update quantity stock in mysql column

I'm trying to create a basic storefront through node.js and have to update the stock_quantity based on user input, but i am having trouble figuring out how to not set the value, but subtract from the current value.

I have provided a function of my code that would run to get this information. I am able to set the value through workbench, but unable through node.js I just get a OKPacket {fieldCount:0, affectedRows:0...etc).Any help is appreciated.

products = table, stock_quantity = column, item_id = column/ primary key

//updates the inventory and gives the total price and item description
function checkOut(answer, amount) {
        var item_id = answer;
        var quantity = amount;
        var stock_quantity = stock_quantity - quantity;
        // answer and amount have successfully moved!!
        console.log("\n(consolelog)quantity: " + quantity + "\n(consolelog)item_id: " + item_id);

        connection.query("UPDATE products SET stock_quantity = ? WHERE item_id = ?",
         ['stock_quantity', 'item_id'], function(error, results) {
                if (error) throw error;
                // console.log("\nProduct Name: " + results.product_name + "\nQuantity: " + results.stock_quantity);
                console.log(results);
        // shopAgain();
        });
}       

via Christian A