The jQuery library provides a general-purpose abstraction layer for common web scripting, and it is therefore useful in almost every scripting situation. Its extensible nature means that we could never cover all the possible uses and functions in a single book, as plugins are constantly being developed to add new abilities. The core features, though, assist us in accomplishing the following tasks:

• Access elements in a document: Without a JavaScript library, web developers often need to write many lines of code to traverse the Document Object Model (DOM) tree and locate specific portions of an HTML document's structure. With jQuery, developers have a robust and efficient selector mechanism at their disposal, making it easy to retrieve the exact piece of the document that needs to be inspected or manipulated.

$('div.content').find('p'); 

• Modify the appearance of a web page: CSS offers a powerful method of influencing the way a document is rendered, but it falls short when not all web browsers support the same standards. With jQuery, developers can bridge this gap, relying on the same standards support across all browsers. In addition, jQuery can change the classes or individual style properties applied to a portion of the document even after the page has been rendered.

$('ul > li:first').addClass('active'); 

• Alter the content of a document: Not limited to mere cosmetic changes, jQuery can modify the content of a document itself with a few keystrokes. Text can be changed, images can be inserted or swapped, lists can be reordered, or the entire structure of the HTML can be rewritten and extended--all with a single easy-to-use Application Programming Interface (API).

$('#container').append('<a href="more.html">more</a>'); 

• Respond to a user's interaction: Even the most elaborate and powerful behaviors are not useful if we can't control when they take place. The jQuery library offers an elegant way to intercept a wide variety of events, such as a user clicking on a link, without the need to clutter the HTML code itself with event handlers.

$('button.show-details').click(() => { 
  $('div.details').show(); 
});

• Animate changes being made to a document: To effectively implement such interactive behaviors, a  designer must also provide visual feedback to the user. The jQuery library facilitates this by providing an array of effects such as fades and wipes, as well as a toolkit for crafting new ones.

$('div.details').slideDown(); 

• Retrieve information from a server without refreshing a page: This pattern is known as Ajax, which originally stood for Asynchronous JavaScript and XML, but has since come to represent a much greater set of technologies for communicating between the client and the server. The jQuery library removes the browser-specific complexity from this process, allowing developers to focus on the server-side functionality.

$('div.details').load('more.html #content');

With the resurgence of interest in dynamic HTML comes a proliferation of JavaScript frameworks. Some are specialized, focusing on just one or two of the tasks previously mentioned. Others attempt to catalog every possible behavior and animation and serves these up prepackaged. To maintain the wide range of features outlined earlier while remaining relatively compact, jQuery employs several strategies:

• Leverage knowledge of CSS: By basing the mechanism for locating page elements on CSS selectors, jQuery inherits a terse yet legible way of expressing a document's structure. The jQuery library becomes an entry point for designers who want to add behaviors to their pages because a prerequisite for doing professional web development is knowledge of CSS syntax.
• Support extensions: In order to avoid "feature creep", jQuery relegates special-case uses to plugins. The method for creating new plugins is simple and well documented, which has spurred the development of a wide variety of inventive and useful modules. Even most of the features in the basic jQuery download are internally realized through the plugin architecture and can be removed if desired, yielding an even smaller library.
• Abstract away browser quirks: An unfortunate reality of web development is that each browser has its own set of deviations from published standards. A significant portion of any web application can be relegated to handling features differently on each platform. While the ever-evolving browser landscape makes a perfectly browser-neutral codebase impossible for some advanced features, jQuery adds an abstraction layer that normalizes the common tasks, reducing the size of code while tremendously simplifying it.
• Always work with sets: When we instruct jQuery to find all elements with the class collapsible and hide them, there is no need to loop through each returned element. Instead, methods such as .hide() are designed to automatically work on sets of objects instead of individual ones. This technique, called implicit iteration, means that many looping constructs become unnecessary, shortening code considerably.
• Allow multiple actions in one line: To avoid overuse of temporary variables or wasteful repetition, jQuery employs a programming pattern called chaining for the majority of its methods. This means that the result of most operations on an object is the object itself, ready for the next action to be applied to it.

These strategies keep the file size of the jQuery package small, while at the same time providing techniques for keeping our custom code that uses the library compact as well.

The elegance of the library comes about partly by design and partly due to the evolutionary process spurred by the vibrant community that has sprung up around the project. Users of jQuery gather to discuss not only the development of plugins but also enhancements to the core library. The users and developers also assist in continually improving the official project documentation, which can be found at http://api.jquery.com.

Despite all the efforts required to engineer such a flexible and robust system, the end product is free for all to use. This open source project is licensed under the MIT License to permit free use of jQuery on any site and facilitate its use within proprietary software. If a project requires it, developers can relicense jQuery under the GNU Public License for inclusion in other GNU-licensed open source projects.

