Server expressions are not resolved when they are placed outside of the ASPX page. So if you place them in external JS file you will end up with a string representation of the statement instead of an object.
One workaround for this is to use global variable in which to store the object returned by the statement. Then in your external file to use the $find() function and to pass the global variable as a parameter to this method.
All the best,
the Telerik team
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