I have a helper function that iterates through a binary search tree breadth first, and inserts each 'level' of the binary tree into a subarray, which is then pushed into a 2d array.

For example:

//      a
//    /   \
//   b     c
//  / \     \
// d   e     f

Looks like this:

// [
//   ['a'],
//   ['b', 'c'],
//   ['d', 'e', 'f']
// ]

Here is my helper function:

const myLevels = (root) => {
  const subLevels = [];
  const queue = [{
    node: root,
    currentLevel: 0,
  }];

  while (queue.length > 0) {
    let { node, currentLevel } = queue.shift();

    if (subLevels.length === currentLevel) {
      subLevels.push([node.val]);
    } else {
      subLevels[currentLevel].push(node.val);
    };

    if (node.left !== null) queue.push({ node: node.left, currentLevel: currentLevel + 1 });
    if (node.right !== null) queue.push({ node: node.right, currentLevel: currentLevel + 1 });
  };

  return subLevels;
};

This works, however, I want to use the helper function in another function which will return a specific element(s) out of each level that is returned by the helper function. For example, what if I wanted to return the leftmost and rightmost elements from each level? Or what if I wanted to find the second largest value in each level (assuming that each node value consists of a number), or what if I wanted to return the vowels from each level?

My current approach is to iterate through each sub-array of the 2d array, and then iterate through those elements, as this approach makes the most sense to me. However, I have no idea how to conceptualize keeping track of each level, and telling my function to 'look at this subarray like it's a brand new array'.

This is my main function thus far:

function findXEachLevel(root) {
  const levels = myLevels(root);
  let finalArray = [];

  for (let i = 0; i < levels.length; i++) {
    let currentLevel = levels[i];
    console.log(currentLevel) // Shows the current subarray in the 2d array

    for (let j = 0; j < currentLevel.length; j++) {
      let currentValue = currentLevel[j];
      console.log(currentValue) // Shows the current value in the subarray
    };
  };
};

Would a callback be a viable solution here, or is there a simpler approach?