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- Herausgeber: Packt Publishing
- Kategorie: Fachliteratur
- Sprache: Englisch
JavaScript is an object-oriented programming language that is used for website development. Web pages developed today currently follow a paradigm that has three clearly distinguishable parts: content (HTML), presentation (CSS), and behavior (JavaScript). JavaScript is one important pillar in this paradigm, and is responsible for the running of the web pages.
This book will take your JavaScript skills to a new level of sophistication and get you prepared for your journey through professional web development. Updated for ES6, this book covers everything you will need to unleash the power of object-oriented programming in JavaScript while building professional web applications. The book begins with the basics of object-oriented programming in JavaScript and then gradually progresses to cover functions, objects, and prototypes, and how these concepts can be used to make your programs cleaner, more maintainable, faster, and compatible with other programs/libraries.
By the end of the book, you will have learned how to incorporate object-oriented programming in your web development workflow to build professional JavaScript applications.
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Object-Oriented JavaScript - Third Edition
Object-Oriented JavaScript - Third Edition
Copyright © 2017 Packt Publishing
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First published: July 2008
Second edition: July 2013
Third edition: January 2017
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Credits
Authors
Ved Antani
Stoyan Stefanov
Copy Editor
Zainab Bootwala
Reviewer
Mohamed Sanaulla
Project Coordinator
Ritika Manoj
Commissioning Editor
Wilson Dsouza
Proofreader
Safis Editing
Acquisition Editor
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Indexer
Rekha Nair
Content Development Editor
Arun Nadar
Graphics
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Technical Editor
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Production Coordinator
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About the Authors
Ved Antani has been building scalable server and mobile platforms using JavaScript, Go, and Java since 2005. He is an associate vice president at Myntra and has previously worked at Electronic Arts and Oracle. He is an avid reader and author on several subjects. He has studied computer science and currently lives in Bangalore, India. Ved is passionate about classical music and loves to spend time with his son.
Writing this book required a significant investment of my time, and I would like to thank my parents and family for their support and encouragement during those long days and weekends when I was practically invisible.
Stoyan Stefanov is a Facebook engineer, author, and speaker. He talks regularly about web development topics at conferences, and his blog, www.phpied.com. He also runs a number of other sites, including JSPatterns.com - a site dedicated to exploring JavaScript patterns. Previously at Yahoo!, Stoyan was the architect of YSlow 2.0 and creator of the image optimization tool, Smush.it.
A "citizen of the world", Stoyan was born and raised in Bulgaria, but is also a Canadian citizen, currently residing in Los Angeles, California. In his offline moments, he enjoys playing the guitar, taking flying lessons, and spending time at the Santa Monica beaches with his family.
I'd like to dedicate this book to my wife, Eva, and my daughters, Zlatina and Nathalie. Thank you for your patience, support, and encouragement.
About the Reviewer
Mohamed Sanaulla is a software developer with more than 7 years of experience in developing enterprise applications and Java-based back-end solutions for e-commerce applications.
His interests include Enterprise software development, refactoring and redesigning applications, designing and implementing RESTful web services, troubleshooting Java applications for performance issues, and TDD.
He has strong expertise in Java-based application development, ADF (JSF-based JavaEE web framework), SQL, PL/SQL, JUnit, designing RESTful services, Spring, Struts, Elasticsearch, and MongoDB. He is also a Sun Certified Java Programmer for the Java 6 platform. He is a moderator for JavaRanch.com. He likes to share findings on his blog (http://sanaulla.info).
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Preface
JavaScript has emerged as one of the most robust and versatile programming language around. Modern JavaScript embraces a vast array of time-tested and cutting edge features. Several of these features are slowly giving shape to the next generation of web and server platforms. ES6 introduces very important language constructs, such as promises, classes, arrow functions, and several, much anticipated features. This book takes a detailed look at the language constructs and their practical uses. This book doesn't assume any prior knowledge of JavaScript and works from the ground up to give you a thorough understanding of the language. People who know the language will still find it useful and informative. For people who already know JavaScript and are familiar with ES5 syntax, this book will be a very useful primer for ES6 features.
What this book covers
Chapter 1, Object-Oriented JavaScript, talks briefly about the history, present, and future of JavaScript, and then moves on to explore the basics of object-oriented programming (OOP) in general. You will then learn how to set up your training environment (Firebug) in order to dive into the language on your own, using the book examples as a base.
Chapter 2, Primitive Data Types, Arrays, Loops, and Conditions, discusses the language basics-variables, data types, primitive data types, arrays, loops, and conditionals.
