gt;
  &lt;script&gt;
  
  var width = 960,
      height = 500,
      root;
  
  var force = d3.layout.force()
      .linkDistance(80)
      .charge(-120)
      .gravity(.05)
      .size([width, height])
      .on("tick", tick);
  
  var svg = d3.select("body").append("svg")
      .attr("width", width)
      .attr("height", height);
  
  var link = svg.selectAll(".link"),
      node = svg.selectAll(".node");
  
  d3.json("graph.json", function(error, json) {
    root = json;
    update();
  });
  
  function update() {
    var nodes = flatten(root),
        links = d3.layout.tree().links(nodes);
  
    // Restart the force layout.
    force
        .nodes(nodes)
        .links(links)
        .start();
  
    // Update links.
    link = link.data(links, function(d) { return d.target.id; });
  
    link.exit().remove();
  
    link.enter().insert("line", ".node")
        .attr("class", "link");
  
    // Update nodes.
    node = node.data(nodes, function(d) { return d.id; });
  
    node.exit().remove();
  
    var nodeEnter = node.enter().append("g")
        .attr("class", "node")
        .on("click", click)
        .call(force.drag);
  
    nodeEnter.append("circle")
        .attr("r", function(d) { return Math.sqrt(d.size) / 10 || 4.5; });
  
    nodeEnter.append("text")
        .attr("dy", ".35em")
        .text(function(d) { return d.name; });
  
    node.select("circle")
        .style("fill", color);
  }
  
  function tick() {
    link.attr("x1", function(d) { return d.source.x; })
        .attr("y1", function(d) { return d.source.y; })
        .attr("x2", function(d) { return d.target.x; })
        .attr("y2", function(d) { return d.target.y; });
  
    node.attr("transform", function(d) { return "translate(" + d.x + "," + d.y + ")"; });
  }
  
  function color(d) {
    return d._children ? "#3182bd" // collapsed package
        : d.children ? "#c6dbef" // expanded package
        : "#fd8d3c"; // leaf node
  }
  
  // Toggle children on click.
  function click(d) {
    if (d3.event.defaultPrevented) return; // ignore drag
    if (d.children) {
      d._children = d.children;
      d.children = null;
    } else {
      d.children = d._children;
      d._children = null;
    }
    update();
  }
  
  // Returns a list of all nodes under the root.
  function flatten(root) {
    var nodes = [], i = 0;
  
    function recurse(node) {
      if (node.children) node.children.forEach(recurse);
      if (!node.id) node.id = ++i;
      nodes.push(node);
    }
  
    recurse(root);
    return nodes;
  }
  
  &lt;/script&gt;
  </code></pre>
  
  <p>how i can change the code.</p>