leetcode-637.js

var averageOfLevels = function(root) {
  const valuesByLevel = []
  dfs(root, 0, valuesByLevel)
  
  const result = []
  for (let i=0; i < valuesByLevel.length; i++) {
    const arr = valuesByLevel[i]
    result.push(average(arr))
  }
  return result
};

function average(arr) {
  let sum = 0
  for (let j=0; j < arr.length; j++) {
    sum += arr[j]
  }
  return sum / arr.length
}

function dfs(node, level, valuesByLevel) {
  if (!node) return
  
  if (valuesByLevel[level]) {
    const arr = valuesByLevel[level]
    arr.push(node.val)
  } else {
    valuesByLevel[level] = [node.val]
  }
  
  dfs(node.left, level + 1, valuesByLevel)
  dfs(node.right, level + 1, valuesByLevel)
}

https://leetcode.com/problems/average-of-levels-in-binary-tree/