$(imageobj).attr('src', $(imageobj).attr('src') + '?' + Math.random() );

 var totalimages = 10;
 var loadedimages = 0;
$("<img/>").load(function() {
++loadedimages;
 if(loadedimages == totalimages){
//全部图片加载完成时…..
 }
 });

 var clearSelection = function () {
 if(document.selection && document.selection.empty) {
 document.selection.empty();
 } else if(window.getSelection) {
 var sel = window.getSelection();
 sel.removeAllRanges();
 }
 }

 $(element).bind('dblclick',function(event){
 clearSelection();
 });

 <ul>
 <li>cloud</li>
 <li>sun</li>
 <li>rain</li>
 <li>snow</li>
 </ul

 var items = $('.to_order li').get();

 items.sort(function(a,b){
 var keyA = $(a).text();
 var keyB = $(b).text();
 if (keyA < keyB) return -1;
 if (keyA > keyB) return 1;
 return 0;
 });
 var ul = $('.to_order');
 $.each(items, function(i, li){
 ul.append(li);
 });

 $(document).ready(function(){
 $(document).bind("contextmenu",function(e){
 return false;
 });
 });

 $.extend($.expr[':'], {
 //选择器名
 moreThanAThousand : function (a){
 return parseInt($(a).html()) > 1000;
 }
 });
 $(document).ready(function() {
 $('td:moreThanAThousand').css('background-color', '#ff0000′);
 });

if ($("#elementid").length) {
//……
}

var req = $.ajax({
type: "POST",
url: "someurl",
data: "id=1″,
success: function(){

}
});
//取消ajax请求
req.abort()

$(document).ready(function() {
var dt = new Date();
dt.setSeconds(dt.getSeconds() + 60);
document.cookie = "cookietest=1; expires=" + dt.toGMTString();
var cookiesEnabled = document.cookie.indexOf("cookietest=") != -1;
if(!cookiesEnabled){
//cookies不能用……..
}
});

 $("ul > li").click(function () {
 var index = $(this).prevAll().length;
 });
//如果用的是jquery1.4，明河推荐使用以下方法：
 $("ul > li").click(function () {
 var index = $(this).index();
 });

jQuery.fn.center = function () {
this.css("position","absolute");
this.css("top", ( $(window).height() – this.height() ) / 2+$(window).scrollTop() + "px");
this.css("left", ( $(window).width() – this.width() ) / 2+$(window).scrollLeft() + "px");
return this;
}
$(element).center();

$(document).ready(function() {
var pathname = window.location.pathname;
});