The Dark Age of DOM Scripting

function setOpacity(elem, newValue) { = newValue;
    //IE hacks: = 1;    //Force hasLayout; needed for filters = 'alpha(opacity=' + (newValue * 100) + ')';

var button = document.getElementById('hoverDemo1');
function onMouseOver() {
    setOpacity(button, 1);
function onMouseOut() {
    setOpacity(button, .5);
setOpacity(button, .5);     //Fade immediately
if (button.addEventListener) {
    button.addEventListener('mouseover', onMouseOver, false);
    button.addEventListener('mouseout', onMouseOut, false);
} else {                    //More IE Hacks:
    button.attachEvent('onmouseover', onMouseOver);
    button.attachEvent('onmouseout', onMouseOut);

jQuery in Action

jQuery is an open-source Javascript library that takes the pain out of DOM manipulation, hiding browser differences from developers, and providing a single uniform and consistent API that works in every major browser.

By switching to jQuery, these 20 lines can be reduced to just 4!

    function() { $(this).css('opacity', 1.0); },
    function() { $(this).css('opacity', 0.5); }
).css('opacity', 0.5);  //Set initial opacity

Using jQuery, we can also easily add additional features, such as animation:

    function() { $(this).animate({ opacity: 1.0 }); },
    function() { $(this).animate({ opacity: 0.5 }); }
).css('opacity', 0.5);  //Set initial opacity

Who's Using jQuery?

Google Microsoft Wordpress
Nokia Amazon NetFlix
Twitter Dell AOL

Downloading jQuery


When creating your site, you should use the Development version, which is nicely formatted and commented.

After your site is finished, you should switch to the Production version, which is three times smaller, but impossible to debug.

Not Downloading jQuery

Actually, you don't even need to download jQuery.

Google graciously provides free hosted versions of jQuery (and other libraries), so you can just add a <script> tag that references jQuery from Google's servers without downloading anything.

<script type="text/javascript" 

If you don't remember the URL, just search Google for jQuery Google and you'll find it in the first result.
You can also find the URLs for Google's hosted libraries at

Before releasing your site, change jquery.js to jquery.min.js to get the production version.

Advanced Javascript

More on Functions

Unlike many other languages, Javascript treats functions as first-class values. Functions can be passed around put in variables, compared, and more.

function createMultiplier(by) {
    var inner = function(n) { return n * by; }  
    return inner;      
var doubler = createMultiplier(2); 
alert("doubler(2) returns " + doubler(2));

Javascript functions can take any number of arguments, regardless of what parameters the function declared.

Extra arguments can be retrieved from the arguments keyword, which is an array-like object.

function showArgs() {
    alert("I got " + arguments.length + " arguments!\n"
        + "The third one is " + arguments[2]);
showArgs(1, 2, 3);


Javascript supports objects, which are essentially bags of member values. These members can include simple values, functions, or more objects.

Objects inherit properties (including methods) from prototypes, which are similar to classes in classical object-oriented languages.
This feature is beyond the scope of this presentation.

Every value you can work with in Javascript, except for null and undefined, is actually an object, and has properties.
For example, writing 42.toString() accesses the toString property (which is inherited from Number.prototype) and calls the function that the property points to.

You can create your own objects by writing { }, and optionally putting properties between the braces.

var emptyObject = { };
emptyObject.state = "Not empty anymore";

var person = { name: "שמואל", age: 42 };
person.age++;        //Happy birthday!
alert( + " is " + person.age);

The this Keyword

All Javascript functions take a hidden parameter called the this keyword.

The this keyword indicates the context on which the function was invoked.

If you call a function normally, this will be the global window object.

If you call a function on an object, this will be the object that you called it on.

function whatsThis() {
} = "the window object";
alert("When called normally, this is " + whatsThis());

var myObject = { 
    name: "MyObject", 
    whatsThis: whatsThis
alert("When called on an object, this is " + myObject.whatsThis());
Using jQuery

jQuery Objects

Most of jQuery is built around the jQuery object.

A jQuery object is an immutable Javascript object which wraps a set of DOM elements and contains jQuery methods that manipulate the elements.

For those of you who know Java or similar languages, a jQuery object can be thought of as an instance of the jQuery class, which has a List<DOMElement> and some methods that do things with the elements.

You can create jQuery objects by calling the jQuery function.

The jQuery Function

The jQuery function takes a variety of parameters and returns a jQuery object.

