16. Implement retry(fn, retries, delay) — Retry a Promise

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Retry-with-backoff is a real-world async pattern (flaky network calls) and a clean recursion-over-promises question.

The idea

Try fn. On rejection, either give up (out of retries → rethrow) or wait, then recurse with one fewer retry and a doubled delay. Doubling is exponential backoff — it stops a struggling server from getting hammered.

Follow-ups

"Cap the delay?" → Math.min(delay * 2, maxDelay).
"Jitter?" → add a random fraction to avoid synchronized retries (the thundering herd).
"Only retry some errors?" → take a shouldRetry(err) predicate.
"Abort support?" → thread an AbortSignal and reject early if it fires.

What to practice

Write the recursive version, then add a maxDelay cap and random jitter.

More questions

index.js

function retry(fn, retries = 3, delay = 100) {
  // fn returns a Promise; resolve on success, give up after `retries`
}

let n = 0;
retry(() => (++n < 3 ? Promise.reject('fail') : Promise.resolve('ok')))
  .then(console.log); // "ok" on the 3rd attempt

⟂ Tests

Test Code

Enter JavaScript that runs after your solution. It should return a value or a Promise.