How do you implement a custom Promise.any polyfill?

Q: How do you implement a custom Promise.any polyfill?

**Target Audience:** Engineers preparing for JavaScript async/Promise interview questions. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term: . Every result shown below is **real, captured output** from actually running the code in this doc on Node.js v24.19.0 — not illustrative sample output. ## 1. Why This Even Matters — A Story First Imagine calling five different pizza places at once and going with whichever one actually answers the phone and says yes, even if two others already said "sorry, we are closed" a few seconds earlier. Those early no-answers do not matter at all — only the first genuine yes does. `Promise.any` is exactly that: the first fulfillment wins, and early rejections are simply ignored, right up until every single option has said no. ## 2. The Core Idea 📌 **Interview term:** `Promise.any` returns a promise that fulfills as soon as any one of the input promises ful

The problem

Question presented to candidate: "Promise.any takes an array of promises and resolves as soon as ANY one of them fulfills — it only rejects if every single one of them fails, and in that case it rejects with a special AggregateError containing all the individual failure reasons. Implement a polyfill for it. What happens if a rejection genuinely arrives before a fulfillment does? And what should the order of the errors be inside the AggregateError?"

What a strong answer should cover:

The outer promise resolves the instant ANY item fulfills — a rejection arriving first, even much faster than the eventual winner, must not affect the outcome at all.
The outer promise only rejects once EVERY item has rejected; that is the opposite failure condition from Promise.all.
The rejection reason is a real AggregateError, a built-in Error subclass added specifically for this purpose, whose .errors property is an array of every individual rejection reason.
That .errors array should preserve INPUT order (which promise was originally where), not the order the rejections actually happened to arrive in — this is the exact same indexing discipline Promise.all's polyfill needs for its results array.
An empty input array is a genuine edge case: it must reject immediately with an AggregateError containing zero errors, since there is nothing that could possibly fulfill.
AggregateError's constructor signature is new AggregateError(errorsIterable, message) — the errors come first, the message second, the reverse of what some engineers expect from plain Error.

Clarifying questions expected:

"Should the polyfill construct a real AggregateError, or is a plain Error with an attached .errors array acceptable if the environment predates AggregateError?" — a real interview-worthy compatibility question, since AggregateError is a newer addition than Promise itself.
"Do all the rejection reasons need to be captured even after the first one arrives, or can we stop tracking once we know the outer promise cannot possibly succeed?" — clarifies whether early-exit optimizations are in scope.

Code / implementation expected: Yes — a full, runnable polyfill plus a real test proving a fulfillment wins over a faster rejection, and that AggregateError.errors preserves input order.

promises
polyfill
async

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Engineers preparing for JavaScript async/Promise interview questions. Difficulty: Medium

How to read this doc: Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every result shown below is real, captured output from actually running the code in this doc on Node.js v24.19.0 — not illustrative sample output.

1. Why This Even Matters — A Story First

Imagine calling five different pizza places at once and going with whichever one actually answers the phone and says yes, even if two others already said "sorry, we are closed" a few seconds earlier. Those early no-answers do not matter at all — only the first genuine yes does. Promise.any is exactly that: the first fulfillment wins, and early rejections are simply ignored, right up until every single option has said no.

2. The Core Idea

📌 Interview term: `Pr

The solution is hidden so you can try first

7 min read · 7 sections

Run the code

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

JavaScriptThe polyfill plus a real test proving a fulfillment wins over a faster rejection, and error-order preservation (run directly)