Principles of Object-Oriented Programming in JavaScript
Principles of Object-Oriented Programming in JavaScript
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Introduction

Most developers associate object-oriented programming with languages that are typically taught in school, like C++ and Java, which base object-oriented programming around classes. Before you can do anything in these languages, you need to create a class, even if you’re just writing a simple command-line program. Common design patterns in the industry reinforce class-based concepts as well. But JavaScript doesn’t use classes, and this is part of the reason people get confused when they try learning it after C++ or Java. Object-oriented languages have several characteristics:

  • Encapsulation - Data can be grouped together with functionality that operates on that data. This, quite simply, is the definition of an object.
  • Aggregation - One object can reference another object.
  • Inheritance - A newly created object has the same characteristics as another object without explicitly duplicating its functionality.
  • Polymorphism One interface may be implemented by multiple objects.

JavaScript has all these characteristics, though because the language has no concept of classes, some aren’t implemented in quite the way you might expect. At first glance, a JavaScript program might even look like a procedural program you would write in C. If you can write a function and pass it some variables, you have a working script that seemingly has no objects. A closer look at the language, however, reveals the existence of objects through the use of dot notation.

Many object-oriented languages use dot notation to access properties and methods on objects, and JavaScript is syntactically the same. But in JavaScript, you never need to write a class definition, import a package, or include a header file. You just start coding with the data types that you want, and you can group those together in any number of ways. You could certainly write JavaScript in a procedural way, but its true power emerges when you take advantage of its object-oriented nature. That’s what this book is about.

Make no mistake: A lot of the concepts you may have learned in more traditional object-oriented programming languages don’t necessarily apply to JavaScript. While that often confuses beginners, as you read, you’ll quickly find that JavaScript’s weakly typed nature allows you to write less code to accomplish the same tasks as other languages. You can just start coding without planning the classes that you need ahead of time. Need an object with specific fields? Just create an ad hoc object wherever you want. Did you forget to add a method to that object? No problem - just add it later.

Inside these pages, you’ll learn the unique way that JavaScript approaches object-oriented programming. Leave behind the notions of classes and class-based inheritance and learn about prototype-based inheritance and constructor functions that behave similarly. You’ll learn how to create objects, define your own types, use inheritance, and otherwise manipulate objects to get the most out of them. In short, you’ll learn everything you need to know to understand and write JavaScript professionally. Enjoy!

Who This Book Is for

This book is intended as a guide for those who already understand object-oriented programming but want to know exactly how the concept works in JavaScript. Familiarity with Java, C#, or object-oriented programming in other languages is a strong indicator that this book is for you. In particular, this book is aimed at three groups of readers:

  • Developers who are familiar with object-oriented programming concepts and want to apply them to JavaScript
  • Web application and Node.js developers trying to structure their code more effectively
  • Novice JavaScript developers trying to gain a deeper understanding of the language

This book is not for beginners who have never written JavaScript. You will need a good understanding of how to write and execute JavaScript code to follow along.

Overview

Chapter 1: Primitive and Reference Types introduces the two different value types in JavaScript: primitive and reference. You’ll learn what distinguishes them from each other and how understanding their differences is important to an overall understanding of JavaScript.

Chapter 2: Functions explains the ins and outs of functions in JavaScript. First-class functions are what makes JavaScript such an interesting language.

Chapter 3: Understanding Objects details the makeup of objects in JavaScript. JavaScript objects behave differently than objects in other languages, so a deep understanding of how objects work is vital to mastering the language.

Chapter 4: Constructors and Prototypes expands on the previous discussion of functions by looking more specifically at constructors. All constructors are functions, but they are used a little bit differently. This chapter explores the differences while also talking about creating your own custom types.

Chapter 5: Inheritance explains how inheritance is accomplished in JavaScript. Though there are no classes in JavaScript, that doesn’t mean inheritance isn’t possible. In this chapter, you’ll learn about prototypal inheritance and how it differs from class-based inheritance.

Chapter 6: Object Patterns walks through common object patterns. There are many different ways to build and compose objects in JavaScript, and this chapter introduces you to the most popular patterns for doing so.

