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Can't make JS plugins work with Rails 5.1 and Yarn

I'm trying to install fullpage.js plugin in a Rails 5.1 app following this guide's advice but haven't been successful. The steps I've followed are:
Entered yarn add fullpage.js in the command line
Checked that the plugin is listed in package.json (the plugin is indeed listed under dependencies as "fullpage.js": "^2.9.4")
Declare the plugin in the JS manifest at app/assets/javascripts/application.js using //= require fullpage
Unfortunately I get an exception when loading the webpage (
couldn't find file 'fullpage' with type 'application/javascript'
)
I've tried using //= require fullpage/fullpage and //= require fullpage.js (this last one as I noticed no other dependencies are listed in package.json with their JS extension.
None of those variations have worked.
Could you help me understand what I'm doing wrong?

I found what was the problem.
It was simply the way I was referencing the asset files in the manifests. To do it correctly I had to go into the node_modules folder (in the root directory) find the folder where fullpage.js was installed and check inside in which folder the js and css file where exactly located.
In this case they happened to be located in node_modules/fullpage.js/dist/ so I had to reference them in the manifest as following:
#the js file
//= require fullpage.js/dist/jquery.fullpage
#the css file
*= require fullpage.js/dist/jquery.fullpage
Note that the name of the file I was referencing were "jquery.fullpage.js" and "jquery.fullpage.css" so I'm just leaving the extensions out as usual.

Why do we have to require the JS files inside application.js in rails?

I am new to rails so pardon the question. Now, why do we have to require the JS files inside the application.js file or the css files inside the application.css file? As far as I have read that when we launch the server rails loads all the javascript and css files from the directory into one file, so if it already loads all the files from the directory why there is a need to write inside the application.js or application.css file?
For example:
//= require abc
//= require xyz
If I already have abc.js and xyz.js file, why should I require them inside application.js file?

You are misunderstanding the concept. Let me explain the process. As you know, when you launch the server, rails first precompiles the files inside the assets folder with the help of sprockets-rails gem, but it does so by following the directives specified inside the manifest files i.e application.js and application.css.
Now inside the application.js you have "//= require_tree .", this tells sprockets to load all the files inside the javascript directory, process them, compress and combine them to produce one master Javascript file, this helps to reduce the page load time of the website. Now,here is your question, since "//= require_tree ." directive already takes all the javascript files present inside the javascript directory, why there is a need to specify the javascript files inside application.js? The answer is "Order".
"//= require_tree ." it loads, compress and combine all the JS files in an unspecified order or random order. Now, if you are a web developer or have started now, you might know or will come to know that many times you have to load JS files in some specific order, otherwise there may arise some conflict when we implement them, they might not work as we want them to.
One such famous combo is jquery and bootstrap. In order to use bootstrap JS part it needs jQuery, so you have to initialise jquery first and then bootstrap. Precisely for this reason in rails you require the files inside application.js specifying the order in which you want the sprockets to load,compress and combine into one master JS file. As sprockets processess the directives from top to bottom in the order specified in the application.js file, it becomes important to require the files in application.js file. If you have 2 javascript or css files which in no way connflict with each other then, there is no need for you to require the files inside application.js or application.css file and they will still work fine.
For example:
//= require jquery
//= require jquery_ujs
//= require bootstrap.min
//= require_tree .
Above, the sprockets-rails gem will first load jquery.js file then jquery_ujs.js file and then bootstrap.min.js file in that order. There is no need to add the extension as it assumes that all the files will be of type javascript only.
The above whole explanation also applies for the precompilation of css files specified inside application.css.
For more information, I advise you to visit http://guides.rubyonrails.org/asset_pipeline.html and read about rails asset pipeline.

