function storeProvision(stock, ordered) {
    let shop = {};
    addToShop(ordered, addToShop(stock, shop));
    function addToShop(arr, obj) {
        for (let i = 0; i < arr.length; i += 2) {
            let product = arr[i];
            let quantity = Number(arr[i + 1]);
            if (!obj.hasOwnProperty(product)) obj[product] = 0;
            obj[product] += quantity;
        }
        return obj;
    }
    Object.entries(shop).forEach(el => console.log(`${el[0]} -> ${el[1]}`));
}