Beginning JavaScript

Jeremy McPeak

• Editore: Wrox
• Anno: 2015
• ISBN:
  9781118903742
• Edizione: 5
• Argomento: Computer Science
• Categoria: Programming in JavaScript

The bestselling JavaScript guide, updated with current features and best practices

Beginning JavaScript 5th Edition shows you how to work effectively with JavaScript frameworks, functions, and modern browsers, and teaches more effective coding practices using HTML5. This new edition has been extensively updated to reflect the way JavaScript is most commonly used today, introducing you to the latest tools and techniques available to JavaScript developers. Coverage includes modern coding practices using HTML5 markup, the JSON data format, DOM APIs, the jQuery framework, and more. Exercises with solutions provide plenty of opportunity to practice, and the companion website offers downloadable code for all examples given in the book.

• Learn JavaScript using the most up to date coding style
• Understand JSON, functions, events, and feature detection
• Utilize the new HTML5 elements and the related API
• Explore new features including geolocation, local storage, and more

JavaScript has shaped the Web from a passive medium into one that is rich, dynamic, and interactive. No matter the technology on the server side, it's JavaScript that makes it come alive in the browser. To learn JavaScript the way it's used today, Beginning JavaScript, 5th Edition is your concise guide.

1
Introduction to JavaScript and the Web

WHAT YOU WILL LEARN IN THIS CHAPTER:

• Adding JavaScript to your web pages
• Referencing external JavaScript files
• Changing the background color of a web page

WROX.COM CODE DOWNLOADS FOR THIS CHAPTER

You can find the wrox.com code downloads for this chapter at http://www.wiley.com/go/BeginningJavaScript5E on the Download Code tab. You can also view all of the examples and related files at http://beginningjs.com.

In this introductory chapter, you look at what JavaScript is, what it can do for you, and what you need in order to use it. With these foundations in place, you will see throughout the rest of the book how JavaScript can help you to create powerful web applications for your website.

The easiest way to learn something is by actually doing it, so throughout the book you create a number of useful example programs using JavaScript. This process starts in this chapter, by the end of which you will have created your first piece of JavaScript code.

INTRODUCTION TO JAVASCRIPT

In this section you take a brief look at what JavaScript is, where it came from, how it works, and what sorts of useful things you can do with it.

What Is JavaScript?

Having bought this book, you are probably already well aware that JavaScript is some sort of computer language, but what is a computer language? Put simply, a computer language is a series of instructions that tell the computer to do something. That something can be one of a wide variety of things, including displaying text, moving an image, or asking the user for information. Normally, the instructions, or what is termed code, are processed from the top line downward. This simply means that the computer looks at the code you’ve written, works out what action you want it to take, and then takes that action. The act of processing the code is called running or executing it.

In natural English, here are instructions, or code, you might write to make a cup of instant coffee:

1. Put coffee crystals in cup.
2. Fill kettle with water.
3. Put kettle on to boil.
4. Has the kettle boiled? If so, then pour water into cup; otherwise, continue to wait.
5. Drink coffee.

You’d start running this code from the first line (instruction 1), and then continue to the next (instruction 2), then the next, and so on until you came to the end. This is pretty much how most computer languages work, JavaScript included. However, on some occasions you might change the flow of execution or even skip over some code, but you see more of this in Chapter 3.

JavaScript is an interpreted language rather than a compiled language. What is meant by the terms interpreted and compiled?

Well, to let you in on a secret, your computer doesn’t really understand JavaScript at all. It needs something to interpret the JavaScript code and convert it into something that it understands; hence it is an interpreted language. Computers understand only machine code, which is essentially a string of binary numbers (that is, a string of zeros and ones). As the browser goes through the JavaScript, it passes it to a special program called an interpreter, which converts the JavaScript to the machine code your computer understands. It’s a bit like having a translator translate English to Spanish, for example. The important point to note is that the conversion of the JavaScript happens at the time the code is run; it has to be repeated every time this happens. JavaScript is not the only interpreted language; others exist, including PHP and Ruby.

The alternative compiled language is one in which the program code is converted to machine code before it’s actually run, and this conversion has to be done only once. The programmer uses a compiler to convert the code that he wrote to machine code, and this machine code is run by the program’s user. Compiled languages include C#, Java, and many others. Using a real-world analogy, it’s a bit like having a Spanish translator verbally tell you in English what a Spanish document says. Unless you change the document, you can use it without retranslation as much as you like.

Perhaps this is a good place to dispel a widespread myth: JavaScript is not the script version of the Java language. In fact, although they share the same name, that’s virtually all they do share. Particularly good news is that JavaScript is much, much easier to learn and use than Java. In fact, languages like JavaScript are the easiest of all languages to learn, but they are still surprisingly powerful.

JavaScript and the Web

For most of this book you look at JavaScript code that runs inside a web page loaded into a browser. All you need to create these web pages is a text editor—for example, Windows Notepad—and a web browser, such as Chrome, Firefox, or Internet Explorer (IE), with which you can view your pages. These browsers come equipped with JavaScript interpreters (more commonly known as JavaScript engines).

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NOTE Throughout this book, we use the terms “IE” and “Internet Explorer” interchangeably when referring to Microsoft’s Internet Explorer browser.

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In fact, the JavaScript language first became available in Netscape’s Navigator 2. Initially, it was called LiveScript, but because Java was the hot technology of the time, Netscape decided that JavaScript sounded more exciting. When JavaScript really took off, Microsoft decided to add its own dialect of JavaScript, called JScript, to Internet Explorer 3.

In 1997, JavaScript was standardized by Ecma International, a membership-based non-profit organization, and renamed to ECMAScript. Today’s browser makers look to the ECMAScript standard to implement the JavaScript engines included in their respective browsers, but that doesn’t necessarily mean that all browsers support the same features. JavaScript support among today’s browsers is certainly more unified than it has ever been, but as you see in future chapters, developers still have to cope with older, and in many cases non-standard, JavaScript implementations.

The ECMAScript standard controls various aspects of the language and helps ensure that different versions of JavaScript are compatible. However, although Ecma sets standards for the actual language, it doesn’t specify how it’s used in particular hosts. By host, we mean hosting environment; in this book, that is the web browser. Other hosting environments include PDF files, web servers, and many, many other places. In this book, we discuss only its use within the web browser. The organization that sets the standards for web pages is the World Wide Web Consortium (W3C). It not only sets standards for HTML and CSS, but also for how JavaScript interacts with web pages inside a web b...