多件商品购买数量增加减少的js

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>商品数量调整</title>
<script>
function adjustQuantity(itemId, operation) {
    let quantityElement = document.getElementById(`quantity-${itemId}`);
    let currentQuantity = parseInt(quantityElement.value);
    if (!isNaN(currentQuantity)) {
        if (operation === 'increase') {
            quantityElement.value = currentQuantity + 1;
        } else if (operation === 'decrease' && currentQuantity > 1) {
            quantityElement.value = currentQuantity - 1;
        }
    }
}
</script>
</head>
<body>

<div id="item1">
    <button onclick="adjustQuantity('item1', 'decrease')">-</button>
    <input type="text" id="quantity-item1" value="1">
    <button onclick="adjustQuantity('item1', 'increase')">+</button>
</div>

</body>
</html>

function validateQuantity(itemId) {
    let quantityElement = document.getElementById(`quantity-${itemId}`);
    let quantity = parseInt(quantityElement.value);
    if (isNaN(quantity) || quantity < 1) {
        alert("请输入有效的数量！");
        quantityElement.value = 1; // 重置为默认值
    }
}