JavaScript to implement the back to top button

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;
            /* Hide the button first */
            /*display: none;*/
        }

<a href="javascript:;" class="backtotop" id="backtotop">Back to<br>Top</a> 

.backtotop {
    /* Hide the button first */
    display: none;
}

(function() {
      // 1. Get the element to be operated let oBackBtn = document.querySelector("#backtotop"); 
      // 2. Monitor the scrolling of the web page window.onscroll = function() {
                // Get the scroll distance let offsetY = getPageScroll().y;
                // console.log(offsetY);
 
                // Determine whether to display the rollback button if (offsetY >= 200) {
                    oBackBtn.style.display = "block";
                } else {
                    oBackBtn.style.display = "none";
                }
            }
 
            let timerId = null;
 
            // 3. Listen for clicks on the rollback button oBackBtn.onclick = function() {
 
                //Set the table to close first to prevent timer conflict clearInterval(timerId);
 
                //Set the timer timerId = setInterval(function() {
                    let begin = getPageScroll().y; //Current position let target = 0; //Target position let step = (target - begin) * 0.3;
                    begin += step;
 
                    //Judge whether to end if (Math.abs(Math.floor(step)) <= 1) {
 
                        //Clear timer clearInterval(timerId);
 
                        // window.scrollTo(x, y);
                        // x indicates the position to which the webpage should be scrolled horizontally // y indicates the position to which the webpage should be scrolled vertically 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
                }
            }
        })();