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Book Excerpt: jQuery Essentials | Part 1

An introduction to jQuery

This excerpt is from the book Murach's JavaScript and jQuery by Mike Murach and Zak Ruvalcaba.

Now that you have the JavaScript skills that you need for using jQuery, you're ready to learn jQuery. So, in this excerpt, you'll learn a working subset of jQuery that will get you off to a fast start.

When you complete this section, you'll have all the jQuery skills that you need for developing professional web pages. You can also go on to any of the three sections that follow because they are written as independent modules. If, for example, you want to learn how to use Ajax next, skip to section 4.

Get off to a fast start with jQuery
In this excerpt you'll quickly see how jQuery makes JavaScript programming easier. Then, you'll learn a working subset of jQuery that will get you off to a fast start. Along the way, you'll study four complete applications that will show you how to apply jQuery.

Introduction to jQuery
In this introduction, you'll learn what jQuery is, how to include it in your applications, and how jQuery, jQuery UI, and plugins can simplify JavaScript development.

What jQuery is
As Figure 1 summarizes, jQuery is a free, open-source, JavaScript library that provides dozens of methods for common web features that make JavaScript programming easier. Beyond that, the jQuery functions are coded and tested for cross-browser compatibility, so they will work in all browsers.

What jQuery offers

Dozens of methods that make it easier to add JavaScript features to your web pages

ŸMethods that are tested for cross-browser compatibility

How to include the jQuery file after you've downloaded it to your computer

<script src="jquery-1.8.2.min.js"></script>

How to include the jQuery file from a Content Delivery Network (CDN)

<script src="http://code.jquery.com/jquery-latest.min.js"></script>

Description

ŸjQuery is a free, open-source, JavaScript library that provides methods that make JavaScript programming easier. Today, jQuery is used by more than 50% of the 10,000 most-visited web sites, and its popularity is growing rapidly.

ŸThe jQuery download comes in two versions. One version (min) is a compressed version that is relatively small and loads fast. The other version is uncompressed so you can use it to study the JavaScript code in the library.

ŸIf you include the jQuery file from a Content Delivery Network (CDN), you don't have to provide it from your own server, but then you can't work offline.

ŸThe jQuery CDN now provides a link that will always deliver the latest version of jQuery. That's the way the script element is coded in all of the applications in this book.

ŸIf you download the jQuery file to your system, you can change the filename so it's simpler, but then you may lose track of what version you're using.

Figure 1: What jQuery is and how to include it in your applications

Those are two of the reasons why jQuery is used by more than half of the 10,000 most-visited web sites today. And that's why jQuery is commonly used by professional web developers. In fact, you can think of jQuery as one of the four technologies that every web developer should know how to use: HTML, CSS, JavaScript, and jQuery. But don't forget that jQuery is actually JavaScript.

How to include jQuery in your web pages
If you go to the web site that's shown in this figure, you'll find a download button that lets you download the single file that contains the jQuery core library. By default, the version that's downloaded is a compressed version that today is around 32KB. As a result, this version loads quickly into browsers, which is another reason why developers like jQuery.

The other version is uncompressed and currently about 247KB. If you download this version, you can study the JavaScript code that's used in the library. But beware, this code is extremely complicated.

Once you've downloaded the compressed version of the core library, you can include it in a web page by coding a script statement like the first one in this figure. Then, if you store the file on your own computer or a local web server, you'll be able to develop jQuery applications without being connected to the Internet. For production applications, though, you'll need to deploy the file to your Internet web server.

In this script statement, the file name includes the version number, but you can use whatever file name you want. However, if the file name doesn't include the version number, it's easy to lose track of which version you're using.

The other way to include the jQuery library in your web applications and the one we recommend is to get the file from a Content Delivery Network (CDN). A CDN is a web server that hosts open-source software, and the Google, Microsoft, and jQuery web sites are CDNs for getting the jQuery libraries. In the second example in this figure, the script element uses the jQuery CDN with a URL that gets the latest version of jQuery, and that's the way all of the applications in this book include the jQuery library.