Chapter 3, Functions, covers functions that JavaScript uses, and here you will learn to master them all. You will also learn about the scope of variables and JavaScript's built-in functions. An interesting, but often misunderstood, feature of the language-closures-is demystified at the end of the chapter.
Chapter 4, Objects, talks about objects, how to work with properties and methods, and the various ways to create your objects. This chapter also talks about built-in objects such as Array, Function, Boolean, Number, and String.
Chapter 5, ES6 Iterators and Generators, introduces the most anticipated features of ES6, Iterators and Generators. With this knowledge, you will proceed to take a detailed look at the enhanced collections constructs.
Chapter 6, Prototype, is dedicated to the all-important concept of prototypes in JavaScript. It also explains how the prototype chain works, hasOwnProperty(), and some gotchas of prototypes.
Chapter 7, Inheritance, discusses how inheritance works. This chapter also talks about a method to create subclasses like other classic languages.
Chapter 8, Classes and Modules, shows that ES6 introduces important syntactical features that makes it easier to write classical object-oriented programming constructs. ES6 class syntax wraps the slightly complex syntax of ES5. ES6 also has full language support for modules. This chapter goes into the details of the classes and module constructs introduced in ES6.
Chapter 9, Promises and Proxies, explains that JavaScript has always been a language with strong support for asynchronous programming. Up until ES5, writing asynchronous programs meant you needed to rely on callbacks-sometimes resulting in callback hell. ES6 promises are a much-awaited feature introduced in the language. Promises provide a much cleaner way to write asynchronous programs in ES6. Proxies are used to define custom behavior to some of the fundamental operations. This chapter looks at practical uses of both promises and proxies in ES6.
Chapter 10, The Browser Environment, is dedicated to browsers. This chapter also covers BOM (Browser Object Model), DOM (W3C's Document Object Model), browser events, and AJAX.
Chapter 11, Coding and Design Patterns, dives into various unique JavaScript coding patterns, as well as several language-independent design patterns, translated to JavaScript from the Book of Four, the most influential work of software design patterns. This chapter also discusses JSON.
Chapter 12, Testing and Debugging, talks about how Modern JavaScript is equipped with tools that support Test Driven Development and Behavior Driven Development. Jasmine is one of the most popular tools available at the moment. This chapter discusses TDD and BDD using Jasmine as the framework.
Chapter 13, Reactive Programming and React, explains that with the advent of ES6, several radical ideas are taking shape. Reactive programming takes a very different approach to how we manage change of states using data flows. React, however, is a framework focusing on the View part of MVC. This chapter discusses these two ideas.
Appendix A, Reserved Words, lists the reserved words in JavaScript.
Appendix B, Built-in Functions, is a reference of built-in JavaScript functions together with sample uses.
Appendix C, Built-in Objects, is a reference that provides details and examples of the use of every method and property of every built-in object in JavaScript.
Appendix D, Regular Expressions, is a regular expressions pattern reference.
Appendix E, Answers to Exercise Questions, has solutions for all the exercises mentioned at the end of the chapters.
What you need for this book
You need a modern browser-Google Chrome or Firefox are recommended, and an optional Node.js setup. Most of the code in this book can be executed in http://babeljs.io/repl/ or http://jsbin.com/. To edit JavaScript, you can use any text editor of your choice.
Who this book is for
This book is for anyone who is starting to learn JavaScript, or who knows JavaScript but isn't very good at the object-oriented part of it. This book can be a useful primer for ES6 if you are already familiar with the ES5 features of the language.
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Chapter 1. Object-Oriented JavaScript
Ever since the early days of the web, there has been a need for more dynamic and responsive interfaces. While it's OK to read static HTML pages of text, and even better when they are beautifully presented with the help of CSS, it's much more fun to engage with applications in our browsers, such as e-mail, calendars, banking, shopping, drawing, playing games, and text editing. All that is possible thanks to JavaScript, the programming language of the web. JavaScript started with simple one-liners embedded in HTML, but is now used in much more sophisticated ways. Developers leverage the object-oriented nature of the language to build scalable code architectures made up of reusable pieces.
If you look at the past and present buzzwords in web development, DHTML, Ajax, Web 2.0, HTML5, they all essentially mean HTML, CSS, and JavaScript-HTML for content, CSS for presentation, and JavaScript for behavior. In other words, JavaScript is the glue that makes everything work together so that we can build rich web applications.
However, that's not all; JavaScript can be used for more than just the web.
