Retry Strategies with Fetch: Exponential Backoff and Idempotency Checks

Applications retry failed requests including POST/PATCH requests that may have partially succeeded server-side, causing duplicate operations (double charges, duplicate records).

Only automatically retry idempotent requests (GET, PUT, DELETE) or requests with a server-recognized idempotency key. Use exponential backoff.

async function fetchWithRetry(url, options = {}, retries = 3) {
  const idempotent = ['GET', 'HEAD', 'PUT', 'DELETE'].includes(
    (options.method || 'GET').toUpperCase()
  );

  for (let attempt = 0; attempt <= retries; attempt++) {
    try {
      const res = await fetch(url, options);
      if (res.status === 429) {
        const retryAfter = parseInt(res.headers.get('Retry-After') || '1', 10);
        await delay(retryAfter * 1000);
        continue;
      }
      if (res.status >= 500 && idempotent && attempt < retries) {
        await delay(Math.min(1000 * 2 ** attempt, 30000));
        continue;
      }
      return res;
    } catch (err) {
      if (attempt === retries || !idempotent) throw err;
      await delay(Math.min(1000 * 2 ** attempt, 30000));
    }
  }
}

const delay = (ms) => new Promise(r => setTimeout(r, ms));

Retrying POST without idempotency keys can cause financial or data integrity issues. The RFC recommends using Idempotency-Key headers (stripe's pattern) for POST requests that must be retried safely.