toTop(){

  let timer = null;

  cancelAnimationFrame(timer);

  timer = requestAnimationFrame(function fn(){

    var oTop = document.body.scrollTop || document.documentElement.scrollTop;

    if(oTop > 0){

      document.body.scrollTop = document.documentElement.scrollTop = oTop - 50;

      timer = requestAnimationFrame(fn);

    }else{

      cancelAnimationFrame(timer);

    }   

  });

},