JavaScript programs run inside a host environment. The web browser is the most common environment, but it's not the only one. Using JavaScript, you can create all kinds of widgets, application extensions, and other pieces of software, as you'll see in a bit. Taking the time to learn JavaScript is a smart investment; you learn one language and can then write all kinds of different applications running on multiple platforms, including mobile and server-side applications. These days, it's safe to say that JavaScript is everywhere.
This book starts from zero, and does not assume any prior programming knowledge other than some basic understanding of HTML. Although there is one chapter dedicated to the web browser environment, the rest of the book is about JavaScript in general, so it's applicable to all environments.
Let's start with the following:
A bit of history
Initially, the web was not much more than just a number of scientific publications in the form of static HTML documents connected together with hyperlinks. Believe it or not, there was a time when there was no way to put an image in a page. However, that soon changed. As the web grew in popularity and size, the webmasters who were creating HTML pages felt they needed something more. They wanted to create richer user interactions, mainly driven by the desire to save server round trips for simple tasks such as form validation. Two options came up-Java applets and LiveScript, a language conceived by Brendan Eich at Netscape in 1995 and later included in the Netscape 2.0 browser under the name of JavaScript.
The applets didn't quite catch on, but JavaScript did. The ability to use short code snippets embedded in HTML documents and alter otherwise static elements of a web page was embraced by the webmaster community. Soon, the competing browser vendor, Microsoft, shipped Internet Explorer (IE) 3.0 with JScript, which was a reverse engineered version of JavaScript plus some IE-specific features. Eventually, there was an effort to standardize the various implementations of the language, and this is how ECMAScript was born. European Computer Manufacturers Association (ECMA) created the standard called ECMA-262, which describes the core parts of the JavaScript programming language without browser and web page-specific features.
You can think of JavaScript as a term that encompasses the following three pieces:
While there is one chapter in this book dedicated to the browser, the DOM, and the BOM, most of this book describes the core language and teaches you skills you can use in any environment where JavaScript programs run.
Browser wars and renaissance
For better or for worse, JavaScript's instant popularity happened during the period of the browser wars I (approximately 1996 to 2001). Those were the times during the initial Internet boom when the two major browser vendors, Netscape and Microsoft, were competing for market share. Both were constantly adding more bells and whistles to their browsers and their versions of JavaScript, DOM, and BOM, which naturally led to many inconsistencies. While adding more features, the browser vendors were falling behind on providing proper development and debugging tools and adequate documentation. Often, development was a pain; you would write a script while testing in one browser, and once you're done with development, you test in the other browser, only to find that your script simply fails for no apparent reason, and the best you can get is a cryptic error message, such as operation aborted.
Inconsistent implementations, missing documentation, and no appropriate tools painted JavaScript in such a light that many programmers simply refused to bother with it.
On the other hand, developers who did try to experiment with JavaScript got a little carried away, adding too many special effects to their pages without much regard of how usable the end results were. Developers were eager to make use of every new possibility the browsers provided, and ended up enhancing their web pages with things such as animations in the status bar, flashing colors, blinking texts, objects stalking your mouse cursor, and many other innovations that actually hurt the user experience. These various ways to abuse JavaScript are now mostly gone, but they were one of the reasons why the language had something of a bad reputation. Many serious programmers dismissed JavaScript as nothing but a toy for designers to play around with, and dismissed it as a language unsuitable for serious applications. The JavaScript backlash caused some web projects to completely ban any client-side programming and trust only their predictable and tightly controlled server. And really, why would you double the time to deliver a finished product and then spend additional time debugging problems with the different browsers?
Everything changed in the years following the end of the browser wars I. A number of events reshaped the web development landscape in a positive way. Some of them are given as follows:
In this healthier environment, developers started finding out new and better ways to use the instruments that were already available. After the public release of applications such as Gmail and Google Maps, which were rich on client-side programming, it became clear that JavaScript is a mature, unique in certain ways, and powerful prototypal object-oriented language. The best example of its rediscovery was the wide adoption of the functionality provided by the XMLHttpRequest object, which was once an IE-only innovation, but was then implemented by most other browsers. XMLHttpRequest object allows JavaScript to make HTTP requests and get fresh content from the server in order to update some parts of a page without a full page reload. Due to the wide use of the XMLHttpRequest object, a new breed of desktop-like web applications, dubbed Ajax applications, was born.
