Classify Errors Into Retryable And Fatal

export type ErrorKind = "retryable" | "fatal" | "unknown";

export interface ErrorDecision {
  readonly kind: ErrorKind;
  readonly retryable: boolean;
  readonly reason: string;
}

const RETRYABLE_CODES = new Set(["ECONNRESET", "ETIMEDOUT", "EAI_AGAIN", "EPIPE"]);

const FATAL_CODES = new Set(["EACCES", "ENOENT", "EINVAL"]);

function statusOf(error: unknown): number | null {
  if (!error || typeof error !== "object") return null;
  const record = error as Record<string, unknown>;
  const status = record.status ?? record.statusCode;
  return typeof status === "number" ? status : null;
}

function codeOf(error: unknown): string | null {
  if (!error || typeof error !== "object") return null;
  const record = error as Record<string, unknown>;
  return typeof record.code === "string" ? record.code : null;
}

export function classifyError(error: unknown): ErrorDecision {
  const code = codeOf(error);
  if (code && RETRYABLE_CODES.has(code)) {
    return { kind: "retryable", retryable: true, reason: `transient system error code ${code}` };
  }
  if (code && FATAL_CODES.has(code)) {
    return { kind: "fatal", retryable: false, reason: `local error code ${code} will not fix itself` };
  }
  const status = statusOf(error);
  if (status !== null) {
    if (status >= 500) {
      return { kind: "retryable", retryable: true, reason: `server status ${status} is transient` };
    }
    if (status >= 400) {
      return { kind: "fatal", retryable: false, reason: `client status ${status} will fail again` };
    }
  }
  return {
    kind: "unknown",
    retryable: false,
    reason: "no recognised code or status; defaulting to non-retryable",
  };
}