invertTree.js

const nullNode = null;

const node1 = {
  left: nullNode,
  right: nullNode,
  value: 1
};

const node3 = {
  left: nullNode,
  right: nullNode,
  value: 3
};

const node6 = {
  left: nullNode,
  right: nullNode,
  value: 6
};

const node9 = {
  left: nullNode,
  right: nullNode,
  value: 9
};

const node2 = {
  left: node1,
  right: node3,
  value: 2
};

const node7 = {
  left: node6,
  right: node9,
  value: 7
};

const node4 = {
  left: node2,
  right: node7,
  value: 4
};

function invertTree(tree) {
  // base recursion case
  if (!tree.left) return tree;

  const reversedLeft = invertTree(tree.left)
  const reversedRight = invertTree(tree.right)

  const reversedNode = {
    left: reversedRight,
    right: reversedLeft,
    value: tree.value
  };

  return reversedNode;
}

invertTree(node4)