[BinaryTree] Node depths

depths.js

const assert = require("assert");

// O(n) time | O(h) space - h is height.
function nodeDepths(root, depth = 0) {
  if (root === null) {
    return 0;
  }
  return depth + nodeDepths(root.left, depth + 1) + nodeDepths(root.right, depth + 1);
}

// This is the class of the input binary tree.
class BinaryTree {
  constructor(value) {
    this.value = value;
    this.left = null;
    this.right = null;
  }
}

const root = new BinaryTree(1);
root.left = new BinaryTree(2);
root.left.left = new BinaryTree(4);
root.left.left.left = new BinaryTree(8);
root.left.left.right = new BinaryTree(9);
root.left.right = new BinaryTree(5);
root.right = new BinaryTree(3);
root.right.left = new BinaryTree(6);
root.right.right = new BinaryTree(7);
const actual = nodeDepths(root);

assert.deepStrictEqual(actual, 16);