DOM_tree_height.js

function treeHeight(tree) {
  if (tree === null) return 0;
  let subHeights = Array.from(tree.children).map(treeHeight)
  return Math.max(...subHeights, 0) + 1
}

// Another option is to traverse DOM layer by layer
function treeHeight2(tree) {
  if (tree === null) return 0;
  const queue = [tree]
  let height = 0
  while (queue.length) {
    let count = queue.length
    height += 1
    while (count) {
      const head = queue.shift()
      queue.push(...head.children)
      count -= 1
    }
  }
  return height
}

// Example
const node = document.getElementById("main");
console.log(treeHeight(mainNode));
console.log(treeHeight2(mainNode));