The present
An interesting thing about JavaScript is that it always runs inside a host environment. The web browser is just one of the available hosts. JavaScript can also run on the server, on the desktop, and on mobile devices. Today, you can use JavaScript to do all of the following:
The future
We can only speculate what the future will be, but it's quite certain that it will include JavaScript. For quite some time, JavaScript may have been underestimated and underused (or maybe overused in the wrong ways), but every day, we witness new applications of the language in much more interesting and creative ways. It all started with simple one-liners, often embedded in HTML tag attributes, such as onclick. Nowadays, developers ship sophisticated, well-designed and architected, and extensible applications and libraries, often supporting multiple platforms with a single codebase. JavaScript is indeed taken seriously, and developers are starting to rediscover and enjoy its unique features more and more.
Once listed in the nice-to-have sections of job postings, today, knowledge of JavaScript is often a deciding factor when it comes to hiring web developers. Common job interview questions you can hear today include-Is JavaScript an object-oriented language? Good. Now, how do you implement inheritance in JavaScript? After reading this book, you'll be prepared to ace your JavaScript job interview and even impress your interviewers with some bits that, maybe, they didn't know.
ECMAScript 5
The last most important milestone in ECMAScript revisions was ECMAScript 5 (ES5), officially accepted in December 2009. ECMAScript 5 standard is implemented and supported on all major browsers and server-side technologies.
ES5 was a major revision because apart from several important syntactic changes and additions to the standard libraries, ES5 also introduced several new constructs in the language.
For instance, ES5 introduced some new objects and properties, and also the so-called strict mode. Strict mode is a subset of the language that excludes deprecated features. The strict mode is opt-in and not required, meaning that if you want your code to run in the strict mode, you will declare your intention using (once per function, or once for the whole program) the following string:
"use strict";This is just a JavaScript string, and it's ok to have strings floating around unassigned to any variable. As a result, older browsers that don't speak ES5 will simply ignore it, so this strict mode is backwards compatible and won't break older browsers.
For backwards compatibility, all the examples in this book work in ES3, but at the same time, all the code in the book is written so that it will run without warnings in ES5's strict mode. Additionally, any ES5-specific parts will be clearly marked. Appendix C, Built-in Objects, lists the new additions to ES5 in detail.
Strict mode in ES6
While strict mode is optional in ES5, all ES6 modules and classes are strict by default. As you will see soon, most of the code we write in ES6 resides in a module; hence, strict mode is enforced by default. However, it is important to understand that all other constructs do not have implicit strict mode enforced. There were efforts to make newer constructs, such as arrow and generator functions, to also enforce strict mode, but it was later decided that doing so would result in very fragmented language rules and code.
Object-oriented programming
Before diving into JavaScript, let's take a moment to review what people mean when they say object-oriented, and what the main features of this programming style are. Here's a list of concepts that are most often used when talking about object-oriented programming (OOP):
Let's take a closer look into each one of these concepts. If you're new to the object-oriented programming lingo, these concepts might sound too theoretical, and you might have trouble grasping or remembering them from one reading. Don't worry, it does take a few tries, and the subject can be a little dry at a conceptual level. However, we'll look at plenty of code examples further on in the book, and you'll see that things are much simpler in practice.
Objects
As the name object-oriented suggests, objects are important. An object is a representation of a thing (someone or something), and this representation is expressed with the help of a programming language. The thing can be anything, a real-life object, or a more convoluted concept. Taking a common object, a cat, for example, you can see that it has certain characteristics-color, name, weight, and so on and can perform some actions-meow, sleep, hide, escape, and so on. The characteristics of the object are called properties in OOP-speak, and the actions are called methods.
The analogy with the spoken language are as follows:
Take the sentence "The black cat sleeps on the mat" as an example. "The cat" (a noun) is the object, "black" (adjective) is the value of the color property, and "sleep" (a verb) is an action or a method in OOP. For the sake of the analogy, we can go a step further and say that "on the mat" specifies something about the action "sleep", so it's acting as a parameter passed to the sleep method.
Classes
In real life, similar objects can be grouped based on some criteria. A hummingbird and an eagle are both birds, so they can be classified as belonging to some made-up Birds class. In OOP, a class is a blueprint or a recipe for an object. Another name for object is instance, so we can say that the eagle is one concrete instance of the general Birds class. You can create different objects using the same class because a class is just a template, while the objects are concrete instances based on the template.
There's a difference between JavaScript and the classic OO languages such as C++ and Java. You should be aware right from the start that in JavaScript, there are no classes; everything is based on objects. JavaScript has the notion of prototypes, which are also objects (we'll discuss them later in detail). In a classic OO language, you'd say something like-create a new object for me called Bob, which is of class Person. In a prototypal OO language, you'd say-I'm going to take this object called Bob's dad that I have lying around (on the couch in front of the TV?) and reuse it as a prototype for a new object that I'll call Bob.