Acknowledgments

I’d like to thank Kate Matsudaira for convincing me that self-publishing an ebook was the best way to get this information out. Without her advice, I’d probably still be trying to figure out what I should do with the information contained in this book.

Thanks to Rob Friesel for once again providing excellent feedback on an early copy of this book, and Cody Lindley for his suggestions. Additional thanks to Angus Croll for his technical review of the finished version — his nitpicking made this book much better.

Thanks as well to Bill Pollock, whom I met at a conference and who started the ball rolling on publishing this book with No Starch Press.

Help and Support

If you have questions, comments, or other feedback about this book, please visit the mailing list at: http://groups.google.com/group/zakasbooks.

Chapter 1: Primitive and Reference Types

Most developers learn object-oriented programming by working with class-based languages such as Java or C#. When these developers start learning JavaScript, they get disoriented because JavaScript has no formal support for classes. Instead of defining classes from the beginning, with JavaScript you can just write code and create data structures as you need them. Because it lacks classes, JavaScript also lacks class groupings such as packages. Whereas in languages like Java, package and class names define both the types of objects you use and the layout of files and folders in your project, programming in JavaScript is like starting with a blank slate: You can organize things any way you want. Some developers choose to mimic structures from other languages, while others take advantage of JavaScript’s flexibility to come up with something completely new. To the uninitiated, this freedom of choice can be overwhelming, but once you get used to it, you’ll find JavaScript to be an incredibly flexible language that can adapt to your preferences quite easily.

To ease the transition from traditional object-oriented languages, JavaScript makes objects the central part of the language. Almost all data in JavaScript is either an object or accessed through objects. In fact, even functions (which languages traditionally make you jump through hoops to get references to) are represented as objects in JavaScript, which makes them first-class functions.

Working with and understanding objects is key to understanding JavaScript as a whole. You can create objects at any time and add or remove properties from them whenever you want. In addition, JavaScript objects are extremely flexible and have capabilities that create unique and interesting patterns that are simply not possible in other languages.

This chapter focuses on how to identify and work with the two primary JavaScript data types: primitive types and reference types. Though both are accessed through objects, they behave in different ways that are important to understand.

What are types?

Although JavaScript has no concept of classes, it still uses two kinds of types: primitive and reference. Primitive types are stored as simple data types. Reference types are stored as objects, which are really just references to locations in memory.

The tricky thing is that JavaScript lets you treat primitive types like reference types in order to make the language more consistent for the developer.

While other programming languages distinguish between primitive and reference types by storing primitives on the stack and references in the heap, JavaScript does away with this concept completely: It tracks variables for a particular scope with a variable object. Primitive values are stored directly on the variable object, while reference values are placed as a pointer in the variable object, which serves as a reference to a location in memory where the object is stored. However, as you’ll see later in this chapter, primitive values and reference values behave quite differently although they may initially seem the same.

Of course, there are other differences between primitive and reference types.

Primitive Types

Primitive types represent simple pieces of data that are stored as is, such as true and 25. There are five primitive types in JavaScript:

  • Boolean - true or false
  • Number - Any integer or floating-point numeric value
  • String - A character or sequence of characters delimited by either single or double quotes (JavaScript has no separate character type)
  • Null - A primitive type that has only one value, null.
  • Undefined - A primitive type has only one value, undefined (undefined is the value assigned to a variable that is not initialized)

The first three types (Boolean, number, and string) behave in similar ways, while the last two (null and undefined) work a bit differently, as will be discussed throughout this chapter. All primitive types have literal representations of their values. Literals represent values that aren’t stored in a variable, such as a hardcoded name or price. Here are some examples of each type using its literal form:

// strings
var name = "Nicholas";
var selection = "a";

// numbers
var count = 25;
var cost = 1.51;

// boolean
var found = true;

// null
var object = null;

// undefined
var flag = undefined;
var ref;    // assigned undefined automatically

In JavaScript, as in many other languages, a variable holding a primitive directly contains the primitive value (rather than a pointer to an object). When you assign a primitive value to a variable, the value is copied into that variable. This means that if you set one variable equal to another, each variable gets its own copy of the data. For example:

var color1 = "red";
var color2 = color1;

Here, color1 is assigned the value of "red". The variable color2 is then assigned the value of color1, which stores "red" in color2. Even though color1 and color2 contain the same value, they are completely separate from each another, and you can change the value in color1 without affecting color2 and vice versa. That’s because there are two different storage locations, one for each variable. Figure 1-1 illustrates the variable object for this snippet of code.

