# Quick question from your JS book

**URL:** <https://forum.kirupa.com/t/quick-question-from-your-js-book/634203>\
**Category:** programming\
**Created:** [February 2, 2016, 10:41am UTC](https://forum.kirupa.com/t/quick-question-from-your-js-book/634203 "2016-02-02T10:41:28Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![bstagy](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/bstagy/32/7351_2.png) [@bstagy](https://forum.kirupa.com/u/bstagy)\
**Post date:** [February 2, 2016, 10:41am UTC](https://forum.kirupa.com/t/quick-question-from-your-js-book/634203/1 "2016-02-02T10:41:28Z")

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In your JS book you devote a chapter about Iffy’s and provide an example - content slider. What’s driving me crazy is a variable called link that uses a property called [itemID]. Is that a built in property to the window object?

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**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:** [February 2, 2016, 4:26pm UTC](https://forum.kirupa.com/t/quick-question-from-your-js-book/634203/2 "2016-02-02T16:26:52Z")

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The `itemID` property is a custom one that I defined directly on `link`. In JavaScript, you can quickly add properties to other objects (you’ve created) by just using them like in that example. For extending built-in objects, you shouldn’t do this. You should extend the `prototype` as shown later in the book 🙂

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**Author:** ![bstagy](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/bstagy/32/7351_2.png) [@bstagy](https://forum.kirupa.com/u/bstagy)\
**Post date:** [February 2, 2016, 4:42pm UTC](https://forum.kirupa.com/t/quick-question-from-your-js-book/634203/3 "2016-02-02T16:42:26Z")

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I am learning so much from you…I can’t believe. Thank you for being you.

Sincerely,  
Brad

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**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:** [February 2, 2016, 9:22pm UTC](https://forum.kirupa.com/t/quick-question-from-your-js-book/634203/4 "2016-02-02T21:22:39Z")

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Haha, thanks for the compliment 😛