WooCommerce purchase page add-to-cart with quantity and AJAX, by customising the add-to-cart template in the WooCommerce loop. See blog post for details: http://snippets.webaware.com.au/snippets/woocommerce-add-to-cart-with-quantity-and-ajax/

add-to-cart.php

<?php
/**
* Loop Add to Cart -- with quantity and AJAX
* requires associated JavaScript file qty-add-to-cart.js
*
* @link http://snippets.webaware.com.au/snippets/woocommerce-add-to-cart-with-quantity-and-ajax/
* @link https://gist.github.com/mikejolley/2793710/
*/

// add this file to folder "woocommerce/loop" inside theme

global $product;

if( $product->get_price() === '' && $product->product_type != 'external' ) return;

// script for add-to-cart with qty
wp_enqueue_script('qty-add-to-cart', get_stylesheet_directory_uri() . '/js/qty-add-to-cart.js', array('jquery'), '1.0.1', true);

?>

<?php if ( ! $product->is_in_stock() ) : ?>

    <a href="<?php echo get_permalink($product->id); ?>" class="button"><?php echo apply_filters('out_of_stock_add_to_cart_text', __('Read More', 'woocommerce')); ?></a>

<?php else : ?>

    <?php

        switch ( $product->product_type ) {
            case "variable" :
                $link   = get_permalink($product->id);
                $label  = apply_filters('variable_add_to_cart_text', __('Select options', 'woocommerce'));
            break;
            case "grouped" :
                $link   = get_permalink($product->id);
                $label  = apply_filters('grouped_add_to_cart_text', __('View options', 'woocommerce'));
            break;
            case "external" :
                $link   = get_permalink($product->id);
                $label  = apply_filters('external_add_to_cart_text', __('Read More', 'woocommerce'));
            break;
            default :
                $link   = esc_url( $product->add_to_cart_url() );
                $label  = apply_filters('add_to_cart_text', __('Add to cart', 'woocommerce'));
            break;
        }

        //printf('<a href="%s" rel="nofollow" data-product_id="%s" class="button add_to_cart_button product_type_%s">%s</a>', $link, $product->id, $product->product_type, $label);

        if ( $product->product_type == 'simple' ) {

            ?>
            <form action="<?php echo esc_url( $product->add_to_cart_url() ); ?>" class="cart" method="post" enctype='multipart/form-data'>

                <?php woocommerce_quantity_input(); ?>

                <button type="submit" data-quantity="1" data-product_id="<?php echo $product->id; ?>"
                    class="button alt ajax_add_to_cart add_to_cart_button product_type_simple"><?php echo $label; ?></button>

            </form>
            <?php

        } else {

            printf('<a href="%s" rel="nofollow" data-product_id="%s" class="button add_to_cart_button product_type_%s">%s</a>', $link, $product->id, $product->product_type, $label);

        }

    ?>

<?php endif; ?>

qty-add-to-cart.js

/*!
script for WooCommerce add to cart with quantity, via AJAX
Author: support@webaware.com.au
Author URI: http://snippets.webaware.com.au/
License: GPLv2 or later
Version: 1.0.1
*/

// @link http://snippets.webaware.com.au/snippets/woocommerce-add-to-cart-with-quantity-and-ajax/
// @link https://gist.github.com/mikejolley/2793710/

// add this file to folder "js" inside theme

jQuery(function ($) {

    /* when product quantity changes, update quantity attribute on add-to-cart button */
    $("form.cart").on("change", "input.qty", function() {
        if (this.value === "0")
            this.value = "1";

        $(this.form).find("button[data-quantity]").data("quantity", this.value);
    });

    /* remove old "view cart" text, only need latest one thanks! */
    $(document.body).on("adding_to_cart", function() {
        $("a.added_to_cart").remove();
    });

});

i'm using Woocommerce 4.8 and i got this:
" Fatal error: Uncaught Error: Call to a member function get_price() on null "

Hello,
I was wondering if it was possible when you click on the "-" button (in my archive page) that removes the quantity of the product in the cart (in ajax)?

I want to reproduce the same system as in the basket, add or delete the ajax quantities with the minus and the plus.

Do you have an idea please?