Figure 1-1: Variable Object
Figure 1-1: Variable Object

Because each variable containing a primitive value uses its own storage space, changes to one variable are not reflected on the other. For example:

var color1 = "red";
var color2 = color1;

console.log(color1); // "red"
console.log(color2); // "red"

color1 = "blue";

console.log(color1); // "blue"
console.log(color2); // "red"

In this code, color1 is changed to "blue" and color2 retains its original value of "red".

Identifying Primitive Types

The best way to identify primitive types is with the typeof operator, which works on any variable and returns a string indicating the type of data. The typeof operator works well with strings, numbers, Booleans, and undefined. The following shows the output when using typeof on different primitive values:

console.log(typeof "Nicholas");     // "string"
console.log(typeof 10);             // "number"
console.log(typeof 5.1);            // "number"
console.log(typeof true);           // "boolean"
console.log(typeof undefined);      // "undefined"

As you might expect, typeof returns “string” when the value is a string; “number” when the value is a number (regardless of integer or floating-point values); “boolean” when the value is a Boolean; and “undefined” when the value is undefined.

The tricky part involves null.

You wouldn’t be the first developer to be confused by the result of this line of code:

console.log(typeof null); // "object"

When you run typeof null, the result is “object”. But why an object when the type is null? (In fact, this has been acknowledged as an error by TC-39, the committee that designs and maintains JavaScript. You could reason that null is an empty object pointer, making “object” a logical return value, but that’s still confusing.)

The best way to determine if a value is null is to compare it against null directly, like this:

console.log(value === null); // true or false

Primitive Methods

Despite the fact that they’re primitive types, strings, numbers, and Booleans actually have methods. (The null and undefined types have no methods.) Strings, in particular, have numerous methods to help you work with them. For example:

var name = "Nicholas";
var lowercaseName = name.toLowerCase();    // convert to lowercase
var firstLetter = name.charAt(0);          // get first character
var middleOfName = name.substring(2, 5);   // get characters 2-4

var count = 10;
var fixedCount = count.toFixed(2);         // convert to "10.00"
var hexCount = count.toString(16);         // convert to "a"

var flag = true;
var stringFlag = flag.toString();          // convert to "true"

Reference Types

Reference types represent objects in JavaScript and are the closest things to classes that you will find in the language. Reference values are instances of reference types and are synonymous with objects (the rest of this chapter refers to reference values simply as objects). An object is an unordered list of properties consisting of a name (always a string) and a value. When the value of a property is a function, it is called a method. Functions themselves are actually reference values in JavaScript, so there’s little difference between a property that contains an array and one that contains a function except that a function can be executed.

Of course, you must create objects before you can begin working

Creating Objects

It sometimes helps to think of JavaScript objects as nothing more than hash tables, as shown in Figure 1-2.

Figure 1-2: Structure of an object
Figure 1-2: Structure of an object

There are a couple of ways to create, or instantiate, objects. The first is to use the new operator with a constructor. (A constructor is simply a function that uses new to create an object—any function can be a constructor.) By convention, constructors in JavaScript begin with a capital letter to distinguish them from nonconstructor functions. For example, this code instantiates a generic object and stores a reference to it in object:

var object = new Object();

Reference types do not store the object directly into the variable to which it is assigned, so the object variable in this example doesn’t actually contain the object instance. Instead, it holds a pointer (or reference) to the location in memory where the object exists. This is the primary difference between objects and primitive values, as the primitive is stored directly in the variable.

When you assign an object to a variable, you’re actually assigning a pointer. That means if you assign one variable to another, each variable gets a copy of the pointer, and both still reference the same object in memory. For example:

var object1 = new Object();
var object2 = object1;

This code first creates an object (with new) and stores a reference in object1. Next, object2 is assigned the value of object1. There is still only the one instance of the object that was created on the first line, but both variables now point to that object, as illustrated in Figure 1-3.

