js判断滚动条是否至页面顶部

window.onscroll = function () {
    let flag = document.body.scrollHeight > (window.innerHeight || document.documentElement.clientHeight);
    if (flag) {
        let scrollTop = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop || 0;
        if (scrollTop == 0) { //此时滚动条处于页面的顶部
            mui.toast("到达顶部")
        }
    }
}