tree-sum.js

const tree = {
  value: 1,
  children: [
    {
      value: 10,
      children: [
        {
          value: 2,
        },
      ],
    },
    {
      value: 5,
    },
  ],
};

function treeSum(root) {
  const reducer = (acc, item) =>
    item.children?.length > 0 ? acc + treeSum(item) : acc + item.value;

  return root.children.reduce(reducer, root.value);
}

console.log(treeSum(tree));