Figure 1-3: Two variables pointing to one object
Figure 1-3: Two variables pointing to one object

Dereferencing Objects

JavaScript is a garbage-collected language, so you don’t really need to worry about memory allocations when you use reference types. However, it’s best to dereference objects that you no longer need so that the garbage collector can free up that memory. The best way to do this is to set the object variable to null.

var object1 = new Object();

// do something

object1 = null; // dereference

Here, object1 is created and used before finally being set to null. When there are no more references to an object in memory, the garbage collector can use that memory for something else. (Dereferencing objects is especially important in very large applications that use millions of objects)

Adding or Removing Properties

Another interesting aspect of objects in JavaScript is that you can add and remove properties at any time. For example:

var object1 = new Object();
var object2 = object1;

object1.myCustomProperty = "Awesome!";
console.log(object2.myCustomProperty); // "Awesome!"

Here, myCustomProperty is added to object1 with a value of “Awesome!”. That property is also accessible on object2 because both object1 and object2 point to the same object.

In addition to generic object reference types, JavaScript has several other built-in types that are at your disposal.

Instantiating Built-in Types

You’ve seen how to create and interact with generic objects created with new Object(). The Object type is just one of a handful of built-in reference types that JavaScript provides. The other built-in types are more specialized in their intended usage and can be instantiated at any time.

The built-in types are:

  • Array - An ordered list of numerically indexed values
  • Date - A date and time
  • Error - A runtime error (there are also several more specific
  • Function - A function
  • Object - A generic object
  • RegExp - A regular expression

You can instantiate each built-in reference type using new, as shown here:

var items = new Array();
var now = new Date();
var error = new Error("Something bad happened.");
var func = new Function("console.log('Hi');");
var object = new Object();
var re = new RegExp("\\d+");

Literal Forms

Several built-in reference types have literal forms. A literal is syntax that allows you to define a reference value without explicitly creating an object, using the new operator and the object’s constructor. (Earlier in this chapter, you saw examples of primitive literals including string literals, numeric literals, Boolean literals, the null literal, and the undefined literal.)

Object and Array Literals

To create an object with object literal syntax, you can define the properties of a new object inside braces. Properties are made up of an identifier or string, a colon, and a value, with multiple properties separated by commas.

For example:

var book = {
    name: "The Principles of Object-Oriented JavaScript",
    year: 2014
};

You can also use string literals as property names, which is useful when you want a property name to have spaces or other special characters:

var book = {
    "name": "The Principles of Object-Oriented JavaScript",
    "year": 2014
};

This example is equivalent to the previous one despite the syntactic differences. Both examples are also logically equivalent to the following:

var book = new Object();
book.name = "The Principles of Object-Oriented JavaScript";
book.year = 2014;

The outcome of each of the previous three examples is the same: an object with two properties. The choice of pattern is up to you because the functionality is ultimately the same.

You can define an array literal in a similar way by enclosing any number of comma-separated values inside square brackets. For example:

var colors = [ "red", "blue", "green" ];
console.log(colors[0]); // "red"

This code is equivalent to the following:

var colors = new Array("red", "blue", "green")
console.log(colors[0]); // "red"

Function Literals

You almost always define functions using their literal form. In fact, using the Function constructor is typically discouraged given the challenges of maintaining, reading, and debugging a string of code rather than actual code, so you’ll rarely see it in code.

Creating functions is much easier and less error prone when you use the literal form. For example:

function reflect(value) {
    return value;
}

// is the same as

var reflect = new Function("value", "return value;");

This code defines the reflect() function, which returns any value passed to it. Even in the case of this simple function, the literal form is easier to write and understand than the constructor form. Further, there is no good way to debug functions that are created in the constructor form: These functions aren’t recognized by JavaScript debuggers and therefore act as a black box in your application.

