Script you can execute on a BigCommerce Stencil storefront to delete existing cart for current session.

findAndDeleteCart.js

const myStorefrontUrl = "http://my-storefront.com"
findAndDeleteCart(myStorefrontUrl);

function findAndDeleteCart(storefrontUrl) {
    fetch(storefrontUrl + "/api/storefront/carts", {
        "method": "GET",
        "headers": {
          "Content-Type": "application/json"
        }
      })
      .then(response => response.json())
      .then(cart => {
        if (cart.length < 1) return;

        return fetch(storefrontUrl + "/api/storefront/carts/" + cart[0].id ,{
            "method": "DELETE",
            "headers": {
              "Content-Type": "application/json"
            }
        });
      })
      .then(response => console.log(response))
      .catch(err => console.log(err));
}