使用按钮f增加和减少数量

<button id="increaseBtn">增加</button>
<span id="quantity">0</span>
<button id="decreaseBtn">减少</button>

var increaseBtn = document.getElementById("increaseBtn");
var decreaseBtn = document.getElementById("decreaseBtn");
var quantityElement = document.getElementById("quantity");

increaseBtn.addEventListener("click", function() {
  var quantity = parseInt(quantityElement.textContent);
  quantity++;
  quantityElement.textContent = quantity;
});

decreaseBtn.addEventListener("click", function() {
  var quantity = parseInt(quantityElement.textContent);
  if (quantity > 0) {
    quantity--;
    quantityElement.textContent = quantity;
  }
});