jQuery UI " $("#datepicker").datepicker is not a function"
Solution 1:
I struggled with a similar problem for hours. It then turned out that jQuery was included twice, once by the program that I was adding a jQuery function to and once by our in-house debugger.
Solution 2:
If there is another library that is using the $ variable, you can do this:
var $j = jQuery.noConflict();
$j("#datepicker").datepicker();
Also make sure your javascript includes are in the correct order so the jquery core library is defined before the jquery.ui. I've had that cause issues.