Developer forum

You need two calls for this: one that empties the cart and one that add the current product to the cart.

One way to do this (using jQuery) is like this

<button id="onlyThisInCart">Only this in cart</button>
<script>(function($) {
    var url = '<!--@Global:Pageview.Url.Raw.JSEncoded()-->';
    $('#onlyThisInCart').on('click', function () {
        // Empty the cart
        $.post(url, {
            'CartCmd': 'emptycart'
        })
            .done(function() {
                // Add current product to cart. Assumes that productId is in current url
                // Note: we should actually make a POST when adding the product, but it's just an example
                document.location.href = url+(url.indexOf('?') < 0 ? '?' : '&')+ 'CartCmd=add';
            })
    });
}(jQuery))</script>

Best regards,
Mikkel