Increment or decrement quantity of product in cart

line-item-controls.twig

{% set incrementable = incrementable ?? true %}
{% set decrementable = decrementable ?? true %}

<div class="line-item__controls">
    {% if incrementable or decrementable %}
        <div class="line-item__quantity">
            <form
                method="POST"
                data-ajax>
                {{ csrfInput() }}
                {{ actionInput('commerce/cart/update-cart') }}
                {{ hiddenInput("lineItems[#{lineItem.id}][qty]", lineItem.qty - 1) }}

                <button
                    class="line-item__quantity-button line-item__quantity-button--decrement"
                    type="submit"
                    title="Remove One"
                    {% if not decrementable %}disabled{% endif %}>&minus;</button>
            </form>

            <div class="line-item__quantity-readout">{{ lineItem.qty }}</div>

            <form
                method="POST"
                data-ajax>
                {{ csrfInput() }}
                {{ actionInput('commerce/cart/update-cart') }}
                {{ hiddenInput("lineItems[#{lineItem.id}][qty]", lineItem.qty + 1) }}

                <button
                    class="line-item__quantity-button line-item__quantity-button--increment"
                    type="submit"
                    title="Add One"
                    {% if not incrementable %}disabled{% endif %}>&plus;</button>
            </form>
        </div>
    {% endif %}

    <div class="line-item__remove">
        <form
            method="POST"
            data-ajax>
            {{ csrfInput() }}
            {{ actionInput('commerce/cart/update-cart') }}
            {{ hiddenInput("lineItems[#{lineItem.id}][remove]", 'Yes, please!') }}

            <button
                class="line-item__remove-button"
                type="submit">Remove</button>
        </form>
    </div>
</div>

line-item.twig

{# Info about your line item... #}

{# Then, include the partial (optionally passing `incrementable` or `decrementable` booleans, depending on whether it's possible to modify quantity) #}
{% include '__blocks/line-item__controls' with {
    lineItem: lineItem
} only %}