A silly way to get values from leaf bottom up. https://codepen.io/jasonhsieh/pen/PdGxGR?editors=0012

sumTreeFromLeafNodes.js

var tree = [
  {
    label: "a",
    children: [
      {
        label: "a-2-1",
        children: [
          {
            label: "a-3-1",
            value: 3
          }
        ]
      },
      {
        label: "a-2-2",
        children: [
          {
            label: "a-3-2",
            value: 2
          },

          {
            label: "a-3-3",
            value: 3
          }
        ]
      }
    ]
  },
  {
    label: "b",
    children: [
      {
        label: "b-2-1",
        children: [
          {
            label: "b-3-1",
            value: 5
          }
        ]
      },
      {
        label: "b-2-2",
        children: [
          {
            label: "b-3-2",
            value: 2
          },

          {
            label: "b-3-3",
            value: 4
          }
        ]
      }
    ]
  }
];

const sum = (node, ...sums) => {
  if (node.children) {
    const tempSums = [];
    for (let n of node.children) {
      n.sum = {
        count: 0
      };
      sum(n, ...sums, n.sum);
    }
  } else {
    for (let s of sums) {
      s.count += node.value;
    }
  }
  return node;
};
var dumpHead = {
  children: tree
};

console.log(JSON.stringify(sum(dumpHead), null, 2)); //pretty console