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Implement Promise.race()

Q: Implement Promise.race()

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Easy > **How to read this doc:** the sharpest, most commonly-missed part of this question — that a REJECTION can win the race just as validly as a resolution — was verified directly, not assumed from the method's name. ## 1. The problem, restated Implement `Promise.race(promises)`: returns a new promise that settles the instant the FIRST input promise settles, adopting that promise's own outcome (value if it resolved, reason if it rejected). ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | First to resolve, or first to settle at all? | The real, native `Promise.race()` means first to SETTLE — a fast rejection genuinely wins over a slower resolution, a real, common misconception worth surfacing before coding. | | Can inputs be plain, non-promise values? | The real spec wraps every input with `Promise.resolve()`, so a plain value "set

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The problem

Problem, as an interviewer would state it: "Implement Promise.race() yourself. Given an array of promises, return a new promise that settles — resolves OR rejects — the moment the FIRST input promise settles, matching whatever that promise did."

Examples:

Code

const fast = new Promise(r => setTimeout(() => r("fast"), 20));
const slow = new Promise(r => setTimeout(() => r("slow"), 100));
await Promise.race([slow, fast]); // "fast" - it settled first

Clarifying questions expected:

Does "first" mean first to RESOLVE specifically, or first to SETTLE at all (resolve OR reject)?
Can the input array contain non-promise values (plain values)?
What happens with an empty input array?

Code / implementation expected: Yes — real, direct proof that both a fast resolution and, separately, a fast rejection each correctly win a real race against a slower opposite outcome.

promise
promiseRace

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Solution

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

How to read this doc: the sharpest, most commonly-missed part of this question — that a REJECTION can win the race just as validly as a resolution — was verified directly, not assumed from the method's name.

1. The problem, restated

Implement Promise.race(promises): returns a new promise that settles the instant the FIRST input promise settles, adopting that promise's own outcome (value if it resolved, reason if it rejected).

2. Clarifying questions a strong candidate asks

Question	Why it matters
First to resolve, or first to settle at all?	The real, native `Promise.race()` means first to SETTLE — a fast rejection genuinely wins over a slower resolution, a real, common misconception worth surfacing before coding.
Can inputs be plain, n

The solution is hidden so you can try first

6 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.race() correctly lets a fast rejection win over a slower resolution, matching native behavior — verified directly

Practice

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