簡單實現點選回到頂部

 var btn = document.getElementById('btn');
        var timer = null;
        btn.onclick = function () {
            timer = window.setInterval(() => {
                // document.documentElement.csrollTop = 0;
                if (document.documentElement.scrollTop == 0) {
                    clearInterval(timer);
                    return
                }
                document.documentElement.scrollTop -= 20;
            }, 5)
        }
        window.onmousewheel = function () {//滑鼠滾輪事件
            console.log(document.documentElement.scrollTop)
            clearInterval(timer);
        }

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