Regular Expression Literals

JavaScript also has regular expression literals that allow you to define regular expressions without using the RegExp constructor. Regular expression literals look very similar to regular expressions in Perl: The pattern is contained between two slashes, and any additional options are single characters following the second slash. For example:

var numbers = /\d+/g;

// is the same as

var numbers = new RegExp("\\d+", "g");

The literal form of regular expressions in JavaScript is a bit easier to deal with than the constructor form because you don’t need to worry about escaping characters within strings.When using the RegExp constructor, you pass the pattern in as a string, so you have to escape any backslashes. (That’s why \d is used in the literal and \\d is used in the constructor.) Regular expression literals are preferred over the constructor form in Java Script except when the regular expression is being constructed dynamically from one or more strings.

That said, with the exception of Function, there really isn’t any right or wrong way to instantiate built-in types. Many developers prefer literals, while some prefer constructors. Choose whichever method you find more comfortable to use.

Property Access

Properties are name/value pairs that are stored on an object. Dot notation is the most common way to access properties in JavaScript (as in many object-oriented languages), but you can also access properties on JavaScript objects by using bracket notation with a string.

For example, you could write this code, which uses dot notation:

var array = [];
array.push(12345);

With bracket notation, the name of the method is now included in a string enclosed by square brackets, as in this example:

var array = [];
array["push"](12345);

This syntax is very useful when you want to dynamically decide which property to access. For example, here bracket notation allows you to use a variable instead of the string literal to specify the property to access.

var array = [];
var method = "push";
array[method](12345);

In this listing, the variable method has a value of "push", so push() is called on the array. This capability is quite useful, as you’ll see throughout this book. The point to remember is that, other than syntax, the only difference—performance or otherwise—between dot notation and bracket notation is that bracket notation allows you to use special characters in property names. Developers tend to find dot notation easier to read, so you’ll see it used more frequently than bracket notation.

Identifying Reference Types

A function is the easiest reference type to identify because when you use the typeof operator on a function, the operator should return "function":

function reflect(value) {
    return value;
}

console.log(typeof reflect); // "function"

Other reference types are trickier to identify because, for all reference types other than functions, typeof returns "object". That’s not very helpful when you’re dealing with a lot of different types. To identify reference types more easily, you can use JavaScript’s instanceof operator.

The instanceof operator takes an object and a constructor as parameters. When the value is an instance of the type that the constructor specifies, instanceof returns true; otherwise, it returns false, as you can see here:

var items = [];
var object = {};

function reflect(value) {
    return value;
}

console.log(items instanceof Array); // true
console.log(object instanceof Object); // true
console.log(reflect instanceof Function); // true

In this example, several values are tested using instanceof and a constructor. Each reference type is correctly identified by using instanceof and the constructor that represents its true type (even though the constructor wasn’t used in creating the variable).

The instanceof operator can identify inherited types. That means every object is actually an instance of Object because every reference type inherits from Object.

To demonstrate, the following listing examines the three references previously created with instanceof:

var items = [];
var object = {};

function reflect(value) {
    return value;
}

console.log(items instanceof Array);       // true
console.log(items instanceof Object);      // true
console.log(object instanceof Object);     // true
console.log(object instanceof Array);      // false
console.log(reflect instanceof Function);  // true
console.log(reflect instanceof Object);    // true

Each reference type is correctly identified as an instance of Object, from which all reference types inherit.

Identifying Arrays

Although instanceof can identify arrays, there is one exception that affects web developers: JavaScript values can be passed back and forth between frames in the same web page. This becomes a problem only when you try to identify the type of a reference value, because each web page has its own global context—its own version of Object, Array, and all other built-in types. As a result, when you pass an array from one frame to another, instanceof doesn’t work because the array is actually an instance of Array from a different frame.

To solve this problem, ECMAScript 5 introduced Array.isArray(), which definitively identifies the value as an instance of Array regardless of the value’s origin. This method should return true when it receives a value that is a native array from any context. If your environment is ECMAScript 5 compliant, Array.isArray() is the best way to identify arrays:

var items = [];

console.log(Array.isArray(items)); // true

The Array.isArray() method is supported in most environments, both in browsers and in Node.js. This method isn’t supported in Internet Explorer 8 and earlier.

