.new{
        color: #FFD113;
    }

    <span class="old">首页</span>

    var arr = document.getElementsByTagName("span");
    arr[i].classList.add("new"); /*添加样式*/
    arr[j].classList.remove("new");/*删除样式*/

    var old= element.className;
    /*加上空格, 不然想查询"old"，若原本有"older"的类名的就会有问题*/
    old = " " + old + " ";
    
    /*查询类名中是否含有某个class*/
    old.indexOf(" ClassName ") === -1 ? false : true;//同样的查询时也要带上空格*2
    
    /*增加*/
    1.  new=old+" new";//注意空格*1
    
    2.  new = old.concat(" ClassName");//注意空格*1
        element.setAttribute("class", new);
    
    /*删除*/
    new = old.replace(" ClassName "," ");//注意空格*3
    element.setAttribute("class",new);
    
    /*修改*/
    new = old.replace(" targetClassName "," yourClassName ");//注意空格*4
    element.setAttribute("class",new );