Linked to from: https://support.boldcommerce.com/hc/en-us/articles/360000120406

bold-checkout-loader.liquid

<script type="application/javascript">
    if (typeof BOLD === 'undefined') {
        BOLD = {};
    }
    BOLD.checkout = {
        getCookie: function getCookie(name) {
            var value = '; ' + document.cookie;
            var parts = value.split('; ' + name + '=');
            if (parts.length === 2) {
                return parts.pop().split(';').shift();
            }
        },
        getPath: function getPath(url) {
            var parser = document.createElement('a');
            parser.href = url;
            return parser.pathname;
        },
        isCheckoutButton: function isCheckoutButton(elem) {
            if (elem.form) {
                var action = elem.form.getAttribute('action');
                if (action.search(/^\/checkout\b/) !== -1) {
                    return true;
                }
                if (action.search(/^\/cart\b/) !== -1 && elem.getAttribute('name') === 'checkout') {
                    return true;
                }
                var path = BOLD.checkout.getPath(action);
                if (path.search(/^\/checkout\b/) !== -1) {
                    return true;
                }
                if (path.search(/^\/cart\b/) !== -1 && elem.getAttribute('name') === 'checkout') {
                    return true;
                }
            }
            return false;
        },
        initialize: function initialize() {
            document.addEventListener('click', function (event) {
                if (BOLD.checkout.isCheckoutButton(event.target)) {
                    event.target.form.action = '/apps/checkout/begin-checkout?shop=' + Shopify.shop;
                    var cartCookie = BOLD.checkout.getCookie('cart');
                    var element = document.createElement('INPUT');
                    element.type = 'HIDDEN';
                    element.name = 'cart_id';
                    element.value = cartCookie;
                    event.target.form.appendChild(element);
                  if (typeof BOLDCURRENCY !== 'undefined') {
                    var currencyElement = document.createElement('INPUT');
                    currencyElement.type = 'HIDDEN';
                    currencyElement.name = 'bold_cart_params[currency_iso_name]';
                    currencyElement.value = BOLDCURRENCY.currencyCookie.read();
                    event.target.form.appendChild(currencyElement);
                  }
                }
            });
        }
    };
    (function () {
        BOLD.checkout.initialize();
    })();
</script>