how to combine ruby code with javascript in rails 7

The condition is set and can be checked on the server and the view is rendered on the server. That means you can simple use ERB to check the condition and then add a <script> block when it is true. The script block will run on the client.

# in the controller
def index
  @posts = Post.all.order(id: :desc)
  @post_loaded = @posts.length > 0
end

# in the view
<h1 class="text-center m-4 text-success">Posts</h1>
<div id="posts" data-controller="like" data-like-url-value="<%= like_posts_path %>">
  <div class="row">
    <% @posts.each do |post| %>
        <%= render post %>
    <% end %>
  </div>
</div>

<% if @post_loaded %>
<script>
  console.log("All Posts has been loaded successfully.")
</script>
<% end %>

I have a solution for the above question, Which you have asked.

app/controllers/posts_controller.rb

def index
   @posts = Post.all.order(id: :desc)
   if @posts.length > 0
     session[:post] = "some value"
   end
end

app/views/posts/index.html.erb

<h1 class="text-center m-4 text-success">Posts</h1>
<div id="posts" data-controller="like" data-like-url-value="<%= like_posts_path %>">
    <div class="row">
        <% @posts.each do |post| %>
            <%= render post %>
        <% end %>
    </div>
</div>
<script>
   let post = "<%= session[:post] %>";
   console.log(post);
</script>

You can also try this; if your application has a lot of code, it will become easier to handle that and improve code quality as well.

app/controllers/posts_controller.rb

def index
   @posts = Post.all.order(id: :desc)
   if @posts.length > 0
     session[:post] = true
   end
end

app/views/posts/index.html.erb

<%= javascript_include_tag "my_javascript" %>
<h1 class="text-center m-4 text-success">Posts</h1>
<div id="posts" data-controller="like" data-like-url-value="<%= like_posts_path %>">
    <div class="row">
        <% @posts.each do |post| %>
            <%= render post %>
        <% end %>
    </div>
</div>

app/javascript/my_javascript.js

if(some condition){
  console.log("All Posts has been loaded successfully.")
}