js实现首尾移动轮播效果

<script>
    var bt=document.getElementsByTagName("button");
    var oli=document.getElementsByTagName("li");
    var oul=document.getElementsByTagName("ul")[0];
    var x=-100;
    bt[1].onclick=function right() {
        var m=0
      var sj=  setInterval(function () {
            x-=10;
          m++;
            oul.style.left=x+"px";
            if(m==10){
                clearInterval(sj)
            } if(x<=-500){
              x=-100;
              oul.style.left=x+"px";
          }
        },100)
    }
    bt[0].onclick=function () {
        var m=0
        var sj=  setInterval(function () {
            x+=10;
            m++;
            oul.style.left=x+"px";
            if(m==10){
                clearInterval(sj)
            } if(x>=0){
                x=-400;
                oul.style.left=x+"px";
            }
        },100)
    }
</script>