The changes introduced in jQuery 3 are quite subtle compared to the changes introduced in jQuery 2. Most of what's changed is under the hood. Let's take a brief look at some changes and how they're likely to impact an existing jQuery project. You can review the fine-grained details (https://jquery.com/upgrade-guide/3.0) while reading this book.

The biggest change with browser support in jQuery 3 is Internet Explorer. Having to support older versions of this browser is the bane of any web developer's existence. jQuery 3 has taken a big step forward by only supporting IE9+. The support policy for other browsers is the current version and the previous version.

The Deferred object was introduced in jQuery 1.5 as a means to better manage asynchronous behavior. They were kind of like ES2015 promises, but different enough that they weren't interchangeable. Now that the ES2015 version of JavaScript is commonplace in modern browsers, the Deferred object is fully compatible with native Promise objects. This means that quite a lot has changed with the old Deferred implementation.

The idea that the document-ready callback function is executed asynchronously might seem counterintuitive at first. There are a couple of reasons this is the case in jQuery 3. First, the $(() => {}) expression returns a Deferred instance, and these now behave like native promises. The second reason is that there's a jQuery.ready promise that resolves when the document is ready. As you'll see later on in this book, you can use this promise alongside other promises to perform other asynchronous tasks before the DOM is ready to render.

There are a number of other breaking changes to the API that were introduced in jQuery 3 that we won't dwell on here. The upgrade guide that I mentioned earlier goes into detail about each of these changes and how to deal with them. However, I'll point out functionality that's new or different in jQuery 3 as we make our way through this book.

Now that we have covered the range of features available to us with jQuery, we can examine how to put the library into action. To get started, we need to download a copy of jQuery.

No installation is required. To use jQuery, we just need a publicly available copy of the file, no matter whether that copy is on an external site or our own. Since JavaScript is an interpreted language, there is no compilation or build phase to worry about. Whenever we need a page to have jQuery available, we will simply refer to the file's location from a <script> element in the HTML document.

The official jQuery website (http://jquery.com/) always has the most up-to-date stable version of the library, which can be downloaded right from the home page of the site. Several versions of jQuery may be available at any given moment; the most appropriate for us as site developers will be the latest uncompressed version of the library. This can be replaced with a compressed version in production environments.

As jQuery's popularity has grown, companies have made the file freely available through their Content Delivery Networks (CDNs). Most notably, Google (https://developers.google.com/speed/libraries/devguide), Microsoft (http://www.asp.net/ajaxlibrary/cdn.ashx), and the jQuery project itself (http://code.jquery.com) offer the file on powerful, low-latency servers distributed around the world for fast download, regardless of the user's location. While a CDN-hosted copy of jQuery has speed advantages due to server distribution and caching, using a local copy can be convenient during development. Throughout this book, we'll use a copy of the file stored on our own system, which will allow us to run our code whether we're connected to the Internet or not.

There are three pieces to most examples of jQuery usage: the HTML document, CSS files to style it, and JavaScript files to act on it. For our first example, we'll use a page with a book excerpt that has a number of classes applied to portions of it. This page includes a reference to the latest version of the jQuery library, which we have downloaded, renamed jquery.js, and placed in our local project directory:

<!DOCTYPE html> 

<html lang="en"> 
  <head> 
    <meta charset="utf-8"> 
    <title>Through the Looking-Glass</title> 

    <link rel="stylesheet" href="01.css"> 

    <script src="jquery.js"></script> 
    <script src="01.js"></script> 
  </head> 

  <body>   
    <h1>Through the Looking-Glass</h1> 
    <div class="author">by Lewis Carroll</div> 

    <div class="chapter" id="chapter-1"> 
      <h2 class="chapter-title">1. Looking-Glass House</h2> 
      <p>There was a book lying near Alice on the table, 
        and while she sat watching the White King (for she 
        was still a little anxious about him, and had the 
        ink all ready to throw over him, in case he fainted 
        again), she turned over the leaves, to find some 
        part that she could read, <span class="spoken"> 
        "&mdash;for it's all in some language I don't know," 
        </span> she said to herself.</p> 
      <p>It was like this.</p> 
      <div class="poem"> 
        <h3 class="poem-title">YKCOWREBBAJ</h3> 
        <div class="poem-stanza"> 
          <div>sevot yhtils eht dna ,gillirb sawT'</div> 
          <div>;ebaw eht ni elbmig dna eryg diD</div> 
          <div>,sevogorob eht erew ysmim llA</div> 
          <div>.ebargtuo shtar emom eht dnA</div> 
        </div> 
      </div> 
      <p>She puzzled over this for some time, but at last 
        a bright thought struck her. <span class="spoken"> 
        "Why, it's a Looking-glass book, of course! And if 
        I hold it up to a glass, the words will all go the 
        right way again."</span></p> 
      <p>This was the poem that Alice read.</p> 
      <div class="poem"> 
        <h3 class="poem-title">JABBERWOCKY</h3> 
        <div class="poem-stanza"> 
          <div>'Twas brillig, and the slithy toves</div> 
          <div>Did gyre and gimble in the wabe;</div> 
          <div>All mimsy were the borogoves,</div> 
          <div>And the mome raths outgrabe.</div> 
        </div> 
      </div> 
    </div> 
  </body> 
</html> 

