$ Is Not Defined, Uncaught Referenceerror While Using Select2 Plugin With Rails

I'm trying to use the Select2 plugin to add a search bar to my select box. I get the following error when looking at the page console : Uncaught ReferenceError: $ is not defined

Solution 1:

dont use //= require_tree if you already include all you scripts manualy. this can be the cause of errors. remove this string and try to reload page

p.s i also think what you are using incorrect sintax

//= require_tree

correct version contain dot

//= require_tree .

p.p.s. not about your main question, but you are using old version of ruby. check the newer

one more important thing what i didn't mentiond before. you are using defer="defer" attribute for deffering main script loading. inline javascript what you type on page loaded BEFORE main scripts