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<ul class="nav">

    <li class="active">栏目1</li>

    <li>栏目2</li>

    <li>栏目3</li>

</ul>

<script type="text/javascript">

    // 获取到当前的li元素

    let activeLi = document.querySelector(".active");

    // 打印classList属性

    console.log(activeLi.classList)

    // 控制台输出：

    //     DOMTokenList ['active', value: 'active']

</script>

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<ul class="nav">

    <li class="item active">栏目1</li>

    <li class="item">栏目2</li>

    <li class="item">栏目3</li>

</ul>

<script type="text/javascript">

    let currentElement = document.querySelector(".nav").firstElementChild;

    // 遍历li元素设置点击事件

    while (currentElement) {

        currentElement.onclick = function() {

            // 移除激活li的active

            document.querySelector(".active").classList.remove("active");

            // 当前li类增加active

            this.classList.add("active");

        }

        currentElement = currentElement.nextElementSibling;

    }

</script>