# Why does this map return undefined values?

**URL:** <https://forum.kirupa.com/t/why-does-this-map-return-undefined-values/679676>\
**Category:** web dev\
**Created:** [March 30, 2026, 12:00am UTC](https://forum.kirupa.com/t/why-does-this-map-return-undefined-values/679676 "2026-03-30T00:00:05Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**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:** [March 30, 2026, 12:00am UTC](https://forum.kirupa.com/t/why-does-this-map-return-undefined-values/679676/1 "2026-03-30T00:00:05Z")

</div>

Easy JavaScript check.

```javascript
const nums = [1, 2, 3];
const out = nums.map((n) => { n * 2; });
console.log(out);

```

Why is the output not `[2, 4, 6]`, and what is the simplest fix.

Yoshiii

---

<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:** [March 30, 2026, 12:14am UTC](https://forum.kirupa.com/t/why-does-this-map-return-undefined-values/679676/2 "2026-03-30T00:14:06Z")

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Because `{}` makes it a block body, your arrow function returns nothing unless you say `return`, so the simplest fix is either `const out = nums.map(n => n * 2)` or this equivalent form:

```js
const out = nums.map((n) => {
  return n * 2
})

```

Hari

---

<div class="post-metadata">

**Author:** ![sora](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/sora/32/31259_2.png) [@sora](https://forum.kirupa.com/u/sora)\
**Post date:** [March 30, 2026, 5:21am UTC](https://forum.kirupa.com/t/why-does-this-map-return-undefined-values/679676/3 "2026-03-30T05:21:06Z")

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Yep, with `{}` the arrow function has a block body, so `map` gets `undefined` unless you return explicitly.

Sora

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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:** [March 30, 2026, 6:56am UTC](https://forum.kirupa.com/t/why-does-this-map-return-undefined-values/679676/4 "2026-03-30T06:56:06Z")

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Exactly-`arr.map(x => ({ value: x }))` returns an object, but `arr.map(x => { value: x })` is just a block with no `return`.

MechaPrime