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Rails 5 - Rails-ujs has already been loaded

I have put the tag javascript into my body application.
In my console I am getting this error:
Rails-ujs has already been loaded.
In development mode the app works, however it doesn't work in production.
I have deployed my rails application on Heroku.
I try to move the tag javascript into the head, but then all function js is not working.
Any solution?

If you're using webpack and a separate front-end, you might have two instances of your javascript tag in application.html.
= javascript_include_tag 'application', 'data-turbolinks-eval' => false
= javascript_pack_tag 'application'
You'll want to remove the first instance; similarly if you've used pack_tag to render your stylesheets, you may be experiencing CSS classes being called twice (you'll see stacked classes on an element in dev tools).

I solved bringing my javascript into the head and simply including all my functions below:
$ (document).on('turbolinks: load', function () {
})

If you do not use webpack then use as like:
<%= javascript_include_tag 'application', 'data-turbolinks-track': 'reload', 'data-turbolinks-eval': false %>
<%= javascript_include_tag 'frontend_application', 'data-turbolinks-track': 'reload' %>
</body>
in application.js file
//= require rails-ujs
//= require activestorage
//= require turbolinks
in frontend_application.js file
//= require compress.min
//= require custom

Rails_Javascript loads only if I pass by index home page

I am facing a javascript issue on Rails;
I used this web app in order to publish a book rating system
I've noticed that when I go to my homepage (index), then I click on the book that I want to rate (, the script loads and everything works fine, but when I go directly to the book without passing by the home page, the script of rating will not load
on application.js
//= require jquery
//= require jquery_ujs
//= require bootstrap-sprockets
//= require turbolinks
//= require_tree .
on the head of application.html.erb
<%= stylesheet_link_tag 'application', media: 'all', 'data-turbolinks-track' => true %>
<%= javascript_include_tag 'application', 'data-turbolinks-track' => true %>
<%= csrf_meta_tags %>
What am I missing ?

It may be caused by turbolinks which uses AJAX to view templates to speed things up. But it will cause JavaScripts to not load without a page refresh. You can try removing the turbolinks gem from your gemfile and run bundle install. Alternatively you can wrap the raty javascript with the following. So put the first line at the beginning of your javascript and close your javascript with the last line.
$(document).on('turbolinks:load', function() {
...your jQuery/javascript goes here...
});

Rails Javascript onchange works with <script> but not externally

In my index.html.erb file I have a form that successfully submits onChange. However, this only works if I use <script> tags placed after my form. Why is the same js in welcome.js not running? Index loads as a /welcome route.
# app/assets/javascripts/application.js
//= require jquery
//= require jquery_ujs
//= require jquery-ace.min
//= require jquery.sortable
//= require chartkick
//= require hammer
//= require_tree .
External js file
# app/assets/javascripts/welcome.js
$('.chart-radio-icon').change(function(){
$('#mega_form').submit();
});
View
# views/welcome/index.html.erb
<%= form_tag mega_chart_path, id: 'mega_form', remote: true do %>
<%= radio_button_tag :metric_type, :followers_count, true, :class => 'chart-radio-icon'%>
<%= label_tag(:followers, 'Followers', :class => 'chart-label chart-label-social') %>
Again, the js works when placed in <script> tags in the view, but not externally.

EDIT:
Since we've determined that your asset files are not being included in development. For a sanity check try this and restart your server:
Rails.application.config.assets.precompile = %w( application.js application.css )
END EDIT
Since you are using a require_tree ., and since the welcome.js file is in app/assets/javascripts, it will be included on all pages in your application. See here.
I think that the problem is that on page load, the event listener is not being assigned to the DOM element. Try wrapping your JS code in:
#app/assets/javascripts/welcome.js
$(document).ready(function(){
....
});

Rails 3.2.13 Javascript Asset Pipeline

I am working on a Rails 3.2.13 app, and encountering the following problem:
The Javascript files I wrote in the app/assets/javascripts/ directory don't seem to be run.
I have JS code in a file called menu_list.js in the app/assets/javascripts/ directory, but it does not do anything. Whereas when I put the same code on the view page in a tag, it works.
<script>$("#sortable").sortable();</script>
Here's my application.js file:
//= require jquery
//= require jquery_ujs
//= require jquery.purr
//= require jquery-ui
//= require best_in_place
//= require bootstrap
//= require bootstrap-switch
//= require_tree .
my menu_list.js file:
$(function() {
$("#sortable").sortable();
});
It does not make sense to me, as I thought //= require_tree . would include all the javascript files in the directory. I tried other javascript files too, but they don't seem to have any effects.

$("#sortable").sortable();
If this truly is all you have in app/assets/javascripts/menu_list.js it will run as soon as the script is loaded, which is not what you want. You want to run the above code at least after the DOM has fully loaded into the page and not before.
$(function() {
$("#sortable").sortable();
});
If the environment you're running in is :development, and you properly have the following in your layout
<%= javascript_include_tag "application" %>
you will see a separate <script> tag for every Javascript file Sprockets is loading. The //= require_tree . will pick up your app/assets/javascripts/menu_list.js file.
I suggest also making sure you don't have any precompiled version of your assets in place by running the following in shell
rake assets:clean
You can also force debug mode (individual <script> tags for each file) by adding a :debug option to the above include tag
<%= javascript_include_tag "application", debug: true %>

Most common cause for this is that the call to javascript_include_tag that is on the default layout, then you create another layout, forget to add the include there and then nothing works in controllers where you use this new layout.

Rails javascript files (other than application.js) not working

I'm working on implementing jQuery into a rails project, and have this Javascript tag in the head of my application.html.erb file:
<%= javascript_include_tag "application" %>
And if I put jQuery code into my application.js file, it works as expected in my application. However, if I put the jQuery code into any other .js files in my assets/javascripts folder (home.js, for example), it doesn't work as it should in my application.
Here's my application.js file:
//= require jquery
//= require jquery_ujs
//= require jquery-ui
//= require_tree .
It was my impression that the last line of that meant that all my other js files would be compiled into application.js, so having just the <%= javascript_include_tag "application" %> in my application.html.erb file would be sufficient to make all my js files work -- is this a faulty assumption?
These are the contents of my home.js file -- this function works as it should when I put it into my application.js file directly but when it's in my home.js file instead it doesn't work.
$(document).ready(function(){
$("h2").click(function(){
$(this).hide();
});
});

Try to add //=my home in application.js file above //=require_tree .
Hope this helps.