Cache a request

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Check for a saved value before fetching a new one.

async function weather(city) {
  const saved = cache.get(city)
  if (saved !== undefined) return saved

  const response = await fetch(`/weather/${city}`)
  if (!response.ok) throw new Error('Request failed')

  const value = await response.json()
  cache.set(city, value)
  return value
}

Only successful responses are stored. Concurrent misses still make separate requests.