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):

• Object, method, and property
• Class
• Encapsulation
• Aggregation
• Reusability/inheritance
• Polymorphism

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 them or remembering them from one reading. Don't worry, it does take a few tries, and the subject could be a little dry at a conceptual level. But, 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 like 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.

There is also an analogy with the spoken language:

• Objects are most often named using nouns (book, person, and so on)
• Methods are verbs (read, run, and so on)
• Values of the properties are adjectives

Take the sentence "The black cat sleeps on the mat". "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 say that the eagle is one concrete instance of the general class Birds. 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 me a new object 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) both:

• Data (stored in properties)
• The means to do something with the data (using methods)

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 sub-objects. 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 functionality walk, talk, sleep, and eat. Then, you figure out that you need another object called Programmer. You could re-implement all the methods and properties that Person has, but it would be smarter to just say that Programmer inherits Person, 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, since 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, 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.