Rails include javascripts assets folder recursively
//= require_tree .
will require everything in the current directory of your application.js
//= require_tree ./js_library
will require everything in the js_library
subdirectory if it is under app/assets/javascripts
If you are are trying to load javascripts under vendor/assets/javascripts, try:
//= require_tree ../../../vendor/assets/javascripts/js_library