The benefit to using a CDN is that you don't have to download the jQuery file. This works especially well with the jQuery CDN and the "latest" URL. Then, you don't have to change the URL when a new release becomes available. The disadvantage is that you have to be connected to the Internet to use a CDN.

How jQuery can simplify JavaScript development
To show you how jQuery can simplify JavaScript development, Figure 2 shows both the JavaScript and the jQuery for the FAQs application that you learned how to develop in chapter 6. If you're like most people who are learning JavaScript, you probably found the JavaScript code for this application both complicated and confusing. That's because it is.

The JavaScript for the application

var $ = function (id) {
return document.getElementById(id);
}
window.onload = function () {
var faqs = $("faqs");
var h2Elements = faqs.getElementsByTagName("h2");
var h2Node;
for (var i = 0; i < h2Elements.length; i++ ) {
h2Node = h2Elements[i];
// Attach event handler
h2Node.onclick = function () {
var h2 = this;         // h2 is the current h2Node object
if (h2.getAttribute("class") == "plus") {
h2.setAttribute("class", "minus");   
}
else {
h2.setAttribute("class", "plus");
}
if (h2.nextElementSibling.getAttribute("class") == "closed") {
h2.nextElementSibling.setAttribute("class", "open");
}
else {
h2.nextElementSibling.setAttribute("class", "closed");
}
}
}
}

The jQuery for the application

$(document).ready(function() {
$("#faqs h2").toggle(
function() {
$(this).addClass("minus");
$(this).next().show();
},
function() {
$(this).removeClass("minus");
$(this).next().hide();
}
);  // end toggle
}); // end ready

Figure 2: How jQuery can simplify JavaScript development

In contrast, the jQuery code takes less than half as many lines of code. You'll also find that it is much easier to understand once you learn how to use the JQuery selectors, methods, and event methods. And you'll start learning those skills right after this introduction.

Incidentally, jQuery uses CSS selectors to select the HTML elements that the methods should be applied to. For instance,

$("#faqs h2")

is a jQuery selector for the CSS selector

#faqs h2

which selects all of the h2 elements in the element with "faqs" as its id. In fact, jQuery supports all of the CSS selectors including the CSS3 selectors, even in browsers that don't support all of the CSS3 selectors. This is another reason why developers like jQuery.

How jQuery can affect testing and debugging
When you use jQuery to develop applications, you can use the same testing and debugging skills that you learned in chapter 4. Remember, though, that a call to a jQuery method is a call to the jQuery file that you specify in a script element in the head section of the HTML. In other words, when your code executes a jQuery method, it is the JavaScript in the jQuery file that does the processing.

Then, if the jQuery code can't be executed, the display in Firebug or in the error console of the browser will point to a statement in the jQuery file. This can happen, for example, because a faulty parameter is passed to a jQuery method. Unfortunately, the information in Firebug or the error console doesn't tell you which statement in your code caused the problem. Instead, you have to use the other debugging techniques to find the bug and fix it.

In most cases, though, Firebug and the error console of the browser do provide information that helps you find and fix the bug. So this is a minor problem that is more than compensated for by the benefits that you get from using jQuery.

How jQuery UI and plugins can simplify JavaScript development
Besides the core jQuery library, jQuery provides the jQuery UI (User Interface) library. The functions in this library use the core jQuery library to build advanced features that can be created with just a few lines of code. These features include themes, effects, widgets, and mouse interactions.

For instance, the browser display in Figure 3 shows the FAQs application as a jQuery UI widget known as an accordion. To implement this widget, you just need the three lines of JavaScript code that are highlighted, and that also applies the formatting that's shown.

To use jQuery UI, you include a jQuery UI library file in your web pages in much the same way that you include the jQuery core library. You also include a jQuery UI CSS file that provides the themes for jQuery UI. In section 3, you'll learn how to do that, and you'll learn how to use the features of jQuery UI.

