Product IDs not being added to json ajax?

Question

JavaScript

When I click on the button, I try to pass the id of this product and send an ajax request. Ajax should give me json
and whether there is already such an id or not. But ajax throws an error, it doesn't like dataType: "json".
https://codepen.io/anon/pen/LJdvgQ

$(function () {
  var arr = [];
    $(document).on('click', '.product-btn', function (event) {
        event.preventDefault();

        var id_product = $(this).data("id");
        arr.push(id_product);

       $.ajax({
            type: 'POST',
            url: 'ajax.php',
            contentType: 'application/json',
            data: JSON.stringify(arr),
            dataType: "json",
            success: function (data, id_product) {
            	
                        if(arr.indexOf(id_product) != -1){ 
                                    alert("Такой товар уже существует");
                        } else {
                                    arr.push(id_product);
                                    alert("Товар успешно добавлен");
                        }
             },
            error: function (data) {
                console.error("Ошибка при работе с избранным");
            }
        });

    });
});

Answer 1

Try making var arr = [] - an array, not an object var arr = {} like you have now.