Encapsulation
Encapsulation is another OOP related concept, which illustrates the fact that an object contains (encapsulates) the following:
One other term that goes together with encapsulation is information hiding. This is a rather broad term and can mean different things, but let's see what people usually mean when they use it in the context of OOP.
Imagine an object, say, an MP3 player. You, as the user of the object, are given some interface to work with, such as buttons, display, and so on. You use the interface in order to get the object to do something useful for you, like play a song. How exactly the device is working on the inside, you don't know, and, most often, don't care. In other words, the implementation of the interface is hidden from you. The same thing happens in OOP when your code uses an object by calling its methods. It doesn't matter if you coded the object yourself or it came from some third-party library; your code doesn't need to know how the methods work internally. In compiled languages, you can't actually read the code that makes an object work. In JavaScript, because it's an interpreted language, you can see the source code, but the concept is still the same-you work with the object's interface without worrying about its implementation.
Another aspect of information hiding is the visibility of methods and properties. In some languages, objects can have public, private, and protected methods and properties. This categorization defines the level of access the users of the object have. For example, only the methods of the same object have access to the private methods, while anyone has access to the public ones. In JavaScript, all methods and properties are public, but we'll see that there are ways to protect the data inside an object and achieve privacy.
Aggregation
Combining several objects into a new one is known as aggregation or composition. It's a powerful way to separate a problem into smaller and more manageable parts (divide and conquer). When a problem scope is so complex that it's impossible to think about it at a detailed level in its entirety, you can separate the problem into several smaller areas, and possibly then separate each of these into even smaller chunks. This allows you to think about the problem on several levels of abstraction.
Take, for example, a personal computer. It's a complex object. You cannot think about all the things that need to happen when you start your computer. But, you can abstract the problem saying that you need to initialize all the separate objects that your Computer object consists of the Monitor object, the Mouse object, the Keyboard object, and so on. Then, you can dive deeper into each of the subobjects. This way, you're composing complex objects by assembling reusable parts.
To use another analogy, a Book object can contain (aggregate) one or more Author objects, a Publisher object, several Chapter objects, a TOC (table of contents), and so on.
Inheritance
Inheritance is an elegant way to reuse existing code. For example, you can have a generic object, Person, which has properties such as name and date_of_birth, and which also implements the walk, talk, sleep, and eat functionality. Then, you figure out that you need another object called Programmer. You can reimplement all the methods and properties that a Person object has, but it will be smarter to just say that the Programmer object inherits a Person object, and save yourself some work. The Programmer object only needs to implement more specific functionality, such as the writeCode method, while reusing all of the Person object's functionality.
In classical OOP, classes inherit from other classes, but in JavaScript, as there are no classes, objects inherit from other objects.
When an object inherits from another object, it usually adds new methods to the inherited ones, thus extending the old object. Often, the following phrases can be used interchangeably-B inherits from A and B extends A. Also, the object that inherits can pick one or more methods and redefine them, customizing them for its own needs. This way, the interface stays the same and the method name is the same, but when called on the new object, the method behaves differently. This way of redefining how an inherited method works is known as overriding.
Polymorphism
In the preceding example, a Programmer object inherited all of the methods of the parent Person object. This means that both objects provide a talk method, among others. Now imagine that somewhere in your code, there's a variable called Bob, and it just so happens that you don't know if Bob is a Person object or a Programmer object. You can still call the talk method on the Bob object and the code will work. This ability to call the same method on different objects, and have each of them respond in their own way, is called polymorphism.
OOP summary
Here's a quick table summarizing the concepts discussed so far:
Feature
Illustrates concept
Bob is a man (an object).
Objects
Bob's date of birth is June 1, 1980, gender - male, and hair - black.
Properties
Bob can eat, sleep, drink, dream, talk, and calculate his own age.
Methods
Bob is an instance of the Programmer class.
Class (in classical OOP)
Bob is based on another object called Programmer.
Prototype
(in prototypal OOP)
Bob holds data, such as birth_date, and methods that work with the data, such as calculateAge().
Encapsulation
You don't need to know how the calculation method works internally. The object might have some private data, such as the number of days in February in a leap year. You don't know, nor do you want to know.
Information hiding
Bob is part of a WebDevTeam object together with Jill, a Designer object, and Jack, a ProjectManager object.
