function addClass(obj, cls){
        var obj_class = obj.className,//获取 class 内容.
            blank = (obj_class != ‘‘) ? ‘ ‘ : ‘‘;//判断获取到的 class 是否为空, 如果不为空在前面加个‘空格‘.
        var added = obj_class + blank + cls;//组合原来的 class 和需要添加的 class.
        obj.className = added;//替换原来的 class.
    }

    function removeClass(obj, cls){
        var obj_class = ‘ ‘+obj.className+‘ ‘;//获取 class 内容, 并在首尾各加一个空格. ex) ‘abc    bcd‘ -> ‘ abc    bcd ‘
        obj_class = obj_class.replace(/(\s+)/gi, ‘ ‘),//将多余的空字符替换成一个空格. ex) ‘ abc    bcd ‘ -> ‘ abc bcd ‘
            removed = obj_class.replace(‘ ‘+cls+‘ ‘, ‘ ‘);//在原来的 class 替换掉首尾加了空格的 class. ex) ‘ abc bcd ‘ -> ‘bcd ‘
        var removed = removed.replace(/(^\s+)|(\s+$)/g, ‘‘);//去掉首尾空格. ex) ‘bcd ‘ -> ‘bcd‘
        obj.className = removed;//替换原来的 class.
    }

    function hasClass(obj, cls){
        var obj_class = obj.className,//获取 class 内容.
            obj_class_lst = obj_class.split(/\s+/);//通过split空字符将cls转换成数组.
        var x = 0;
        for(x in obj_class_lst) {
            if(obj_class_lst[x] == cls) {//循环数组, 判断是否包含cls
                return true;
            }
        }
        return false;
    }