Callbacks & Callback Hell

function batch(operations, size, callback) {
  const results = [];
  let completed = 0;
  
  for (let i = 0; i < operations.length; i += size) {
    const batch = operations.slice(i, i + size);
    parallel(batch, (err, batchResults) => {
      if (err) return callback(err);
      results.push(...batchResults);
      completed += batch.length;
      if (completed === operations.length) {
        callback(null, results);
      }
    });
  }
}