How to connect javascript files in Rails application?

Question

I am writing a simple Ruby on Rails training application and using semantic-UI CSS framework. From the dropdown menu need to write a little code for jquery. If I write .js code directly in HTML code, it works. If I put it in a separate file app/assets/javascripts/dropdown.js this code stop working. But! If use .coffee, the code works again! How to connect js files to Rails?

So, i have javascript/dropdown.js. I tried to include it in an application.js file, and although this is not necessary, it did not work. I write this code directly in application.js - not work