a {
            text-decoration: none;
    }
        
        body {
            height: 5000px;
        }
        
        .backtotop {
            posITion: fixed;
            bottom: 80px;
            right: 80px;
            width: 80px;
            height: 80px;
            background-color: #ccc;
            font-Size: 20px;
            text-align: center;
            padding-top: 12px;
            box-sizing: border-box;
            cursor: pointer;
            color: #000;
            /* 先隐藏按钮 */
            /*display: none;*/
        }

<a href="javascript:;" class="backtotop" id="backtotop">返回<br>顶部</a>

.backtotop {
    /* 先隐藏按钮 */
    display: none;
}

(function() {
      // 1.拿到需要操作的元素
      let oBackBTn = document.querySelector("#backtotop"); 
      // 2.监听网页的滚动
      window.onscroll = function() {
                // 获取滚动出去的距离
                let offsetY = getPageScroll().y;
                // console.LOG(offsetY);
 
                // 判断是否需要显示回滚按钮
                if (offsetY >= 200) {
                    oBackBtn.style.display = "block";
                } else {
                    oBackBtn.style.display = "none";
                }
            }
 
            let timerId = null;
 
            // 3.监听回滚按钮的点击
            oBackBtn.onclick = function() {
 
                //设表先关，防止定时器冲突
                clearInterval(timerId);
 
                //设置定时器
                timerId = setInterval(function() {
                    let begin = getPageScroll().y; //当前位置
                    let target = 0; //目标位置
                    let step = (target - begin) * 0.3;
                    begin += step;
 
                    //判断是否结束
                    if (Math.abs(Math.floor(step)) <= 1) {
 
                        //清除定时器
                        clearInterval(timerId);
 
                        // window.scrollTo(x, y);
                        // x表示让网页在水平方向滚动到什么位置
                        // y表示让网页在垂直方向滚动到什么位置
                        window.scrollTo(0, 0);
                        return;
                    }
 
                    window.scrollTo(0, begin);
 
                }, 50);
            };
 
            function getPageScroll() {
                let x, y;
                if (window.pageXOffset) {
                    x = window.pageXOffset;
                    y = window.pageYOffset;
                } else if (document.COMpatMode === "BackCompat") {
                    x = document.body.scrollLeft;
                    y = document.body.scrollTop;
                } else {
                    x = document.documentElement.scrollLeft;
                    y = document.documentElement.scrollTop;
                }
                return {
                    x: x,
                    y: y
                }
            }
        })();