Ruby on Rails

I past and copy below the content of my application.js file.
The lines '//= require jquery' and '//= require jquery_ujs' should assure that the jquery and jquery-ujs files provided by jquery-rails gem will be added to the asset pipeline and available for use. "The require_tree directive tells Sprockets to recursively include all JavaScript files in the specified directory into the output. These paths must be specified relative to the manifest file".(quoted from guides.rubyonrails.org). So the line '//= require_tree .' is telling Sprokets to include all files which are in the same directory of the manifest file ( . ), included my custom.js file.

// This is a manifest file that'll be compiled into application.js, which will include all the files
// listed below.
//
// Any JavaScript/Coffee file within this directory, lib/assets/javascripts, vendor/assets/javascripts,
// or any plugin's vendor/assets/javascripts directory can be referenced here using a relative path.
//
// It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
// compiled file.
//
// Read Sprockets README (https://github.com/rails/sprockets#sprockets-directives) for details
// about supported directives.
//
//= require jquery
//= require jquery_ujs
//= require bootstrap
//= require turbolinks
//= require_tree .