very short version:
[html]
复制代码 代码如下:
$('#myDiv').css({top:'50%',left:'50%',margin:'-'+($('#myDiv').height() / 2)+'px 0 0 -'+($('#myDiv').width() / 2)+'px'});
$('#myDiv').css({top:'50%',left:'50%',margin:'-'+($('#myDiv').height() / 2)+'px 0 0 -'+($('#myDiv').width() / 2)+'px'});

short version:
[html]
复制代码 代码如下:
(function($){
$.fn.extend({
center: function () {
return this.each(function() {
var top = ($(window).height() - $(this).outerHeight()) / 2;
var left = ($(window).width() - $(this).outerWidth()) / 2;
$(this).css({position:'absolute', margin:0, top: (top > 0 ? top : 0)+'px', left: (left > 0 ? left : 0)+'px'});
});
}
});
})(jQuery);
(function($){
$.fn.extend({
center: function () {
return this.each(function() {
var top = ($(window).height() - $(this).outerHeight()) / 2;
var left = ($(window).width() - $(this).outerWidth()) / 2;
$(this).css({position:'absolute', margin:0, top: (top > 0 ? top : 0)+'px', left: (left > 0 ? left : 0)+'px'});
});
}
});
})(jQuery);

Activated by this code :
复制代码 代码如下:
$('#mainDiv').center();

[javascript]
复制代码 代码如下:
(function($){
$.fn.extend({
center: function (options) {
var options = $.extend({ // Default values
inside:window, // element, center into window
transition: 0, // millisecond, transition time
minX:0, // pixel, minimum left element value
minY:0, // pixel, minimum top element value
withScrolling:true, // booleen, take care of the scrollbar (scrollTop)
vertical:true, // booleen, center vertical
horizontal:true // booleen, center horizontal
}, options);
return this.each(function() {
var props = {position:'absolute'};
if (options.vertical) {
var top = ($(options.inside).height() - $(this).outerHeight()) / 2;
if (options.withScrolling) top += $(options.inside).scrollTop() || 0;
top = (top > options.minY ? top : options.minY);
$.extend(props, {top: top+'px'});
}
if (options.horizontal) {
var left = ($(options.inside).width() - $(this).outerWidth()) / 2;
if (options.withScrolling) left += $(options.inside).scrollLeft() || 0;
left = (left > options.minX ? left : options.minX);
$.extend(props, {left: left+'px'});
}
if (options.transition > 0) $(this).animate(props, options.transition);
else $(this).css(props);
return $(this);
});
}
});
})(jQuery);
[code]
(function($){
$.fn.extend({
center: function (options) {
var options = $.extend({ // Default values
inside:window, // element, center into window
transition: 0, // millisecond, transition time
minX:0, // pixel, minimum left element value
minY:0, // pixel, minimum top element value
withScrolling:true, // booleen, take care of the scrollbar (scrollTop)
vertical:true, // booleen, center vertical
horizontal:true // booleen, center horizontal
}, options);
return this.each(function() {
var props = {position:'absolute'};
if (options.vertical) {
var top = ($(options.inside).height() - $(this).outerHeight()) / 2;
if (options.withScrolling) top += $(options.inside).scrollTop() || 0;
top = (top > options.minY ? top : options.minY);
$.extend(props, {top: top+'px'});
}
if (options.horizontal) {
var left = ($(options.inside).width() - $(this).outerWidth()) / 2;
if (options.withScrolling) left += $(options.inside).scrollLeft() || 0;
left = (left > options.minX ? left : options.minX);
$.extend(props, {left: left+'px'});
}
if (options.transition > 0) $(this).animate(props, options.transition);
else $(this).css(props);
return $(this);
});
}
});
})(jQuery);

PLUGIN VERSION
[javascript]
复制代码 代码如下:
(function($){
$.fn.extend({
center: function (options) {
var options = $.extend({ // Default values
inside:window, // element, center into window
transition: 0, // millisecond, transition time
minX:0, // pixel, minimum left element value
minY:0, // pixel, minimum top element value
withScrolling:true, // booleen, take care of the scrollbar (scrollTop)
vertical:true, // booleen, center vertical
horizontal:true // booleen, center horizontal
}, options);
return this.each(function() {
var props = {position:'absolute'};
if (options.vertical) {
var top = ($(options.inside).height() - $(this).outerHeight()) / 2;
if (options.withScrolling) top += $(options.inside).scrollTop() || 0;
top = (top > options.minY ? top : options.minY);
$.extend(props, {top: top+'px'});
}
if (options.horizontal) {
var left = ($(options.inside).width() - $(this).outerWidth()) / 2;
if (options.withScrolling) left += $(options.inside).scrollLeft() || 0;
left = (left > options.minX ? left : options.minX);
$.extend(props, {left: left+'px'});
}
if (options.transition > 0) $(this).animate(props, options.transition);
else $(this).css(props);
return $(this);
});
}
});
})(jQuery);

复制代码 代码如下:
(function($){
$.fn.extend({
center: function (options) {
var options = $.extend({ // Default values
inside:window, // element, center into window
transition: 0, // millisecond, transition time
minX:0, // pixel, minimum left element value
minY:0, // pixel, minimum top element value
withScrolling:true, // booleen, take care of the scrollbar (scrollTop)
vertical:true, // booleen, center vertical
horizontal:true // booleen, center horizontal
}, options);
return this.each(function() {
var props = {position:'absolute'};
if (options.vertical) {
var top = ($(options.inside).height() - $(this).outerHeight()) / 2;
if (options.withScrolling) top += $(options.inside).scrollTop() || 0;
top = (top > options.minY ? top : options.minY);
$.extend(props, {top: top+'px'});
}
if (options.horizontal) {
var left = ($(options.inside).width() - $(this).outerWidth()) / 2;
if (options.withScrolling) left += $(options.inside).scrollLeft() || 0;
left = (left > options.minX ? left : options.minX);
$.extend(props, {left: left+'px'});
}
if (options.transition > 0) $(this).animate(props, options.transition);
else $(this).css(props);
return $(this);
});
}
});
})(jQuery);

Activated by this code :
复制代码 代码如下:
$(document).ready(function(){
$('#mainDiv').center();
$(window).bind('resize', function() {
$('#mainDiv').center({transition:300});
});
);

观此人JS代码，让人叹为观止。
简洁明了。却又举一反三。