LargestLevelOfTree.js

var BinaryTree = function (value) {
  this.value = value;
  this.left = null;
  this.right = null;
};

// Starting Level at root = 0
const findLargestLevel = function(node) {
  var queue = [[node, 0]];
  var levelSum = [];
  var largestSum = -Infinity;
  var largestLevel = 0;

  while (queue.length > 0) {
    var tuple = queue.shift();
    var node = tuple[0];
    var depth = tuple[1];

    if (levelSum[depth] === undefined) {
      levelSum.push(node.value);
    } else {
      levelSum[depth] += node.value;
    }

    if (node.left) {
      queue.push([node.left, depth + 1]);
    }
    if (node.right) {
      queue.push([node.right, depth + 1]);
    }
  }

  for (var i = 0; i < levelSum.length; i++) {
    if (levelSum[i] > largestSum) {
      largestSum = levelSum[i];
      largestLevel = i;
    }
  }

  return largestLevel;
};

// --------- TEST 1 ------
// Largest level => 2 
const myTree = new BinaryTree(10);
myTree.left = new BinaryTree(5);
myTree.right = new BinaryTree(15);
myTree.left.left = new BinaryTree(3);
myTree.left.right = new BinaryTree(8);
myTree.right.left = new BinaryTree(12);
myTree.right.right = new BinaryTree(20);
/*
        10
      5    15
    3  8 12  20
*/

// --------- TEST 2 ------ 
// Same Value at level 1 and 2 => should return level 1
const myTree2 = new BinaryTree(10);
myTree2.left = new BinaryTree(5);
myTree2.right = new BinaryTree(30);
myTree2.left.left = new BinaryTree(5);
myTree2.left.right = new BinaryTree(10);
myTree2.right.left = new BinaryTree(15);
myTree2.right.right = new BinaryTree(5);
/*
        10
      5    15
    5  10 15  5
*/

console.log(findLargestLevel(myTree)); // 2
console.log(findLargestLevel(myTree2)); // 1