Promise retries with exponential backoff

withRetries.js

// paste this in any JS REPL to see the execution!
// Note that the fakeAPI is simulated to be flaky so try executin this a few times
// to see both the success and failure case
(() => {
/**
 * Wrap a promise API with a function that will attempt the promise over and over again
 * with exponential backoff until it resolves or reaches the maximum number of retries.
 *   - First retry: 500 ms + <random> ms
 *   - Second retry: 1000 ms + <random> ms
 *   - Third retry: 2000 ms + <random> ms
 * and so forth until maximum retries are met, or the promise resolves.
 */
const withRetries = ({ attempt, maxRetries }) => async (...args) => {
  const slotTime = 500;
  let retryCount = 0;
  do {
    try {
      console.log('Attempting...', Date.now());
      return await attempt(...args);
    } catch (error) {
      const isLastAttempt = retryCount === maxRetries;
      if (isLastAttempt) return Promise.reject(error);
    }
    const randomTime = Math.floor(Math.random() * slotTime);
    const delay = 2 ** retryCount * slotTime + randomTime;
    // Wait for the exponentially increasing delay period before retrying again.
    await new Promise(resolve => setTimeout(resolve, delay));
  } while (retryCount++ < maxRetries);
}

const fakeAPI = (...args) => Math.random() < 0.25 ? Promise.resolve(args) : Promise.reject(new Error('fakeAPI failure'))
const fakeAPIWithRetries = withRetries({ attempt: fakeAPI, maxRetries: 3 });
fakeAPIWithRetries('arg1', 'arg2').then((result) => console.log(result))
})();