MediumFrontend5 min read

Implement Promise.allSettled()

Q: Implement Promise.allSettled()

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the "never rejects overall" claim was verified directly with a real mix of one genuinely rejected and two genuinely fulfilled promises. ## 1. The problem, restated Given an array of promises, return one promise that ALWAYS resolves (never rejects), with an array reporting each input's own real outcome as `{status: "fulfilled", value}` or `{status: "rejected", reason}`. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Can this build on an existing Promise.all? | Recognizing the reuse opportunity avoids reimplementing the concurrent, index-preserving machinery from scratch. | | Exact shape of a settled result object? | Matching the real, documented `{status, value}`/`{status, reason}` shape matters for compatibility with real code expecting it. | | Order preserved like Promise.all? | Yes, genuinely —

738 views

104 of 154

The problem

Problem, as an interviewer would state it: "Implement Promise.allSettled() — unlike Promise.all, it should NEVER reject overall. Every input's real outcome, whether it succeeded or failed, should show up in the results array."

Examples:

Code

await myAllSettled([Promise.resolve(1), Promise.reject("err"), Promise.resolve(3)]);
// [{status:"fulfilled", value:1}, {status:"rejected", reason:"err"}, {status:"fulfilled", value:3}]

Clarifying questions expected:

Can this genuinely be built on top of a real, already-implemented Promise.all, or does it need entirely separate machinery?
What exact shape should a fulfilled vs. rejected result object have?
Does result order still need to match input order, like Promise.all?

Code / implementation expected: Yes — real, direct proof that a mix of fulfilled and rejected inputs produces a correctly-shaped settled-status array, with the overall operation never rejecting.

promise
promiseAllSettled

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 practical JavaScript coding interviews. Difficulty: Medium

How to read this doc: the "never rejects overall" claim was verified directly with a real mix of one genuinely rejected and two genuinely fulfilled promises.

1. The problem, restated

Given an array of promises, return one promise that ALWAYS resolves (never rejects), with an array reporting each input's own real outcome as {status: "fulfilled", value} or {status: "rejected", reason}.

2. Clarifying questions a strong candidate asks

Question	Why it matters
Can this build on an existing Promise.all?	Recognizing the reuse opportunity avoids reimplementing the concurrent, index-preserving machinery from scratch.
Exact shape of a settled result object?	Matching the real, documented `{status, value}`/`{status, reason}` shape matters for

The solution is hidden so you can try first

5 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: Promise.allSettled() genuinely never rejects overall, correctly reporting each input's own real outcome

Practice

Discussion

Other approaches, follow-ups and how it went in real interviews.

No comments yet. Share how you would answer it.

Question 104 of 154 in JavaScript coding

Keep your momentum going.

Mark this question solved to track your progress, then take the next one while it is fresh.

All JavaScript coding questions

Up nextMedium→

Implement deepMerge(target, source) — recursive merge

Continue