JavaScript로 장바구니 결제를 구현하는 방법

<div id="container">
    <ul>
        <li>
            <input type="checkbox" name="liCheck" id="" value="">
            <button type="button" class="decrease" disabled="disabled">-</button>
            <p>
                <span class="piece">0</span>件
            </p>
            <button type="button" class="increase">+</button>
            <p>
                <span class="price">10</span>元
            </p>
            <p>
                小计:
                <input class="smallPrice" type="text" name="" id="" value="0" disabled="disabled">
            </p>
        </li>
        <li>
            <input type="checkbox" name="liCheck" id="" value="">
            <button type="button" class="decrease" disabled="disabled">-</button>
            <p>
                <span class="piece">0</span>件
            </p>
            <button type="button" class="increase">+</button>
            <p>
                <span class="price">20</span>元
            </p>
            <p>
                小计:
                <input class="smallPrice" type="text" name="" id="" value="0" disabled="disabled">
            </p>
        </li>
        <li>
            <input type="checkbox" name="liCheck" id="" value="">
            <button type="button" class="decrease" disabled="disabled">-</button>
            <p>
                <span class="piece">0</span>件
            </p>
            <button type="button" class="increase">+</button>
            <p>
                <span class="price">30</span>元
            </p>
            <p>
                小计:
                <input class="smallPrice" type="text" name="" id="" value="0" disabled="disabled">
            </p>
        </li>
        <li>
            <input type="checkbox" name="liCheck" id="" value="">
            <button type="button" class="decrease" disabled="disabled">-</button>
            <p>
                <span class="piece">0</span>件
            </p>
            <button type="button" class="increase">+</button>
            <p>
                <span class="price">40</span>元
            </p>
            <p>
                小计:
                <input class="smallPrice" type="text" name="" id="" value="0" disabled="disabled">
            </p>
        </li>
        <li>
            <input type="checkbox" name="liCheck" id="" value="">
            <button type="button" class="decrease" disabled="disabled">-</button>
            <p>
                <span class="piece">0</span>件
            </p>
            <button type="button" class="increase">+</button>
            <p>
                <span class="price">50</span>元
            </p>
            <p>
                小计:
                <input class="smallPrice" type="text" name="" id="" value="0" disabled="disabled">
            </p>
        </li>
    </ul>
    <div class="sum">
        <label>
<input type="checkbox" name="" id="checkAll" value="">全选
</label>
        <p>
            商品总计:
            <span id="totalCount">0</span>
        </p>
        <p>
            商品总价:
            <span id="totalPrice">0</span>
        </p>
    </div>
</div>
CSS代码：
html,body,ul,p {
margin:0;
padding:0;
}
ul,li {
list-style:none;
}
ul {
width:450px;
}
li {
display:flex;
justify-content:space-around;
}
.sum {
width:450px;
display:flex;
justify-content:space-around;
}
#container {
width:450px;
margin:100px auto;
}

// 1.页面加载后执行
window.onload = function() {
    // 2.获取元素
    var liCheck = document.getElementsByName("liCheck"); //li里面的复选框
    var decrease = document.getElementsByClassName("decrease"); //减号
    var piece = document.getElementsByClassName("piece"); //件数
    var increase = document.getElementsByClassName("increase"); //加号
    var price = document.getElementsByClassName("price"); //单价
    var smallPrice = document.getElementsByClassName("smallPrice"); //小计
    var checkAll = document.getElementById("checkAll"); //全选复选框
    var totalCount = document.getElementById("totalCount"); //总计
    var totalPrice = document.getElementById("totalPrice"); //总价
    // 3.加减按钮
    for (var i = 0; i < decrease.length; i++) {
        // 4.另存下标
        decrease[i].index = i;
        increase[i].index = i;
        liCheck[i].index = i;
        // 5.减号的点击事件
        decrease[i].onclick = function() {
            // 5-1.判断件数是否大于0
            if (piece[this.index].innerHTML <= 1) {
                this.disabled = true; //当件数小于等于0时, 将减号按钮禁用
            }
            // 5-1-1.当前件数-1
            piece[this.index].innerHTML--;
            // 5-1-2.计算小计
            smallPrice[this.index].value = Number(smallPrice[this.index].value) - Number(price[this.index].innerHTML);
            // 6-4.如果当前条目是被选中状态, 则需要将该商品计入总计和总价
            if (liCheck[this.index].checked) { //选中
                totalCount.innerHTML--;
                totalPrice.innerHTML = Number(totalPrice.innerHTML) - Number(price[this.index].innerHTML);
            }
        }
        // 6.加号的点击事件
        increase[i].onclick = function() {
            // 6-1.将对应的减号解禁
            decrease[this.index].disabled = false;
            // 6-2.当前件数+1
            piece[this.index].innerHTML++;
            // 6-3.计算小计
            smallPrice[this.index].value = Number(smallPrice[this.index].value) + Number(price[this.index].innerHTML);
            // 6-4.如果当前条目是被选中状态, 则需要将该商品计入总计和总价
            if (liCheck[this.index].checked) { //选中
                totalCount.innerHTML++;
                totalPrice.innerHTML = Number(totalPrice.innerHTML) + Number(price[this.index].innerHTML);
            }
        }
        // 7.复选框的状态改变事件
        var count = 0; //存储选中个数
        liCheck[i].onchange = function() {
            // 7-1.判断是否选中
            if (this.checked) { //选中状态
                count++;
                totalCount.innerHTML = Number(totalCount.innerHTML) + Number(piece[this.index].innerHTML); //总计加当前件数
                totalPrice.innerHTML = Number(totalPrice.innerHTML) + Number(smallPrice[this.index].value); //总计加当前件数
                // 7-1-1. 判断选中个数是否与复选框个数一致
            } else { //取消选中状态
                count--;
                totalCount.innerHTML = Number(totalCount.innerHTML) - Number(piece[this.index].innerHTML); //总计加当前件数
                totalPrice.innerHTML = Number(totalPrice.innerHTML) - Number(smallPrice[this.index].value); //总计加当前件数
            }
            count == liCheck.length ? checkAll.checked = true : checkAll.checked = false;
        }
    }
    // 8.全选复选框
    checkAll.onchange = function() {
        totalCount.innerHTML = 0; //总计置为0
        totalPrice.innerHTML = 0; //总价置为0
        for (var j = 0; j < liCheck.length; j++) {
            // 8-1. 将li里面的复选框与全选状态保持一致
            liCheck[j].checked = this.checked;
            // 8-2. 判断是否全选
            if (this.checked) {
                count = liCheck.length;
                totalCount.innerHTML = Number(totalCount.innerHTML) + Number(piece[j].innerHTML);
                totalPrice.innerHTML = Number(totalPrice.innerHTML) + Number(smallPrice[j].value);
            } else {
                count = 0;
            }
        }
    }
}