[JavaScript] How to display an alert when there is an input form omission

Introduction

I am creating an EC site with Rails. When adding an item to the cart, I wanted to display a pop-up like the one below if there was no quantity selected. This time is the memorandum.

Supplementary information

• Ruby 2.5.1
• Ruby on Rails 5.2.4.2

Form contents

The page in the above picture is the following View file.

.showMain
  .showMain__Boxes
    .showMain__Boxes__leftBox
      = image_tag @product.image, width: 450, height: 450, class: "showMain__Boxes__leftBox__image"
    .showMain__Boxes__rightBox
      = form_with url: add_item_carts_path, local: true, name: "formForCart" do |f|
        .showMain__Boxes__rightBox__name
          = @product.name
        .showMain__Boxes__rightBox__namejap
          = @product.namejap
        .showMain__Boxes__rightBox__description
          = @product.description
        .showMain__Boxes__rightBox__orderQuantity
          = f.label :quantity, "quantity", class: "showMain__Boxes__rightBox__orderQuantity__title"
          = f.select :quantity, stock_array_maker(@stock), {include_blank: "Please select"}, {class: "showMain__Boxes__rightBox__orderQuantity__form"}
        .showMain__Boxes__rightBox__line
          .showMain__Boxes__rightBox__line__price
            = "Base price: ¥#{convertToJPY(@product.price)}"
          .showMain__Boxes__rightBox__line__fav
            - if user_signed_in? && current_user
              - if @product.bookmark_by?(current_user)
                = link_to product_bookmark_path(@product.id), class: "showMain__Boxes__rightBox__line__fav__btn.fav", method: :delete, remote: true do
                  %i.fas.fa-star.star
              - else
                = link_to product_bookmarks_path(@product.id), class: "showMain__Boxes__rightBox__line__fav__btn.fav", method: :post, remote: true do
                  %i.far.fa-star.star-o (Add to Favourites)
        .showMain__Boxes__rightBox__line
        .showMain__Boxes__rightBox__addCart
          = f.hidden_field :product_id, value: @product.id
          = f.hidden_field :cart_id, value: @cart.id
          = f.submit "Add to Cart", {class: "showMain__Boxes__rightBox__addCart__btn", id: "addToCart"}

How to write JS

I'm getting two nodes, a submit tag and a quantity select tag.

$(function(){
  $("#addToCart").on('click', function(e){
    if(document.getElementById('quantity').value === ""){
      alert("Please select a quantity.");
      return false;
    }else{
      return true;
    }
  })
});

Set to fire when the submit button of the form is pressed, and if there is no id value of = f.select: quantity ~, it returns false and warns with an alert. If the return value is true, it will be submitted, and if it is false, it will not be submitted. By the way, if there is no "return false", a pop-up will appear, but you will be taken to the next page.

The above JS can also be done by the following method

$(function(){
  $("#addToCart").on('click', function(e){
    if(!(document.getElementById('quantity').value)){
      alert("Please select a quantity.");
      return false;
    }else{
      return true;
    }
  })
});

Reference site

How to check form input with JavaScript [JavaScript] How to check empty strings [Study]