class AncestralTree {
  constructor(name, ancestorNode) {
    this.name = name;
    this.ancestor = ancestorNode;
  }
}

const a = new AncestralTree("A", null);
const b = new AncestralTree("B", a);
const d = new AncestralTree("D", b);
const e = new AncestralTree("E", b);
const c = new AncestralTree("c", a);
const f = new AncestralTree("F", c);
const g = new AncestralTree("G", c);
const h = new AncestralTree("H", d);
const i = new AncestralTree("I", d);

function ancestorFunc(topAnc, desc1, desc2) {
    const list1 = ancList(desc1);
    const list2 = ancList(desc2);
    //list1 = [A, B, D]
    //list2 = [A, B]
    for (let i = 0; i < list1.length; i++) {
        if (list1[i] !== list2[i]) {
          return list1[i - 1].name
        }
    }
}

function ancList (myNode) {
    let node = myNode 
    let ancestors = [];
    while (node){
        ancestors.unshift(node)
        node = node.ancestor
    }
    return ancestors;
}
console.log(ancestorFunc(a, h, i))