The HTML for a jQuery UI accordion

<div id="accordion">
<h3><a href="#">What is jQuery?</a></h3>
<div> <!-- panel contents --> </div>
<h3><a href="#">Why is jQuery becoming so popular?</a></h3>
<div> <!-- panel contents --> </div>
<h3><a href="#">Which is harder to learn: jQuery or JavaScript?></a></h3>
<div> <!-- panel contents --> </div>
</div>

The JavaScript code for the jQuery UI accordion

<script>
$(document).ready(function() {
$("#accordion").accordion();
});
</script>

Some typical plugin functions

ŸData validation

ŸSlide shows

ŸCarousels

Description
jQuery UI is a free, open-source, JavaScript library that provides higher-level effects, widgets, and mouse interactions that can be customized by using themes. In section 3, you'll learn how to use jQuery UI.

A plugin is a JavaScript library that provides functions that work in conjunction with jQuery to make it easier to add features to your web applications. In chapter 11, you'll learn how to use some of the most useful plugins, and you'll also learn how to create your own plugins.

In general, if you can find a plugin or jQuery UI feature that does what you want it to do, you should use it. Often, though, you won't be able to find what you want so you'll need to develop the feature with just the core jQuery library.

Figure 3: How jQuery UI and plugins can simplify JavaScript development

Because jQuery UI can make JavaScript development even easier than it is when using jQuery, it makes sense to use jQuery UI whenever it provides an effect, widget, or mouse interaction that you need. Keep in mind, though, that jQuery UI is limited, so you'll still need jQuery for most of your web applications. As a result, you should think of jQuery UI as an add-on that you should learn how to use after you master jQuery.

Besides jQuery UI, the jQuery web site provides access to dozens of plugins that have been developed for jQuery. These plugins provide higher-level functions like data validation and drop-down menus that require minimal coding for their implementation. To facilitate the development of plugins, jQuery provides specifications that help standardize the way that plugins are implemented. Plugins are one more reason why developers like jQuery.

Like jQuery UI, plugins are libraries that make use of the core jQuery library. In fact, you can think of jQuery UI as a plugin. To use a plugin, you use a script element to include the plugin file in a web page, and you code that script element after the script element for the jQuery core library. In chapter 11, you'll learn how to get the most from plugins, and you'll also learn how to create your own plugins.

In practice, it makes sense to look first for a plugin that implements a feature that you want to add to a web page. If you can find one, you may be able to do in a few hours what would otherwise take a few days. Next, if you can't find a suitable plugin, it makes sense to see whether jQuery UI can facilitate the implementation of the feature. Finally, if neither a plugin nor jQuery UI can help you implement a feature, you have to use jQuery to develop it. That's why you need to master all of the jQuery skills in this section.

More Stories By Mike Murach

As a freelance writer many years ago, Mike Murach decided that he had to develop his own writing methods because the ones that others were using clearly didn’t work. Since then, Mike and his staff have continued to refine those methods, so today every Murach book becomes the best one on its subject. Now, after a long hiatus from writing, Mike has teamed with Zak Ruvalcaba to write Murach’s JavaScript and jQuery.

More Stories By Zak Ruvalcaba

Zak Ruvalcaba has been researching, designing, and developing for the Web since 1995. He holds a BS from San Diego State University and an MS in instructional technology from National University in San Diego.

Zak's skillset includes HTML/HTML5, CSS/CSS3, JavaScript, jQuery, ASP.NET, ADO.NET, Visual Basic, C#, Web Services, and Flash/ActionScript. He is also a Microsoft Certified Application Developer for .NET (MCAD) and a Microsoft Certified Solutions Developer for .NET (MCSD).

In his spare time, Zak teaches web development courses for the San Diego Community College District, Mt. San Jacinto Community College and Palomar Community College.

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