tracks/app/views/todos/update.js.erb

<%- object_name = unique_object_name_for("update_todo_#{@todo.id}") -%>
var <%=object_name%> = {
  <%= render :partial => (@saved ? "update_successful" : "update_has_errors"), locals: {object_name: object_name} %>
}
<%=object_name%>.animate();