Errors

type ApiError = { status: number; code?: string; message: string; retryAfter?: number }

async function parseError(res: Response): Promise<ApiError> {
  const body: any = await res.json().catch(() => ({}))
  const err = body?.error
  const message = typeof err === 'string' ? err : err?.message ?? res.statusText
  const code = body?.code ?? (typeof err === 'object' ? err?.code ?? err?.type : undefined)
  const retry = Number(res.headers.get('retry-after'))
  return { status: res.status, code, message, retryAfter: retry > 0 ? retry : undefined }
}

async function call(url: string, init: RequestInit, attempts = 3): Promise<Response> {
  for (let i = 0; i < attempts; i++) {
    const res = await fetch(url, init)
    if (res.ok) return res
    if (res.status === 429 && res.headers.get('retry-after')) {
      await new Promise((r) => setTimeout(r, Number(res.headers.get('retry-after')) * 1000))
      continue
    }
    if (res.status >= 500 && i < attempts - 1) {
      await new Promise((r) => setTimeout(r, 2 ** i * 1000 + Math.random() * 250))
      continue
    }
    return res // 4xx: the caller decides
  }
  throw new Error('unreachable')
}