Rails应用中的jQuery Uncaught ReferenceError：未定义变量

Question

I am building a Shopping Application with Rails and jQuery. Rails Version 5.1.6. I have an inventories controller and the corresponding inventories.js.

inventories.js

document.addEventListener("turbolinks:load", function() {

   function add_item_to_cart(target_elem) {
     item = $(target_elem).closest('.grid-item')[0].dataset;         
     quantity = $(target_elem).next().val();
     // and some operation wih the quantity element and item.    
   }

    $('.add-to-cart').on('click',function(){
      add_item_to_cart(this);       
    })
})

The problem is whenever I do $(target_elem).next().val() from console I am getting the value from the quantity input box corectly. But when I execute the jquery inventories.js it says

Uncaught ReferenceError: quantity is not defined
    at add_item_to_cart (inventories.self-65782850f63f3dad515dba7e67e72a65016e58fb0751ff2f6342d4183d7d2a0e.js?body=1:10)
at HTMLButtonElement.<anonymous> (inventories.self-65782850f63f3dad515dba7e67e72a65016e58fb0751ff2f6342d4183d7d2a0e.js?body=1:54)
at HTMLButtonElement.dispatch (jquery.self-bd7ddd393353a8d2480a622e80342adf488fb6006d667e8b42e4c0073393abee.js?body=1:5227)
at HTMLButtonElement.elemData.handle (jquery.self-bd7ddd393353a8d2480a622e80342adf488fb6006d667e8b42e4c0073393abee.js?body=1:4879)

Also my jquery always returns something like jQuery.fn.init when selecting an element using a selector. Is this an expected behaviour? because I have not faced anything like this before.

item = $(target_elem).closest('.grid-item')
jQuery.fn.init [div.grid-item, prevObject: jQuery.fn.init(1), context: button.add-to-cart]

In my Gemfile

gem 'jquery-rails'
gem 'jquery-turbolinks'

Answer 1

The behaviour is right, in the javascript you need to denote variable with var or const or let :

var quantity = $(target_elem).next().val();

Otherwise it is undefined variable.