因為我是個前端小白，所以有些問題可能寫的不太恰當，希望大家能夠包容指正。
我要實現的功能是，點選下拉選擇框

html部分：

<div class="selectBox">
<i></i>
<span>PM2.5</span>
<ul class="selectItem">
<li>PM10</li>
<li>O3</li>
<li>SO2</li>
</ul>
 </div>

css部分：

.selectBox
 
 {
        position: absolute;
        top: 4.25rem;
        right: 0.1rem;
        box-sizing: border-box;
        width: 1.6rem;
        height: 0.6rem;
        background: rgba(100, 151, 191, 1);
        border-radius: 0.1rem;
        border: 1px solid rgba(106, 193, 235, 1);
        cursor: pointer;
        line-height
 
: 0.6rem;

        span {
            margin-left: 0.25rem;
        }

        .tranform {
            transform: translateY(-50%) rotate(180deg);
            margin-top: 0px;
        }

        ul {
            display: none;
            width: 100%;
            position: absolute;
            background:
 
 rgba(100, 151, 191, 0.8);
            box-shadow: 0px 0px 4px 0px rgba(140, 140, 140, 0.5);
            border-radius: 0.1rem;
            top: 0.8rem;
            left: 0;
            z-index: 999;
            padding: 5px 0;
            color: #fff;
            text-align: center;
        }

        i {
            position: absolute;
            right: 0.25rem;
            top: 50%;
            margin-top: -0.05rem;
            width: 0;
            height: 0;
            border-right: 0.1rem solid transparent;
            border-left: 0.1rem solid transparent;
            border-top: 0.1rem solid #fff;
            transition: all .2s ease-in-out;
        }
    }

js部分：

$('.selectBox').click(function() {
                $(this).children('i').toggleClass('tranform')
                $(this).children('ul').slideToggle();
            });
            $('.selectItem>li').click(function() {
                let selectText = $(this).text()
                $(this).parent().siblings('span').text(selectText)
            })