Why it is needed $ = jQuery

Question is nearly theoretical. But last time I've encountred some subtle bug. When the 开发者_JAVA技巧page contains child frame and both parent and child frame are used the same js file, if I not set $ = jQuery on the start, functionality of this js file crashed. What the trick could be here?

Thanks!

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If there are other JavaScript libraries included anywhere in the page, they might use $. Mootools and Prototype both use $.

If you include jQuery and then you include Prototype then the $ variable will be over-written, so you'd have to set $ = jQuery before any jQuery code would work:

<link rel="stylesheet" href="jQuery.js" />
<link rel="stylesheet" href="prototype.js" />
<script>$ = jQuery</script>

Equally, as @Guffa says, if you called noConflict anywhere it would unset the $ variable.

Ideally, I don't think you should use the $ variable, as it can cause problems. Just do something like:

jQuery.noConflict();
var jQ = jQuery;

And then use the jQ variable as you would have the $. Then you won't get conflicts.

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That should not be needed unless you removed the reference from the $ variable.

Check if you are using the noConflict method somewhere to decouple jQuery from the $ variable, or have something else (like a function) named $.