Errors

const sleep = (ms) => new Promise((resolve) => setTimeout(resolve, ms))

async function callVirlo(url, attempt = 1) {
  const res = await fetch(url, {
    headers: { Authorization: 'Bearer YOUR_API_KEY' },
  })
  if (res.ok) return res.json()

  // A few 500-range errors arrive as a web page, not JSON.
  // Treat those as a server-side error so they are retried too.
  const err = await res.json().catch(() => ({
    code: res.status >= 500 ? 'service_unavailable' : 'unknown_error',
    message: res.statusText,
  }))
  const canRetry = attempt < 3

  if (err.code === 'rate_limit_exceeded' && canRetry) {
    // Wait as long as the API asks, then try again.
    await sleep((err.retry_after ?? 60) * 1000)
    return callVirlo(url, attempt + 1)
  }
  const serverSide = ['upstream_error', 'service_unavailable', 'request_timeout', 'internal_error']
  if (serverSide.includes(err.code) && canRetry) {
    // Wait 2 seconds, then 4, then give up.
    await sleep(2 ** attempt * 1000)
    return callVirlo(url, attempt + 1)
  }
  if (err.code === 'insufficient_credits') {
    throw new Error(
      `Add funds: this request needs ${err.required_credits} credits, you have ${err.remaining_credits}`,
    )
  }
  // Anything else: fix the cause first (see the table).
  // message can be one sentence or a list of them.
  throw new Error(`${err.code}: ${[].concat(err.message).join('; ')}`)
}