Average of Levels in Binary Tree

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var averageOfLevels = function (root) {
  if (root === null) {
    return null;
  }
  let q = [root];
  let averageArray = [];

  while (q.length) {
    let level = [];
    let sum = 0;
    let count = 0;
    let loopTill = q.length;
    // looping only till the length of q;
    while (loopTill) {
      // removing first element from queue
      let node = q.shift();
      // adding up node val to get sum & also incrementing count tokeep track of nodes at each level
      sum = sum + node.val;
      count = count + 1;
      if (node.left !== null) {
        level.push(node.left);
      }
      if (node.right !== null) {
        level.push(node.right);
      }
      // reducing value as we have remove first element using shift() above
      loopTill--;
    }
    //get average of this level
    let averageOfLevel = sum / count;
    averageArray.push(averageOfLevel);

    q = level;
  }
  return averageArray;
};