原生js之同级元素添加移除class

<div class="tp-bottom-content">
            <div class="tp-nav">
                <ul id="nav">
                    <li class="active"><span>快速跟进</span></li>
                    <li ><span>跟进</span></li>
                    <li><span>联系人(1)</span></li>
                    <li><span>通话记录(1)</span></li>
                    <li><span>订单(1)</span></li>
                    <li><span>合同(1)</span></li>
                </ul>
            </div>
            <div class="content-list">
                <div class="list-item">1</div>
                <div class="list-item" style="display: none;" >2</div>
                <div class="list-item" style="display: none;" >3</div>
                <div class="list-item" style="display: none;" >4</div>
                <div class="list-item" style="display: none;" >5</div>
                <div class="list-item" style="display: none;" >6</div>
            </div>
        </div>

初次完成的js

var li = document.getElementById("nav").getElementsByTagName("li");
            var listItem = document.getElementsByClassName("list-item");
            var length = li.length;
            for (var i = 0; i < length; i++) {
                li[i].index = i;
                li[i].onclick = function() {
                    li[this.index].setAttribute("class","active");
                    for (var j = 0; j < length; j++) {
                        li[j].className = "";
                        listItem[j].style.display = "none";
                    };
                    listItem[this.index].style.display = "block";
                    this.className = "active";
                }
            };