Immediately following the normal HTML preamble, the stylesheet is loaded. For this example, we'll use a simple one:

body { 
  background-color: #fff; 
  color: #000; 
  font-family: Helvetica, Arial, sans-serif; 
}
h1, h2, h3 { 
  margin-bottom: .2em; 
}
.poem { 
  margin: 0 2em; 
} 
.highlight { 
  background-color: #ccc; 
  border: 1px solid #888; 
  font-style: italic; 
  margin: 0.5em 0; 
  padding: 0.5em; 
} 

After the stylesheet is referenced, the JavaScript files are included. It is important that the script tag for the jQuery library be placed before the tag for our custom scripts; otherwise, the jQuery framework will not be available when our code attempts to reference it.

Now, we have a page that looks like this:

We will use jQuery to apply a new style to the poem text.

Our custom code will go in the second, currently empty, JavaScript file, which we included from the HTML using <script src="01.js"></script>. For this example, we only need three lines of code:

$(() => {
  $('div.poem-stanza').addClass('highlight')
});

Now let's step through this script piece by piece to see how it works.

The fundamental operation in jQuery is selecting a part of the document. This is done with the $() function. Typically, it takes a string as a parameter, which can contain any CSS selector expression. In this case, we wish to find all of the <div> elements in the document that have the poem-stanza class applied to them, so the selector is very simple. However, we will cover much more sophisticated options through the course of the book. We will walk through many ways of locating parts of a document in Chapter 2, Selecting Elements.

When called, the $() function returns a new jQuery object instance, which is the basic building block we will be working with from now on. This object encapsulates zero or more DOM elements and allows us to interact with them in many different ways. In this case, we wish to modify the appearance of these parts of the page and we will accomplish this by changing the classes applied to the poem text.

The .addClass() method, like most jQuery methods, is named self descriptively; it applies a CSS class to the part of the page that we have selected. Its only parameter is the name of the class to add. This method, and its counterpart, .removeClass(), will allow us to easily observe jQuery in action as we explore the different selector expressions available to us. For now, our example simply adds the highlight class, which our stylesheet has defined as italicized text with a gray background and a border.

Taken together, $() and .addClass() are enough for us to accomplish our goal of changing the appearance of the poem text. However, if this line of code is inserted alone in the document header, it will have no effect. JavaScript code is run as soon as it is encountered in the browser, and at the time the header is being processed, no HTML is yet present to style. We need to delay the execution of the code until after the DOM is available for our use.

With the $(() => {}) construct (passing a function instead of a selector expression), jQuery allows us to schedule function calls for firing once the DOM is loaded, without necessarily waiting for images to fully render. While this event scheduling is possible without the aid of jQuery, $(() => {}) provides an especially elegant cross-browser solution that includes the following features:

• It uses the browser's native DOM-ready implementations when available and adds a window.onload event handler as a safety net
• It executes functions passed to $() even if it is called after the browser event has already occurred
• It handles the event scheduling asynchronously to allow scripts to delay if necessary

The $() function's parameter can accept a reference to an already defined function, as shown in the following code snippet:

function addHighlightClass()  { 
  $('div.poem-stanza').addClass('highlight'); 
} 

$(addHighlightClass); 

However, as demonstrated in the original version of the script and repeated in Listing 1.2, the method can also accept an anonymous function:

$(() =>
  $('div.poem-stanza').addClass('highlight')
); 

This anonymous function idiom is convenient in jQuery code for methods that take a function as an argument when that function isn't reusable. Moreover, the closure it creates can be an advanced and powerful tool. If you're using arrow functions, you also get lexically bound this as a context, which avoids having to bind functions. It may also have unintended consequences and ramifications of memory use, however, if not dealt with carefully.

Now that our JavaScript is in place, the page looks like this:

The poem stanzas are now italicized and enclosed in boxes, as specified by the 01.css stylesheet, due to the insertion of the highlight class by the JavaScript code.

