var itemId = $(e.currentTarget).data('item-id');
document.cookie = 'clicked_item_id=' + itemId.toString();

cookies[:clicked_item_id]

cookies["clicked_item_id"]

What I wanna do is this:
When users click an item, if he's not logged in,
pop up modal and let him go to login form(save item_id in JS).
After logged in(read item_id in Rails), redirect to the item detail page.
Because the pop up modal is implemented by JS, I must share values between JS and Rails.