js has a classlist interface
MDN link

But you have to consider compatibility.

replace

if (!this.hasClass(obj, class)) obj.className += " " + class;

elem.className
or
elem.classList
I feel classList is better, elem.classList.add('bb') will do
In addition, the jQuery source code implements addClass and removeClass through className
Finally, this bias For basic questions, you need to learn how to Baidu and Google yourself

function addClass(ele,name){
        if(ele && name){
            var cn = ele.className;
            if(!cn){
                ele.className = name;
            }else if(cn.indexOf(name) === -1){
                ele.className = cn.replace(/\s*$/,' '+name);
            }
        }
}
function removeClass(ele,name){
        var reg1 = new RegExp('^\\s*\\b'+ name +'\\b\\s*$');
        var reg2 = new RegExp('^\\s*' + name+'\\b');
        var reg3 = new RegExp('\\b'+ name + '\\s*$');
        var reg4 = new RegExp('\\s*\\b'+name+'\\b\\s*');
        if(ele && name){
            var cn = ele.className;
            if(cn && cn.indexOf(name) > -1){
                if(reg1.test(cn)){
                    ele.className = '';
                }else if(reg2.test(cn)){
                    ele.className = cn.replace(reg2,'');
                }else if(reg3.test(cn)){
                    ele.className = cn.replace(reg3,'');
                }else{
                    ele.className = cn.replace(reg4,' ');
                }
            }
        }
};