D3js: force directed, collapsing internal nodes

Question

In that example of d3js http://bl.ocks.org/mbostock/1062288 , which part of code handle the collapsing internal nodes where they are clicked. The event handler click sets the data for the colour but I don't see which part is handling the collapsing effect. Of course link.exit().remove() and node.exit().remove() remove the points and links. But how the "exit" section is created?

Answer 1

This part does the job added comments to the scriplet:

function click(d) {
  if (!d3.event.defaultPrevented) {
    if (d.children) { 
      //if the node is expanded the d.children will have nodes in it.
      //we are moving the nodes into another variable d._children
      d._children = d.children;
      //making d.children as null so that the exit function will remove the nodes in update.
      d.children = null;
    } else {
      //the node is collapsed state so moving the d._children back into variable d.children
      d.children = d._children;
      //making the d._children null
      d._children = null;
    }
    //after resetting/setting the d.children call update for appending nodes in case of expand(d.children have values) or remove nodes and links when (d.children have no values) 
    update();
  }
}

Hope it helps!