# React book - shouldComponentUpdate

**URL:** <https://forum.kirupa.com/t/react-book-shouldcomponentupdate/639818>\
**Category:** Uncategorized\
**Created:** [May 15, 2019, 3:19pm UTC](https://forum.kirupa.com/t/react-book-shouldcomponentupdate/639818 "2019-05-15T15:19:23Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Martin\_Smith](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/martin_smith/32/9641_2.png) [@Martin\_Smith](https://forum.kirupa.com/u/Martin_Smith)\
**Post date:** [May 15, 2019, 3:19pm UTC](https://forum.kirupa.com/t/react-book-shouldcomponentupdate/639818/1 "2019-05-15T15:19:23Z")

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Page 131, shouldComponentUpdate – what is destination variable – it does not appear to be defined anywhere.

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<div class="post-metadata">

**Author:** ![kirupa](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/kirupa/32/11616_2.png) [@kirupa](https://forum.kirupa.com/u/kirupa)\
**Post date:** [May 15, 2019, 4:26pm UTC](https://forum.kirupa.com/t/react-book-shouldcomponentupdate/639818/2 "2019-05-15T16:26:29Z")

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The destination variable is something defined at the very top:

`var destination = document.querySelector("#container");`

You can see it here: [https://github.com/kirupa/kirupa/blob/master/reactjs/lifecycle.htm#L22](https://github.com/kirupa/kirupa/blob/master/reactjs/lifecycle.htm#L22)

Does that clarify 🙂

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<div class="post-metadata">

**Author:** ![Martin\_Smith](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/martin_smith/32/9641_2.png) [@Martin\_Smith](https://forum.kirupa.com/u/Martin_Smith)\
**Post date:** [May 15, 2019, 6:50pm UTC](https://forum.kirupa.com/t/react-book-shouldcomponentupdate/639818/3 "2019-05-15T18:50:53Z")

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yes thx