The jQuery Function, continued

In addition to creating jQuery objects, the jQuery function has an additional use: it can execute code when the page loads.

Writing jQuery(function() { ... }) will execute the function after the DOM tree is complete (before images finish loading).
If the DOM tree has already been loaded, the function will be executed immediately.

jQuery(function() {
    alert("There are " + jQuery('*').length + " DOM elements!");
    //More code here...

When you write Javascript code at the top of an HTML file, it must be wrapped in this call or it will run before the page body has been parsed.

The jQuery function will be called many times in a typical jQuery-based page. To make code shorter, jQuery also has a $ function which is aliased to the jQuery function.

Thus, all of these examples can be written using $(...) intead of jQuery(...).
Almost all real-world code uses the $ function.


jQuery supports almost all selectors from the CSS3 specification, regardless of the selectors supported by the browser. jQuery also adds its own custom selectors that are not in CSS.

A complete listing of jQuery selectors is beyond the scope of this presentation; for more selectors, see

Common Selectors

Using jQuery Objects

Now that we've seen how to create jQuery objects, what can we do with them?

A jQuery object is an array-like object—it can be used as an array of DOM elements. In other words, all jQuery objects have a length property that indicates the number of elements in the set, and numbered properties (from 0 to length - 1) for the raw DOM elements.

Therefore, you can write $('p')[2] to get the DOM element for the 3rd <p> tag in the page.

Note: This is not a jQuery object; it's a native DOM element. To get a jQuery object containing the 3rd <p> tag in the page, write $('p').eq(2).

In addition to acting like arrays, jQuery objects have nearly 150 methods. I'll cover the more useful ones; for complete documentation, see

jQuery methods fall into three categories:

Informative Methods

These methods retrieve information about an element. They return a Javascript value (typically a string or number) that tells you something useful.

alert($('#slide13 p').text());

Action Methods

These methods modify the elements in the jQuery object. They return the jQuery object that they were called on, allowing you to "chain" method calls—to call multiple methods on the same object.

$('<b> Hi There! </b>')
    .appendTo("#slide14 p:first")

Traversal Methods

These methods create a new jQuery object containing elements based on the contents of the set you called it on.

var slide = $('#slide15');
var items = slide.find('.Item');
alert(items.length + " items");

Property Methods

Many jQuery methods are properties—they can either return something or change it, depending on what parameters they're passed. Thus, they're both informative methods and action methods.

To get the value of a property, call the method with no parameters. It will return the value for the first element in the set.
To set the property, call the method and pass the new value as a parameter.
It will set the property for all elements in the set. Like all other action methods, it will return the jQuery object you called it on, allowing you to chain more methods.

You can also pass a function as the parameter, and jQuery will call the function for each element in the jQuery object and set the property to whatever the function returns.
jQuery will pass two parameters to the function: the index (in the set) of the element it's being called for and the current value of the property for that element.

var items = $('#slide16 p.Item');
    function(index, oldHtml) {
        return (index + 1) + ": " + oldHtml;

Common Informative Methods

Common Properties

Keyed Properties

These methods take an additional parameter for the name of the property to get or set.

alert($('#slide22 .CodeBox').attr('animation'));
$('#slide22 .CodeBox').attr('animation', 'explode');

These methods can also take an object of properties to set.

$('#slide22 p').css({
	fontStyle: "italic",
	color: "Green"

Common Action Methods


jQuery can add event handlers using the .bind(), which takes one or more event names and a callback to handle the event.

$('#eventDemo1').bind("mousemove", function(e) { 
	$(this).text(e.pageX + ", " + e.pageY);

jQuery also has helper methods for common events:

$('#eventDemo2').keyup(function() { 

    function() { $(this).css('opacity', 1.0); },
    function() { $(this).css('opacity', 0.5); }
).css('opacity', 0.5);  //Set initial opacity

Event Pitfalls

There are two common mistakes that people make when creating event handlers.


This code calls alert immediately, then passes its return value to the jQuery click method.
It should be written like this, passing a function expression that calls alert to the click method.

$('#eventDemo4').click(function() { alert("Clicked!"); });

Event Pitfalls

var container = $('#buttonBox1').empty();
for (var i = 1; i <= 9; i++) {
	$('<button>' + i + '</button>')
		.click(function() { alert(i); })
Buttons will appear here!

