javascript——回到顶端按钮的编写

<a href="javascript:void(0);" class="topbtn" title="回到顶端" id="topbtn" onclick="backtop()"></a>

/**
* 当距离顶端的距离超过100px时触发
*/
$(document).ready(function() {
var topbtn = $("#topbtn");
// var pagelookheight = document.documentElement.clientHeight; //　第一屏高度749px
var pagelookheight = 100;

window.onscroll = function() {
var backtop = document.body.scrollTop;
if (backtop >= pagelookheight) {
topbtn.show();
} else {
topbtn.hide();
}
}
});

function backtop() {
var timer = null;
timer = setInterval(function() {
var backtop = document.body.scrollTop;
// 速度调节
var speedtop = backtop / 5;
document.body.scrollTop = backtop - speedtop;
if (backtop == 0) {
clearInterval(timer);
}
}, 30);
}