So, in case you have been stuck in this for hours (like I was), the problem is that you must surround j render statement with double quotes

cart_items.innerHTML = <%= j render 'cartItems'%>

should be

cart_items.innerHTML = "<%= j render 'cartItems'%>"

I have a cart that contains items, in the checkout page these items can be reviews and their quantity cna be changed. I want to add a button that, when clicked, will refresh the partial that is rendering the cart thus updating totals etc.
I have created the button and it changes the innerHtml its supposed to (when I test it with (“test”) instead of the <% j render…%>, however it will not update the cart (reload the partial).
Heres my code

Route
get 'refresh_cart', to: 'carts#refresh'

Controller

def refresh
    @cart = Cart.find_by(user_id: current_user.id)
    respond_to do |format|
      format.js  {}
    end
  end

html

<div class="container-fluid cart-submit" id="cart-item-partial">
  <%= render 'cartItems' %>
</div>

refresh.js.erb

cart_items = document.getElementById("cart-item-partial")
cart_items.innerHTML = <%= j render 'cartItems'%>