theme.html

<script>   
(function () {
        function getCheckoutButtons() {
            return Array.from(document.querySelectorAll('[name=checkout], [href*=checkout]:not([href*=\'/tools/checkout/front_end/login\']), [action*=checkout] [type=submit], [onclick*=checkout], .additional-checkout-button:not(.additional-checkout-button--apple-pay)'));
        }

        function getCart(callback) {
          var xhr = new XMLHttpRequest();
          xhr.open('GET', '/cart.json', true);
          xhr.setRequestHeader('Accept', 'application/json');
          xhr.setRequestHeader('Content-Type', 'application/json');
		  xhr.onreadystatechange = function() {
              if (this.readyState === XMLHttpRequest.DONE && this.status === 200 && callback) {
                  callback(JSON.parse(xhr.responseText));
              }
          }
          
          
          xhr.send();
        }

        function variantsToHave(item) {
            return item.properties && item.properties['_configurationVariants'] && JSON.parse(item.properties['_configurationVariants']) || [];
        }

        function handleQty(qty, variants) {
            return [].concat.apply([], Array(qty).fill(variants));
        }

        function getVariants(items) {
            return items.reduce((acc, val) => acc.concat(val), []).reduce(function (acc, variant) {
                acc[variant] = (acc[variant] || 0) + 1;
                return acc;
            }, {});
        }

        function diff(toHave, current) {
            return Object.keys(toHave).reduce(function (acc, item) {
                acc[item] = toHave[item] - (current[item] || 0);
                return acc;
            }, {});
        }

        function nonZeroQty(cartDiff) {
            return Object.keys(cartDiff).reduce(function (acc, item) {
                if (cartDiff[item]) {
                    acc[item] = cartDiff[item];
                }

                return acc;
            }, {});
        }

        function zeroQty(cartDiff) {
            return Object.keys(cartDiff).reduce(function (acc, item) {
                acc[item] = 0;

                return acc;
            }, {});
        }

        function asCartUpdate(cartDiff) {
            return {
                updates: cartDiff
            };
        }

        function updateCart(toUpdate, callback) {
          var xhr = new XMLHttpRequest();
          xhr.open('POST', '/cart/update.js', true);
          xhr.setRequestHeader('Accept', 'application/json');
          xhr.setRequestHeader('Content-Type', 'application/json');
		  xhr.onreadystatechange = function() {
              if (this.readyState === XMLHttpRequest.DONE && this.status === 200 && callback) {
                  callback();
              }
          }
          
          
          xhr.send(JSON.stringify(toUpdate));
        }

        function toUpdate(cart) {
            var variants = getVariants(cart.items.map(function (item) {
                return handleQty(item.quantity, variantsToHave(item));
            }));
            var currentVariants = cart.items.filter(function (item) {
                return Object.keys(variants).map(function (id) {
                    return Number.parseInt(id);
                }).includes(item.id);
            }).reduce(function (acc, item) {
                acc[item.id] = item.quantity;
                return acc;
            }, {});

            return diff(variants, currentVariants);
        }
      
        function toUpdateZero(cart) {
            var variants = getVariants(cart.items.map(function (item) {
                return handleQty(item.quantity, variantsToHave(item));
            }));
            var currentVariants = cart.items.filter(function (item) {
                return Object.keys(variants).map(function (id) {
                    return Number.parseInt(id);
                }).includes(item.id);
            }).reduce(function (acc, item) {
                acc[item.id] = item.quantity;
                return acc;
            }, {});

            return currentVariants;
        }

        function needUpdate(update) {
            return Object.keys(update.updates).length > 0;
        }
      
        function needUpdateZero(update) {
            const keys = Object.keys(update.updates);
            return keys.length > 0 && keys.every(k => k > 0);
        }

        function needReload() {
            window.location.reload(false);
        }

        document.addEventListener('DOMContentLoaded', function () {
            getCheckoutButtons().forEach(function (button) {
                var zakekeHandled = false;
                button.addEventListener('click', function (e) {
                    if (zakekeHandled) {
                        return;
                    }

                    e.preventDefault();
                    getCart(function(cart) {
                        var cartUpdate = asCartUpdate(nonZeroQty(toUpdate(cart)));
                        if (needUpdate(cartUpdate)) {
                            updateCart(cartUpdate, function () {
                                zakekeHandled = true;
                                e.target.click();
                            });
                        } else {
                        	zakekeHandled = true;
                            e.target.click();
                        }
                    });
                });
            });
        });

      
      if (!window.location.pathname.includes('/cart')) {
        getCart(function(cart) {
          var cartUpdate = asCartUpdate(nonZeroQty(toUpdate(cart)));
          if (needUpdate(cartUpdate)) {
            updateCart(cartUpdate);
          }
        });
      } else {
        getCart(function (cart) {
            var cartUpdate = asCartUpdate(zeroQty(toUpdateZero(cart)));
            if (needUpdate(cartUpdate)) {
                updateCart(cartUpdate, needReload);
            }
        });
        
      }
    })();
</script>