自动显隐回到顶部javascript代码

当拖动网页滚动条的时，页面右下角会自动出现“回到顶部”的图标，点击该图标就可以回到顶部！此代码原理是当页面到达非第一屏时，即显示TOP图标，点击后页面平滑滚动到第一屏！效果很爽！

将如下内容直接保存为 scrolltopcontrol.js ，直接在页面调用即可，代码中的 up.png 图片为向上箭头，可自己替换。

//** jQuery Scroll to Top Control script- (c) Dynamic Drive DHTML code library: http://www.dynamicdrive.com.

var scrolltotop={
	//startline: Integer. Number of pixels from top of doc scrollbar is scrolled before showing control
	//scrollto: Keyword (Integer, or "Scroll_to_Element_ID"). How far to scroll document up when control is clicked on (0=top).
	setting: {startline:100, scrollto: 0, scrollduration:1000, fadeduration:[500, 100]},
	controlHTML: '<img src="up.png" style="width:24px; height:24px" />', //HTML for control, which is auto wrapped in DIV w/ ID="topcontrol"
	controlattrs: {offsetx:5, offsety:5}, //offset of control relative to right/ bottom of window corner
	anchorkeyword: '#top', //Enter href value of HTML anchors on the page that should also act as "Scroll Up" links

	state: {isvisible:false, shouldvisible:false},

	scrollup:function(){
		if (!this.cssfixedsupport) //if control is positioned using JavaScript
			this.$control.css({opacity:0}) //hide control immediately after clicking it
		var dest=isNaN(this.setting.scrollto)? this.setting.scrollto : parseInt(this.setting.scrollto)
		if (typeof dest=="string" && jQuery('#'+dest).length==1) //check element set by string exists
			dest=jQuery('#'+dest).offset().top
		else
			dest=0
		this.$body.animate({scrollTop: dest}, this.setting.scrollduration);
	},

	keepfixed:function(){
		var $window=jQuery(window)
		var controlx=$window.scrollLeft() + $window.width() - this.$control.width() - this.controlattrs.offsetx
		var controly=$window.scrollTop() + $window.height() - this.$control.height() - this.controlattrs.offsety
		this.$control.css({left:controlx+'px', top:controly+'px'})
	},

	togglecontrol:function(){
		var scrolltop=jQuery(window).scrollTop()
		if (!this.cssfixedsupport)
			this.keepfixed()
		this.state.shouldvisible=(scrolltop>=this.setting.startline)? true : false
		if (this.state.shouldvisible && !this.state.isvisible){
			this.$control.stop().animate({opacity:1}, this.setting.fadeduration[0])
			this.state.isvisible=true
		}
		else if (this.state.shouldvisible==false && this.state.isvisible){
			this.$control.stop().animate({opacity:0}, this.setting.fadeduration[1])
			this.state.isvisible=false
		}
	},
	
	init:function(){
		jQuery(document).ready(function($){
			var mainobj=scrolltotop
			var iebrws=document.all
			mainobj.cssfixedsupport=!iebrws || iebrws && document.compatMode=="CSS1Compat" && window.XMLHttpRequest //not IE or IE7+ browsers in standards mode
			mainobj.$body=(window.opera)? (document.compatMode=="CSS1Compat"? $('html') : $('body')) : $('html,body')
			mainobj.$control=$('<div id="topcontrol">'+mainobj.controlHTML+'</div>')
				.css({position:mainobj.cssfixedsupport? 'fixed' : 'absolute', bottom:mainobj.controlattrs.offsety, right:mainobj.controlattrs.offsetx, opacity:0, cursor:'pointer'})
				.attr({title:'Scroll Back to Top'})
				.click(function(){mainobj.scrollup(); return false})
				.appendTo('body')
			if (document.all && !window.XMLHttpRequest && mainobj.$control.text()!='') //loose check for IE6 and below, plus whether control contains any text
				mainobj.$control.css({width:mainobj.$control.width()}) //IE6- seems to require an explicit width on a DIV containing text
			mainobj.togglecontrol()
			$('a[href="' + mainobj.anchorkeyword +'"]').click(function(){
				mainobj.scrollup()
				return false
			})
			$(window).bind('scroll resize', function(e){
				mainobj.togglecontrol()
			})
		})
	}
}

scrolltotop.init()