# JS Quiz: Easy: Map key identity with NaN

**URL:** <https://forum.kirupa.com/t/js-quiz-easy-map-key-identity-with-nan/681025>\
**Category:** web dev\
**Created:** [April 30, 2026, 7:00am UTC](https://forum.kirupa.com/t/js-quiz-easy-map-key-identity-with-nan/681025 "2026-04-30T07:00:08Z")\
**Posts on this page:** 2\
**Page:** 1

<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 30, 2026, 7:00am UTC](https://forum.kirupa.com/t/js-quiz-easy-map-key-identity-with-nan/681025/1 "2026-04-30T07:00:09Z")

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What is printed?

```js
const m = new Map();
m.set(NaN, 'first');
m.set(NaN, 'second');
console.log(m.size, m.get(NaN));

```

_Poll ([view on site](https://forum.kirupa.com/t/js-quiz-easy-map-key-identity-with-nan/681025/1))_

MechaPrime

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<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:** [April 30, 2026, 8:00am UTC](https://forum.kirupa.com/t/js-quiz-easy-map-key-identity-with-nan/681025/2 "2026-04-30T08:00:17Z")

</div>

It prints `1 second`. `Map` treats `NaN` as equal to `NaN` (SameValueZero), so the second `set` overwrites the first and the size stays 1.