$ is not a function is a common error message newbies encounter when trying to use JQuery in Drupal 7.
The explanation is that the usage of $ is disabled in Drupal to avoid conflicts with other libraries. But which other library is using $?
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DOK
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Jérôme Verstrynge
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dupe: [What JavaScript libraries are known to use the global dollar sign: window.$?](http://stackoverflow.com/questions/4632041/what-javascript-libraries-are-known-to-use-the-global-dollar-sign-window) – cliffs of insanity May 13 '12 at 18:47
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At least two major ones:
It is probably because using one char for most commonly used function saves a lot of time. And due to some restrictions for characters in variable name in JavaScript there are actually two candidates for such one-char names: $ and _ (_ is used often for eg. gettext functions or throw-away variables). Thus I would rather expect that if some framework has some shortcut function with one char name, it is probably $.
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Also, using one char for a global method makes your script more compact, it's not just a question of time. – Christophe May 13 '12 at 18:50
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One of the common problems is several different versions of JQuery itself, though I'm not sure that is the reason in Drupal. Another possibility is Mootools
Nanne
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Drupal says it is to avoid conflict between JS libraries themselves. – Jérôme Verstrynge May 13 '12 at 18:30
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I meant that although it sounds like they mean different libraries, the same would be true for different versions of JQuery itself. So while a valid reason, I suspect that wasn't the primary goal in this case. – Nanne May 13 '12 at 18:32
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zepto also uses $, and is a lightweight alternative to jQuery:
In addition, it matches most of the jQuery API.
Christophe
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