Primitive Wrapper Types

Perhaps one of the most confusing parts of JavaScript is the concept of primitive wrapper types. There are three primitive wrapper types (String, Number, and Boolean). These special reference types exist to make working with primitive values as easy as working with objects. (It would be very confusing if you had to use a different syntax or switch to a procedural style just to get a substring of text.)

The primitive wrapper types are reference types that are automatically created behind the scenes whenever strings, numbers, or Booleans are read. For example, in the first line of this listing, a primitive string value is assigned to name. The second line treats name like an object and calls charAt(0) using dot notation.

var name = "Nicholas";
var firstChar = name.charAt(0);
console.log(firstChar);         // "N"

This is what happens behind the scenes:

// what the JavaScript engine does
var name = "Nicholas";
var temp = new String(name);
var firstChar = temp.charAt(0);
console.log(firstChar);         // "N"

Because the second line uses a string (a primitive) like an object, the JavaScript engine creates an instance of String so that charAt(0) will work. The String object exists only for one statement before it’s destroyed (a process called autoboxing). To test this out, try adding a property to a string as if it were a regular object:

var name = "Nicholas";
name.last = "Zakas";

console.log(name.last); // undefined

This code attempts to add the property last to the string name. The code itself is just fine except that the property disappears. What happened? When working with regular objects, you can add properties at any time and they stay until you manually remove them. With primitive wrapper types, properties seem to disappear because the object on which the property was assigned is destroyed immediately afterward.

Here’s what’s actually happening in the JavaScript engine:

// what the JavaScript engine does
var name = "Nicholas";
var temp = new String(name);
temp.last = "Zakas";
temp = null;                    // temporary object destroyed

var temp = new String(name);
console.log(temp.last);         // undefined
temp = null;

Instead of assigning a new property to a string, the code actually creates a new property on a temporary object that is then destroyed. When you try to access that property later, a different object is temporarily created and the new property doesn’t exist there. Although reference values are created automatically for primitive values, when instanceof checks for these types of values the result is false:

var name = "Nicholas";
var count = 10;
var found = false;

console.log(name instanceof String);    // false
console.log(count instanceof Number);   // false
console.log(found instanceof Boolean);  // false

The instanceof operator returns false because a temporary object is created only when a value is read. Because instanceof doesn’t actually read anything, no temporary objects are created, and it tells us the values aren’t instances of primitive wrapper types. You can create primitive wrapper types manually, but there are certain side effects:

var name = new String("Nicholas");
var count = new Number(10);
var found = new Boolean(false);

console.log(typeof name);   // "object"
console.log(typeof count);  // "object"
console.log(typeof found);  // "object"

As you can see, creating an instance of the primitive wrapper type just creates another object, which means that typeof can’t identify the type of data you intend to store.

In addition, you can’t use String, Number, and Boolean objects as you would primitive values. For example, the following code uses a Boolean object. The Boolean object is false, yet console.log(“Found”) still executes because an object is always considered true inside a conditional statement. It doesn’t matter that the object represents false; it’s an object, so it evaluates to true.

var found = new Boolean(false);

if (found) {
    console.log("Found");   // this executes
}

Manually instantiating primitive wrappers can also be confusing in other ways, so unless you find a special case where it makes sense to do so, you should avoid it. Most of the time, using primitive wrapper objects instead of primitives only leads to errors.

Summary

While JavaScript doesn’t have classes, it does have types. Each variable or piece of data is associated with a specific primitive or reference type. The five primitive types (strings, numbers, Booleans, null, and undefined)represent simple values stored directly in the variable object for a given context. You can use typeof to identify primitive types with the exception of null, which must be compared directly against the special value null.

Reference types are the closest thing to classes in JavaScript, and objects are instances of reference types. You can create new objects using the new operator or a reference literal. You access properties and methods primarily using dot notation, but you can also use bracket notation. Functions are objects in JavaScript, and you can identify them with the typeof operator. You should use instanceof with a constructor to identify objects of any other reference type.

To make primitives seem more like references, JavaScript has three primitive wrapper types: String, Number, and Boolean. JavaScript creates these objects behind the scenes so that you can treat primitives like regular objects, but the temporary objects are destroyed as soon as the statement using them is complete. Although you can create your own instances of primitive wrappers, it’s best not to do that because it can be confusing.