# JS Quiz: Medium: Sparse array mapping holes

**URL:** <https://forum.kirupa.com/t/js-quiz-medium-sparse-array-mapping-holes/683372>\
**Category:** web dev\
**Created:** [September 15, 2026, 7:00am UTC](https://forum.kirupa.com/t/js-quiz-medium-sparse-array-mapping-holes/683372 "2026-09-15T07:00:09Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![VaultBoy](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/vaultboy/32/31832_2.png) [@VaultBoy](https://forum.kirupa.com/u/VaultBoy)\
**Post date:** [September 15, 2026, 7:00am UTC](https://forum.kirupa.com/t/js-quiz-medium-sparse-array-mapping-holes/683372/1 "2026-09-15T07:00:09Z")

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What is the final output?

```js
const a = new Array(3);
const b = a.map(() => 1);
console.log(a.length, b.length, 0 in b, b.join('-'));

```

_Poll ([view on site](https://forum.kirupa.com/t/js-quiz-medium-sparse-array-mapping-holes/683372/1))_

---

<div class="post-metadata">

**Author:** ![VaultBoy](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/vaultboy/32/31832_2.png) [@VaultBoy](https://forum.kirupa.com/u/VaultBoy)\
**Post date:** [September 16, 2026, 8:00am UTC](https://forum.kirupa.com/t/js-quiz-medium-sparse-array-mapping-holes/683372/2 "2026-09-16T08:00:05Z")

</div>

**JS Quiz answer:** Option 2 (B).

**Correct choice:** 3 3 false –

**Why:**  
Holes remain holes through map callbacks, so indices are still empty slots.

**Go deeper:**

[Arrays In JavaScript Temp](https://www.kirupa.com/html5/arrays_in_js_temp.htm)

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