#使用 JavaScript 实现带有重试能力的请求工具函数

/**
 * 带超时、重试和指数退避的 fetch 请求工具
 *
 * @param {string | URL} url
 * @param {RequestInit & {
 *   retries?: number;
 *   retryDelay?: number;
 *   timeout?: number;
 *   retryOn?: (response?: Response, error?: unknown) => boolean;
 * }} options
 * @returns {Promise<Response>}
 */
async function requestWithRetry(url, options = {}) {
  const {
    retries = 3,
    retryDelay = 300,
    timeout = 5000,
    retryOn = (response, error) => {
      // 网络错误、超时，以及 408、429 和 5xx 通常适合重试
      return Boolean(
        error ||
          response?.status === 408 ||
          response?.status === 429 ||
          response?.status >= 500
      );
    },
    signal: externalSignal,
    ...fetchOptions
  } = options;

  let lastError;

  for (let attempt = 0; attempt <= retries; attempt++) {
    const controller = new AbortController();
    const timeoutId = setTimeout(() => controller.abort(), timeout);

    // 外部 signal 取消时，同时取消当前请求
    const abortRequest = () => controller.abort();
    externalSignal?.addEventListener("abort", abortRequest, { once: true });

    try {
      const response = await fetch(url, {
        ...fetchOptions,
        signal: controller.signal
      });

      if (!retryOn(response)) {
        return response;
      }

      lastError = new Error(`Request failed with status ${response.status}`);
      lastError.response = response;
    } catch (error) {
      lastError = error;

      // 主动取消不应该继续重试
      if (externalSignal?.aborted) {
        throw error;
      }

      if (!retryOn(undefined, error)) {
        throw error;
      }
    } finally {
      clearTimeout(timeoutId);
      externalSignal?.removeEventListener("abort", abortRequest);
    }

    if (attempt === retries) break;

    // 指数退避 + 随机抖动，避免大量请求同时重试
    const exponentialDelay = retryDelay * 2 ** attempt;
    const jitter = Math.random() * retryDelay;
    await new Promise((resolve) => {
      setTimeout(resolve, exponentialDelay + jitter);
    });
  }

  throw lastError;
}

const response = await requestWithRetry("/api/user", {
  method: "GET",
  retries: 3,
  retryDelay: 200,
  timeout: 3000
});

const data = await response.json();