node.sum()

node.sum(value)

Evaluates the specified value function for this node and each descendant in post-order traversal, and returns this node. The node.value property of each node is set to the numeric value returned by the specified function plus the combined value of all descendants. The function is passed the node’s data, and must return a non-negative number. The value accessor is evaluated for node and every descendant, including internal nodes; if you only want leaf nodes to have internal value, then return zero for any node with children. For example:

root.sum(function(d) { return d.value ? 1 : 0; });

You must call node.sum before invoking a hierarchical layout that requires node.value, such as d3.treemap. Since the API supports method chaining, you can invoke node.sum and node.sort before computing the layout, and then subsequently generate an array of all descendant nodes like so:

var treemap = d3.treemap()
    .size([width, height])
    .padding(2);

var nodes = treemap(root
    .sum(function(d) { return d.value; })
    .sort(function(a, b) { return b.height - a.height || b.value - a.value; }))
  .descendants();

This example assumes that the node data has a value field.