How it works the returns if the specified property is in the specified object. source: **in** operator **true** MDN So? .. it checks if property exists not the value of it. then what can go wrong ? Why I always had something against , mainly because it sounds a bit over promising. The first time I used it I was expecting a behavior more like , just because the word “ is very suggestive to inform you whether is something something else that contains it. **in** **includes** in” **in** So don’t get me wrong, its existence is essential in the language but how it sounds and how some of developer are using is the I met lots situations where I didn’t like the usage of but I just bumped into another situation where browser implementation was a bit misleading. and using made it even more confusing. in **in** firefox console Check the way is detecting touch device we have the same approach implemented in our website and I was debugging that some of the mouse events weren’t firing on Firefox. Modernizer here , More or less here is the key part of detection, **ontouchstart in window** The funny part is that in chrome it’s , and Firefox it’s **undefined** **null.   ** Edit: it turned out to be not that funny, it was a bug in Firefox that under some certain circumstances it shows this particular value **ontouchstart** as null. ref: https://github.com/Modernizr/Modernizr/issues/1731 Check out the following and try to play around with it in your console to grasp the idea. I know … the is consistent with the specs, but from my point of view, I just hate words that can imply different meanings! I would just avoid using (unless you’re using it knowing what your doing) and won’t consider it to be used to check existance of a property. implementation Edit & Correction: Thanks to he pointed out very valid points check it in the comments. I think is the most and I totally missed up on it, which that browser vendors design their APIs that way and Modernizer was doing it the right way. it was a bug on Firefox bug and some other browsers. Jason Rogers here #1: for example it will be undefined on the beginning because no handler was assigned yet. #2: window.onload

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Why you shouldn’t use ‘in’ in javascript

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Why you shouldn’t use ‘in’ in javascript

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