js監聽滾動條實現回到頂部

/**
   * @param {elName} String
   * @param {speed} Number
*/
Up(elName,speed){
      // 直接回到頂部
      // var scrollTop = document.getElementsByClassName('leftBx')[0].scrollTop;
      // document.getElementsByClassName('leftBx')[0].scrollTop = 0
      
      // 緩沖
      timer = setInterval(function() {
	// 再次獲取滾動條距頂部高度
	var osTop = document.getElementsByClassName(elName)[0].scrollTop;
	// 用於設置速度差,產生緩慢效果
	var speed = Math.floor(-osTop / 8);
	document.getElementsByClassName(elName)[0].scrollTop = osTop = osTop + speed
	if (osTop == 0) {
	      clearInterval(timer)
	}
      }, speed)
    }