Drill 18: Trade-off — Hash maps and frequency counting

function topTopic(topics) {
  const counts = new Map()
  for (const topic of topics) {
    counts.set(topic, (counts.get(topic) ?? 0) + 1)
  }

  let best = null
  let bestCount = 0
  for (const [topic, count] of counts) {
    if (count > bestCount) {
      best = topic
      bestCount = count
    }
  }
  return best
}