Even a task as simple as this can be complicated without jQuery at our disposal. In plain JavaScript, we could add the highlight class this way:

window.onload = function() {
  const divs = document.getElementsByTagName('div');
  const hasClass = (elem, cls) =>
    new RegExp(` ${cls} `).test(` ${elem.className} `);

  for (let div of divs) {
    if (hasClass(div, 'poem-stanza') && !hasClass(div, 'highlight')) {
      div.className += ' highlight';
    }
  }
};

Despite its length, this solution does not handle many of the situations that jQuery takes care of for us in Listing 1.2, such as:

• Properly respecting other window.onload event handlers
• Acting as soon as the DOM is ready
• Optimizing element retrieval and other tasks with modern DOM methods

We can see that our jQuery-driven code is easier to write, simpler to read, and faster to execute than its plain JavaScript equivalent.

As this code comparison has shown, jQuery code is typically shorter and clearer than its basic JavaScript equivalent. However, this doesn't mean we will always write code that is free from bugs or that we will intuitively understand what is happening on our pages at all times. Our jQuery coding experience will be much smoother with the assistance of standard development tools.

High-quality development tools are available in all modern browsers. We can feel free to use the environment that is most comfortable to us. Options include the following:

• Microsoft Edge (https://developer.microsoft.com/en-us/microsoft-edge/platform/documentation/f12-devtools-guide/)
• Internet Explorer Developer Tools (http://msdn.microsoft.com/en-us/library/dd565628.aspx)
• Safari Web Development Tools (https://developer.apple.com/safari/tools/)
• Chrome Developer Tools (https://developer.chrome.com/devtools)
• Firefox Developer Tools (https://developer.mozilla.org/en-US/docs/Tools)

Each of these toolkits offers similar development features, including:

• Exploring and modifying aspects of the DOM
• Investigating the relationship between CSS and its effect on page presentation
• Convenient tracing of script execution through special methods
• Pausing execution of running scripts and inspecting variable values

While the details of these features vary from one tool to the next, the general concepts remain the same. In this book, some examples will require the use of one of these toolkits; we will use Chrome Developer Tools for these demonstrations, but development tools for other browsers are fine alternatives.

Up-to-date instructions for accessing and using Chrome Developer Tools can be found on the project's documentation pages at https://developer.chrome.com/devtools. The tools are too involved to explore in great detail here, but a survey of some of the most relevant features will be useful to us.

When Chrome Developer Tools is activated, a new panel appears offering information about the current page. In the default Elements tab of this panel, we can see a representation of the page structure on the left-hand side and details of the selected element (such as the CSS rules that apply to it) on the right-hand side. This tab is especially useful for investigating the structure of the page and debugging CSS issues:

The Sources tab allows us to view the contents of all loaded scripts on the page. By right-clicking on a line number, we can set a breakpoint, set a conditional breakpoint, or have the script continue to that line after another breakpoint is reached. Breakpoints are effective ways to pause the execution of a script and examine what occurs in a step-by-step fashion. On the right-hand side of the page, we can enter a list of variables and expressions we wish to know the value of at any time:

The Console tab will be of most frequent use to us while learning jQuery. A field at the bottom of the panel allows us to enter any JavaScript statement, and the result of the statement is then presented in the panel.

In this example, we perform the same jQuery selector as in Listing 1.2, but we are not performing any action on the selected elements. Even so, the statement gives us interesting information: we see that the result of the selector is a jQuery object pointing to the two .poem-stanza elements on the page. We can use this console feature to quickly try out jQuery code at any time, right from within the browser:

In addition, we can interact with this console directly from our code using the console.log() method:

$(() => {
  console.log('hello');
  console.log(52);
  console.log($('div.poem-stanza'));
});

This code illustrates that we can pass any kind of expression into the console.log() method. Simple values such as strings and numbers are printed directly, and more complicated values such as jQuery objects are nicely formatted for our inspection:

This console.log() function (which works in each of the browser developer tools we mentioned earlier) is a convenient alternative to the JavaScript alert() function, and will be very useful as we test our jQuery code.

In this chapter, we learned how to make jQuery available to JavaScript code on our web page, use the $() function to locate a part of the page that has a given class, call .addClass() to apply additional styling to this part of the page, and invoke $(() => {}) to cause this function to execute upon loading the page. We have also explored the development tools we will be relying on when writing, testing, and debugging our jQuery code.

We now have an idea of why a developer would choose to use a JavaScript framework rather than writing all code from scratch, even for the most basic tasks. We have also seen some of the ways in which jQuery excels as a framework, why we might choose it over other options, and in general, which tasks jQuery makes easier.

The simple example we have been using demonstrates how jQuery works, but is not very useful in real-world situations. In the next chapter, we will expand on this code by exploring jQuery's sophisticated selector language, finding practical uses for this technique.