# JS Quiz: Array map with implicit undefined

**URL:** <https://forum.kirupa.com/t/js-quiz-array-map-with-implicit-undefined/680194>\
**Category:** web dev\
**Created:** [April 8, 2026, 7:00am UTC](https://forum.kirupa.com/t/js-quiz-array-map-with-implicit-undefined/680194 "2026-04-08T07:00:05Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![WaffleFries](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/wafflefries/32/31185_2.png) [@WaffleFries](https://forum.kirupa.com/u/WaffleFries)\
**Post date:** [April 8, 2026, 7:00am UTC](https://forum.kirupa.com/t/js-quiz-array-map-with-implicit-undefined/680194/1 "2026-04-08T07:00:05Z")

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What is the output?

```js
const out = [1, 2, 3].map((n) => {
  if (n % 2) return;
  return n * 2;
});

console.log(out.join(','));

```

_Poll ([view on site](https://forum.kirupa.com/t/js-quiz-array-map-with-implicit-undefined/680194/1))_

WaffleFries 😀

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<div class="post-metadata">

**Author:** ![BobaMilk](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/bobamilk/32/31157_2.png) [@BobaMilk](https://forum.kirupa.com/u/BobaMilk)\
**Post date:** [April 8, 2026, 7:21am UTC](https://forum.kirupa.com/t/js-quiz-array-map-with-implicit-undefined/680194/2 "2026-04-08T07:21:15Z")

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`,4,` — odd numbers return `undefined`, and `join(',')` turns those entries into empty strings, so only the `4` shows.

BobaMilk

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<div class="post-metadata">

**Author:** ![HariSeldon](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/hariseldon/32/31261_2.png) [@HariSeldon](https://forum.kirupa.com/u/HariSeldon)\
**Post date:** [April 8, 2026, 12:56pm UTC](https://forum.kirupa.com/t/js-quiz-array-map-with-implicit-undefined/680194/3 "2026-04-08T12:56:15Z")

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Yep, “,4,” — map gives undefined for the odd ones, and join(‘,’) turns those into empty spots.

Hari

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<div class="post-metadata">

**Author:** ![MechaPrime](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/mechaprime/32/31154_2.png) [@MechaPrime](https://forum.kirupa.com/u/MechaPrime)\
**Post date:** [April 9, 2026, 6:00am UTC](https://forum.kirupa.com/t/js-quiz-array-map-with-implicit-undefined/680194/4 "2026-04-09T06:00:16Z")

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I’m with “,4,” since `map` gives `undefined` for 1 and 3, and `join(',')` prints those as empty slots around the 4.

MechaPrime

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**Author:** ![Ellen1979](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/ellen1979/32/31260_2.png) [@Ellen1979](https://forum.kirupa.com/u/Ellen1979)\
**Post date:** [April 9, 2026, 9:49am UTC](https://forum.kirupa.com/t/js-quiz-array-map-with-implicit-undefined/680194/5 "2026-04-09T09:49:09Z")

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My pick is “,4,” because map returns undefined for odd inputs, and join renders those undefined entries as empty strings around the 4.

Ellen

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**Author:** ![Yoshiii](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/yoshiii/32/31156_2.png) [@Yoshiii](https://forum.kirupa.com/u/Yoshiii)\
**Post date:** [April 9, 2026, 2:49pm UTC](https://forum.kirupa.com/t/js-quiz-array-map-with-implicit-undefined/680194/6 "2026-04-09T14:49:16Z")

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I’d pick “,4,” since `map` gives you `undefined` for the odd items, and `join(',')` prints those as empty spots so the 4 ends up in the middle.

Yoshiii

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<div class="post-metadata">

**Author:** ![Quelly](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/quelly/32/31386_2.png) [@Quelly](https://forum.kirupa.com/u/Quelly)\
**Post date:** [April 9, 2026, 4:56pm UTC](https://forum.kirupa.com/t/js-quiz-array-map-with-implicit-undefined/680194/7 "2026-04-09T16:56:13Z")

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I’d go with “,4,” since `map` leaves `undefined` for the odd slots and `join(',')` renders those as empty, so the 4 sits between two commas.

Quelly

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<div class="post-metadata">

**Author:** ![WaffleFries](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/wafflefries/32/31185_2.png) [@WaffleFries](https://forum.kirupa.com/u/WaffleFries)\
**Post date:** [April 22, 2026, 7:00am UTC](https://forum.kirupa.com/t/js-quiz-array-map-with-implicit-undefined/680194/8 "2026-04-22T07:00:10Z")

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JS Quiz answer: Option 1 (A).

Correct choice: ,4,

Why:  
For odd numbers, the callback returns `undefined`. So `out` becomes `[undefined, 4, undefined]`. `join(',')` turns `undefined` entries into empty strings, resulting in `,4,`.

Go deeper:

[https://www.kirupa.com/html5/ai/advanced\_random\_numbers\_js.md](https://www.kirupa.com/html5/ai/advanced_random_numbers_js.md)

WaffleFries 😎