binary_tree_levels.js

let nodes = [] // list of nodes

const ROOT //binary tree with getChildren function implemented

getNodes(ROOT.getChildren()) // start recursion with no level_index so default is used

function getNodes(level, level_index = 0) { 
  level.forEach(node => { // loop threw nodes in the level
    if (Array.isArray(nodes[level_index])) { 
      nodes[level_index].push(node) // if node already in this level then add
    } else {
      nodes[level_index] = [node] // first node in this level so create array
    }
    // repeat with this nodes children and descend 1 level
    getNodes(node.getChildren(), level_index + 1)
  });
}

nodes[2].join(",") // return list of values from nodes on the 3rd level with comma delimiter 

demo script implementing Breadth-first search algorithm can be seen here.