Aggregation and composition
Designer, ProjectManager, and Programmer are all based on and extend a Person object.
Inheritance
You can call the methods Bob.talk(), Jill.talk(), and Jack.talk(), and they'll all work fine, albeit producing different results. Bob will probably talk more about performance, Jill about beauty, and Jack about deadlines. Each object inherited the method talk from Person and customized it.
Polymorphism and method overriding
Setting up your training environment
This book takes a do-it-yourself approach when it comes to writing code, because I firmly believe that the best way to really learn a programming language is by writing code. There are no cut-and-paste-ready code downloads that you simply put in your pages. On the contrary, you're expected to type in code, see how it works, and then tweak it and play around with it. When trying out the code examples, you're encouraged to enter the code into a JavaScript console. Let's see how you go about doing this.
As a developer, you most likely already have a number of web browsers installed on your system, such as Firefox, Safari, Chrome, or Internet Explorer. All modern browsers have a JavaScript console feature that you'll use throughout the book to help you learn and experiment with the language. More specifically, this book uses WebKit's console, which is available in Safari and Chrome, but the examples should work in any other console.
WebKit's web inspector
This example shows how you can use the console to type in some code that swaps the logo on the google.com home page with an image of your choice. As you can see, you can test your JavaScript code live on any page:
In order to bring up the console in Chrome or Safari, right click anywhere on a page and select Inspect Element. The additional window that shows up is the Web Inspector feature. Select the Console tab, and you're ready to go.
You type code directly into the console, and when you press Enter, your code is executed. The return value of the code is printed in the console. The code is executed in the context of the currently loaded page, so, for example, if you type location.href, it will return the URL of the current page.
The console also has an autocomplete feature. It works in a similar way to the normal command-line prompt in your operating system or autocomplete feature of the full-fledged IDEs. If, for example, you type docu and hit the Tab or right arrow key, docu will be autocompleted to document. Then, if you type . (the dot operator), you can iterate through all the available properties and methods you can call on the document object.
By using the up and down arrow keys, you can go through the list of already executed commands and bring them back in the console.
The console gives you only one line to type in, but you can execute several JavaScript statements by separating them with semicolons. If you need more lines, you can press Shift + Enter to go to a new line without executing the result just yet.
JavaScriptCore on a Mac
On a Mac, you don't actually need a browser; you can explore JavaScript directly from your command line Terminal application.
If you've never used Terminal, you can simply search for it in Spotlight search. Once you've launched it, type the following command:
alias jsc='/System/Library/Frameworks/JavaScriptCore.framework/ Versions/Current/Resources/jsc'This command makes an alias to the little jsc application that stands for JavaScriptCore and is part of the WebKit engine. JavaScriptCore is shipped together with Mac operating systems.
You can add the alias line shown previously to your ~/.profile file so that jsc is always there when you need it.
Now, in order to start the interactive shell, you will simply type jsc from any directory. Then, you can type JavaScript expressions, and when you hit Enter, you'll see the result of the expression. Take a look at the following screenshot:
More consoles
All modern browsers have consoles built in. You have seen the Chrome/Safari console previously. In any Firefox version, you can install the Firebug extension, which comes with a console. Additionally, in newer Firefox releases, there's a console built in and accessible via the Tools | Web Developer | Web Console menu.
Internet Explorer, since version 8, has an F12 Developer Tools feature, which has a console in its Script tab.
It's also a good idea to familiarize yourself with Node.js, and you can start by trying out its console. Install Node.js from http://nodejs.org and try the console in your command prompt (terminal):
As you can see, you can use the Node.js console to try out quick examples. But, you can also write longer shell scripts (test.js in the screenshot) and run them with the scriptname.js node.
Node REPL is a powerful development tool. When you type 'node' on the command line, the REPL invokes. You can try out JavaScript on this REPL:
node > console.log("Hellow World"); Hellow World undefined > a=10, b=10; 10 > console.log(a*b); 100 undefinedSummary
In this chapter, you learned about how JavaScript was born, and where it is today. You were also introduced to object-oriented programming concepts and have seen how JavaScript is not a class-based OO language, but a prototype-based one. Finally, you learned how to use your training environment-the JavaScript console. Now, you're ready to dive into JavaScript and learn how to use its powerful OO features. However, let's start from the beginning.
The next chapter will guide you through the data types in JavaScript (there are just a few), conditions, loops, and arrays. If you think you know these topics, feel free to skip the next chapter, but not before you make sure you can complete the few short exercises at the end of the chapter.
