backTop() {

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

      this.duration -= 3;

      this.duration = this.duration <= 0 ? 1 : this.duration;

      var speed = top / this.duration;

      document.documentElement.scrollTop -= speed;

      if (top > 0) {

        this.timer = setTimeout(() => {

          this.backTop();

        }, 16);

      } else {

        this.duration = 50;

        clearTimeout(this.timer);

      }

    }