binary tree traversal to find common ancestor

commonAncestor.js

const BinaryTree = class {
  constructor(value) {
    this.value = value;
    this.left = null;
    this.right = null;
  }
  addLeftChild(value) {
    if (this.left) {
      console.log('Left child being overwritten');
    }
    this.left = new BinaryTree(value);
  }
  addRightChild(value) {
    if (this.right) {
      console.log('Right child being overwritten');
    }
    this.right = new BinaryTree(value);
  }
}

const jack = new BinaryTree('Jack');
const tom = jack.left = new BinaryTree('Tom');
const matt = jack.right = new BinaryTree('Matt');
const fred = tom.left = new BinaryTree('Fred');
const lorin = fred.right = new BinaryTree('Lorin');
const benny = matt.right = new BinaryTree('Benny');
const yheti = matt.left = new BinaryTree('Yheti');
/*
Tree structure

          jack
         /    \
       tom    matt
      /       /  \
   fred   yheti  benny 
   /
lorin
*/

const commonAncestor = (rootNode, memberA, memberB) => {
  // object of arrays, key is member name
  // value is array of lineage members, index 0 is root
  const ancestry = {};
  const traverse = (node, lineage = []) => {
    lineage.push(node.value);
    ancestry[node.value] = [...lineage];
    if (node.left) {
      traverse(node.left, lineage);
      lineage.pop();
    }
    if (node.right) {
      traverse(node.right, lineage);
      lineage.pop();
    }
  }
  // generate the lineages inside of the ancestry object
  traverse(rootNode);
  const memberALineage = ancestry[memberA.value];
  const memberBLineage = ancestry[memberB.value];
  // one array may be longer than the other
  const length = Math.max(memberALineage.length, memberBLineage.length);
  // iterate through lineages and find the divergence
  for (let i = 0; i < length; i++) {
    let hasLineageDiverged = memberALineage[i] !== memberBLineage[i];
    // double check is previous element the same
    let isPreviousSame = memberALineage[i - 1] === memberBLineage[i - 1];
    if (hasLineageDiverged && isPreviousSame) {
      return memberALineage[i - 1];
    }
  }
  return null;
};

// matt is the first common ancestor
const result = commonAncestor(jack, benny, yheti);
console.log(result); // Matt