This code captures the i variable in the function expression. However, because Javascript doesn't have block scoping, the code shares a single i variable among all of the callbacks.
The callbacks are only run after the for loop finishes (when the user clicks the buttons), when i is 10.

var container = $('#buttonBox2').empty();
for (var i = 1; i <= 9; i++) {
function createButton(i) {
	$('<button>' + i + '</button>')
		.click(function() { alert(i); })
Buttons will appear here!

To fix this code, I move the callbacks to a separate method which takes i as a parameter. Since each callback now comes from a different method invocation, they don't share the same variable.

Live Events

Another common problem with jQuery events is that event handlers are only added to the elements currently in the set. If new similar elements are added later, they won't get event handlers unless they're added separately.

To solve this problem, jQuery supports live events, which bind handlers to all elements that match a selector, no matter when they were created.

$('#liveBox button').live('click', function() { 
    alert("Congratulations!\nYou clicked a button!");

After running this code, all elements that match the selector will have their click event handled.



You can animate a CSS property by calling the animate method.

var p = $('#slide27 p:visible');
    { fontSize: 50 }, 
    2500, //milliseconds
    function() { alert('Done!\n\n' + $(this).text()); }

Animations are queued on each element, meaning that if you call animate twice, the second animation will run after the first one finishes.

var p = $('#slide27 p:visible');
p.animate({ fontSize:  5 })
 .animate({ fontSize: 25 });

Any numeric property can be animated.
jQuery UI adds the ability to animate colors.

Common Traversal Methods


jQuery also has an AJAX library, which hides the differences in XmlHttpRequests between browsers.

To send an AJAX request, call $.ajax:

    url: "/something",
    type: "POST",
    success: function(data, status) {
    error: function() { 

AJAX is asynchronous. When you send an AJAX request, the next line of code will execute immediately, before the server sends a reply. Therefore, any code that needs the server's reply must go in the success callback.


For security reasons, AJAX requests can only be sent to URLs in the same domain that the page is running on.

To send AJAX requests to a different website, you can use the JSONP protocol. (assuming that the other server supports it)
jQuery includes a JSONP client in the getJSON method:

    + "?lat=40.849557&lng=-73.929857&username=demo&callback=?",
    function(data) {
        var celsius = data.weatherObservation.temperature;
        var fahrenheit = 32 + (212 - 32) / 100 * celsius;
        alert("It's " + Math.round(fahrenheit) + "° outside!");

Understanding JSONP

When you make a JSONP request, jQuery will create a callback function with a unique name and add a callback=name parameter to the query string. It will then create <script> tag pointing to the URL.

The server-side script is expected to return Javascript code that calls the function specified by the callback parameter and passes it the requested data.

In this case, jQuery will generate a URL like

This URL returns Javascript code like jQuery1510697_1300041015100({"weatherObservation": { ... }});.
When the new <script> tag executes, this code calls the generated jQuery1510697_1300041015100 method, which will in turn call the callback passed to getJSON.

Extending jQuery

Creating Plugins

You can add your own methods to the jQuery object by adding them to jQuery.fn:

jQuery.fn.double = function() { 
    return this;
var p = $('#slide30 p.Item:first');

These methods are called on jQuery objects, so the this keyword will refer to the jQuery object it was called on.

Thus, I can write this.html() to get the HTML source of the object that the double() method was called on.

In order to allow method chaining, plugin methods should end with return this; this allows you to call other methods after calling the plugin.
Obviously, if you create a plugin that returns information, it should not return this; it should return whatever information it's supposed to. (Like an informative method)

Existing Plugins

jQuery also has a wide ecosystem of free plugins.
Some of the more popular ones include:

Additional Resources


Always check the documentation!
jQuery's documentation is very thorough and understandable, and has lots of examples.


Firebug Inspector Firebug Console


jsFiddle is an HTML sandbox which allows you to quickly try out HTML, CSS, and Javascript code without creating files on your hard disk.

You can type arbitrary code (with syntax highlighting) and execute it on the fly. You can also save your code to a permanent URL so that you can come back to it later or share it with someone else.

Visual Studio

Visual Studio is a great all-around development tool.
Using a vsdoc file, you can even get full IntelliSense for jQuery.

Visual Studio jQuery IntelliSense

Ordinarily, Visual Studio Professional costs $800, but it's available for free to students through Microsoft's DreamSpark program.
Just go to and follow the instructions to verify that you're a student.


StackOverflow is the world's largest online developer community. You can ask programming related questions there and (typically) get an answer within minutes.
Unlike ExpertsExchange, it's completely free.

SLaks - StackOverflow


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