<script type="text/javascript" src="http://ajax.microsoft.com/ajax/jquery/jquery-1.4.2.min.js"></script>
<script type="text/javascript">
if (typeof jQuery == 'undefined') {
    document.write(unescape("%3Cscript src='/js/jquery-1.4.2.min.js' type='text/javascript'%3E%3C/script%3E"));
}
</script>

var $myDiv = $("#myDiv");
$myDiv.click(function(){...});

var $products = $("div.products"); // SLOW
var $products = $(".products"); // FAST

// BAD, a nested query for Sizzle selector engine
var $productIds = $("#products div.id");

// GOOD, #products is already selected by document.getElementById() so only div.id needs to go through Sizzle selector engine
var $productIds = $("#products").find("div.id");

// Unoptimized
$("div.data .gonzalez");

// Optimized
$(".data td.gonzalez");

$(".data table.attendees td.gonzalez");

// Better: Drop the middle if possible.
$(".data td.gonzalez");

// SLOWER because it has to traverse the whole DOM for .class
$('.class');

// FASTER because now it only looks under class-container.
$('.class', '#class-container');

$('div.container > *'); // BAD
$('div.container').children(); // BETTER

$('div.someclass :radio'); // BAD
$('div.someclass input:radio'); // GOOD

$('#outer #inner'); // BAD
$('div#inner'); // BAD
$('.outer-container #inner'); // BAD
$('#inner'); // GOOD, only calls document.getElementById()

var $myList = $("#list-container > ul").detach();
//...a lot of complicated things on $myList
$myList.appendTo("#list-container");

// BAD
var $myList = $("#list");
for(var i = 0; i < 10000; i++){
    $myList.append("<li>"+i+"</li>");
}

// GOOD
var $myList = $("#list");
var list = "";
for(var i = 0; i < 10000; i++){
    list += "<li>"+i+"</li>";
}
$myList.html(list);

// EVEN FASTER
var array = [];
for(var i = 0; i < 10000; i++){
    array[i] = "<li>"+i+"</li>";
}
$myList.html(array.join(''));

// BAD: This runs three functions before it realizes there's nothing in the selection
$("#nosuchthing").slideUp();

// GOOD
var $mySelection = $("#nosuchthing");
if ($mySelection.length) {
    $mySelection.slideUp();
}

$("#myLink").on("click", function(){...}); // BAD

// GOOD
function myLinkClickHandler(){...}
$("#myLink").on("click", myLinkClickHandler);

$(function(){ ... }); // BAD: You can never reuse or write a test for this function.

// GOOD
$(initPage); // or $(document).ready(initPage);
function initPage(){
    // Page load event where you can initialize values and call other initializers.
}

$("#myLink").on("click", myEventHandler); // GOOD

$("#myLink").on("click.mySpecialClick", myEventHandler); // GOOD
// Later on, it's easier to unbind just your click event
$("#myLink").unbind("click.mySpecialClick");

$("#list a").on("click", myClickHandler); // BAD, you are attaching an event to all the links under the list.
$("#list").on("click", "a", myClickHandler); // GOOD, only one event handler is attached to the parent.

// Less readable...
$.ajax({
    url: "something.php?param1=test1&param2=test2",
    ....
});

// More readable...
$.ajax({
    url: "something.php",
    data: { param1: test1, param2: test2 }
});

$("#parent-container").on("click", "a", delegatedClickHandlerForAjax);

$.ajax({ ... }).then(successHandler, failureHandler);

// OR
var jqxhr = $.ajax({ ... });
jqxhr.done(successHandler);
jqxhr.fail(failureHandler);

var jqxhr = $.ajax({
    url: url,
    type: "GET", // default is GET but you can use other verbs based on your needs.
    cache: true, // default is true, but false for dataType 'script' and 'jsonp', so set it on need basis.
    data: {}, // add your request parameters in the data object.
    dataType: "json", // specify the dataType for future reference
    jsonp: "callback", // only specify this to match the name of callback parameter your API is expecting for JSONP requests.
    statusCode: { // if you want to handle specific error codes, use the status code mapping settings.
        404: handler404,
        500: handler500
    }
});
jqxhr.done(successHandler);
jqxhr.fail(failureHandler);

$("#myDiv").addClass("error").show();

$("#myLink")
    .addClass("bold")
    .on("click", myClickHandler)
    .on("mouseover", myMouseOverHandler)
    .show();

$myLink.attr("href", "#").attr("title", "my link").attr("rel", "external"); // BAD, 3 calls to attr()
// GOOD, only 1 call to attr()
$myLink.attr({
    href: "#",
    title: "my link",
    rel: "external"
});

$("#mydiv").css({'color':red, 'font-weight':'bold'}); // BAD

$("#mydiv").addClass("error"); // GOOD

$("#myId"); // is still a little slower than...
document.getElementById("myId");