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Implement promiseRetry(fn, retries, delay) with exponential backoff

Q: Implement promiseRetry(fn, retries, delay) with exponential backoff

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the exact real attempt counts below — 3 for the eventually-succeeding case, and `retries + 1` for the always-failing case — were confirmed with a real counter, not assumed from the recursive structure alone. ## 1. The problem, restated `promiseRetry(fn, retries, delayMs)` calls `fn()`; on rejection, it waits `delayMs`, then tries again — doubling the delay each subsequent attempt — up to `retries` ADDITIONAL attempts beyond the first, finally throwing the last real error if every attempt fails. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | `retries` = total attempts, or additional retries after the first? | A real, common off-by-one ambiguity worth pinning down explicitly. | | Exponential specifically, or fixed delay acceptable? | Exponential is the real, standard choice — worth confirming, not

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The problem

Problem, as an interviewer would state it: "Write promiseRetry(fn, retries, delayMs) — it calls fn, and if it rejects, waits and tries again, up to retries additional times, doubling the delay each time (exponential backoff)."

Examples:

Code

await promiseRetry(flakyFn, 3, 100); // retries up to 3 more times, delays 100/200/400ms

Clarifying questions expected:

Does retries mean total attempts, or additional attempts AFTER the first real try?
Should the delay genuinely double each time (exponential), or is a fixed delay acceptable?
Is there a maximum total delay/timeout across all retries combined, or unbounded?

Code / implementation expected: Yes — real, direct proof of both outcomes: a function that fails its first 2 attempts then succeeds, and a function that genuinely always fails, exhausting all real retries before throwing.

retry
promise
exponential-backoff

Try it yourself

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Solution

Target Audience: Engineers preparing for practical JavaScript coding interviews. Difficulty: Medium

How to read this doc: the exact real attempt counts below — 3 for the eventually-succeeding case, and retries + 1 for the always-failing case — were confirmed with a real counter, not assumed from the recursive structure alone.

1. The problem, restated

promiseRetry(fn, retries, delayMs) calls fn(); on rejection, it waits delayMs, then tries again — doubling the delay each subsequent attempt — up to retries ADDITIONAL attempts beyond the first, finally throwing the last real error if every attempt fails.

2. Clarifying questions a strong candidate asks

Question	Why it matters
`retries` = total attempts, or additional retries after the first?	A real, common off-by-one ambiguity worth pinning down explicitly.
Exponential specific

The solution is hidden so you can try first

4 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal, direct proof: promiseRetry() correctly retries a function that eventually succeeds, and exhausts the exact configured retry count on one that always fails

Practice

Discussion

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