Bootstrap-sass gem Javascript not Working in Rails 4

when using the bootstrap-sass gem i have loaded my css files properly, however have been receiving an error when i try loading the javascript files. I have followed the procedure of loading javascript from https://github.com/twbs/bootstrap-sass however when I view my webpage on localhost I receive the error "couldn't find file 'bootstrap'"
This is what my application.js file looks like - Any help would be much appreciated!
Application.js
// 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 vendor/assets/javascripts of plugins, if any, 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/sstephenson/sprockets#sprockets-directives) for details
// about supported directives.
//
//= require jquery
//= require jquery_ujs
//= require turbolinks
// Loads all Bootstrap javascripts
//= require bootstrap
//= require_tree .

From the Rails 4 the :assets group isn't used anymore, please remove the group from Gemfile:
Rails 4.0 removed the assets group from Gemfile. You'd need to remove that line from your Gemfile when upgrading. You should also update your application file (in config/application.rb):
Bundler.require(:default, Rails.env)

I got it to work by removing gem 'bootstrap-sass', '~> 3.0.3.0' out of the assets group in gemfile! YAY

Did you bundle install and make sure you are loading application.js (from the assets pipeline).

jQuery/Javascript runs on local but not on Heroku

This is my app on Heroku: http://mighty-brushlands-6367.herokuapp.com/ and this is how it SHOULD look, as it does locally:
Well I did some googling. I check this solution: JQuery events are not working on heroku in production but work in development and others.
I went ahead to run heroku run rake assets:precompile but still can't get the effects to kick in on Heroku. This is how my application.js looks like:
// 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 vendor/assets/javascripts of plugins, if any, 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/sstephenson/sprockets#sprockets-directives) for details
// about supported directives.
//
//= require jquery
//= require jquery_ujs
//= require turbolinks
//= require_tree .
//= require bootstrap
//= require textEffect
I need some guidance. I'm a hungry-for-knowledge newbie.

Your Heroku app is getting a 404 error on the application.js & application.css files which leads me to believe it is not getting pushed to Heroku in the first place.
Try running rake assets:precompile in development, then the usual steps git add . , then git commit, and then git push.

The solution was to add
group :production, :staging do
gem 'rails_12factor'
end
To the Gemfile and commit again to Heroku

What can cause Rails 3.1 "= require jquery" to stop working?

I'm porting a Rails 3.0.9 app to Rails 3.1.rc5. My application.js is exactly the same as one generated by Rails 3.1 itself:
// This is a manifest file ...
//
//= require jquery
//= require jquery_ujs
//= require_tree .
But when I run my app and look at the application.js in Firebug or Chrome Developer Tools, all I see is:
// This is a manifest file ...
//
The directives are gone, so it would seem that the file has been processed by Sprockets, but the directives have not been replaced by the contents of jquery et al. There are no errors appearing on the server console or in the logs.
Curiously, when I run a blog app (you know, the canonical tutorial app) it works fine (that is, when I examine application.js in Firebug, it contains the text of jQuery.) This would seem to indicate that something in my app is somehow interfering with Sprockets. Has anyone out there heard of such an issue (and hopefully a workaround)?
Here's my setup:
$ gem list jquery
*** LOCAL GEMS ***
jquery-rails (1.0.12)
$ ruby -v
ruby 1.9.2p290 (2011-07-09) [i386-mingw32]
$ rails -v
Rails 3.1.0.rc5
I'm at a loss as to what might be wrong. I've triple checked my Gemfile; I've run and re-run bundle install and bundle update; I've tried rc3, rc4 and now rc5; I'm running Ruby 1.9.2p290. Any ideas?

One workaround: include JavaScript files with the old-skool tag. For example, in my (Haml) layout:
= javascript_include_tag '/assets/jquery.js'
= javascript_include_tag '/assets/jquery_ujs.js'
= javascript_tag 'jQuery.noConflict();'
The /assets/ prefix tells Rails 3.1.x to look on the asset path, which includes gems, so you'll get the same files as with Sprockets directives. But you won't get concatenation or any other Sprockets preprocessing.
Still looking for better solutions.