Walk a comment thread breadth-first

 1function walkComments(root) {
 2  const queue = [root];
 3  const out = [];
 4  let head = 0;
 5  while (head < queue.length) {
 6    const node = queue[head++];
 7    out.push(node.text);
 8    for (const child of node.replies) queue.push(child);
 9  }
10  return out;
11}