代码如下:









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var tool = {
//此方法为了避免在 ms 段时间内，多次执行func。常用 resize、scoll、mousemove等连续性事件中
buffer: function(func, ms, context){
var buffer;
return function(){
if(buffer) return;
buffer = setTimeout(function(){
func.call(this)
buffer = undefined;
},ms);
};
},
/*读取或设置元素的透明度*/
opacity: function(elem, val){
var setting = arguments.length > 1;
if("opacity" in elem.style){//elem.style["opacity"] 读取不到CSS class中的值
return setting ? elem.style["opacity"] = val : elem.style["opacity"];
}else{
if(elem.filters && elem.filters.alpha) {
return setting ? elem.filters.alpha["opacity"] = val*100 : elem.filters.alpha["opacity"]/100;
}
}
},
//获取或设置文档对象的scrollTop
//function([val])
documentScrollTop: function(val){
var elem = document;
return (val!== undefined) ?
elem.documentElement.scrollTop = elem.body.scrollTop = val :
Math.max(elem.documentElement.scrollTop, elem.body.scrollTop);
}
};
//动画效果
var effect = {
//淡入
fadein: function (elem){
var val = 0;
var interval = 25;
setTimeout(function(){
val += 0.1;
if(val>1){
tool.opacity(elem, 1)
return;
}else {
tool.opacity(elem, val)
setTimeout(arguments.callee, interval);
}
},interval);
},
//淡出
fadeout: function (elem){
var val = 1;
var interval = 25;
setTimeout(function(){
val -= 0.1;
if(val < 0){
tool.opacity(elem, 0)
return;
}else {
tool.opacity(elem,val) ;
setTimeout(arguments.callee, interval);
}
},interval);
},
//减速移动滚动条
move: function(scrollTop){
setTimeout(function(){
scrollTop = Math.floor((scrollTop * 0.65));
tool.documentScrollTop(scrollTop);
if(scrollTop !=0 ){
setTimeout(arguments.callee, 25);
}
},25);
}
};
//主程序
(function(){//gotop
var visible = false;
var elem = document.getElementById("gotop");
function onscroll(){
var scrollTop = tool.documentScrollTop();
if(scrollTop > 0){
if(!visible){
effect.fadein(elem)
visible = true;
}
}else{
if(visible){
effect.fadeout(elem);
visible = false;
}
}
}
function onclick(){
var scrollTop = tool.documentScrollTop();
effect.move(scrollTop);
}
elem.onclick = onclick;
window.onscroll = tool.buffer(onscroll, 200, this);
})();

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