Object-oriented programming in Javascript

JavaScript objects are very simple collections of name-value pairs. There are two ways of creating a simple object in JavaScript. The first way is as follows:

var obj = new Object(); 

And the second way is as follows:

var obj = {}; 

We can also create an object entirely, as follows:

obj = { 
  name: { 
    first: 'Gandalf', 
    last: 'the Grey' 
  }, 
  address: 'Middle Earth' 
}; 

As we can see, to declare a JavaScript object, [key, value] pairs are used, where the key can be considered an attribute of the object and the value is the attribute value. All classes that we will create in this book are JavaScript objects, such as Stack, Set, LinkedList, Dictionary, Tree, Graph, and so on.

In object-oriented programming (OOP), an object is an instance of a class. A class defines the characteristics of the object. For our algorithms and data structures, we will create some classes that will represent them. This is how we can declare a class that represents a book:

function Book(title, pages, isbn){ 
  this.title = title; 
  this.pages = pages; 
  this.isbn = isbn; 
} 

To instantiate this class, we can use the following code:

var book = new Book('title', 'pag',  'isbn'); 

Then, we can access its attributes and update them as follows:

console.log(book.title); //outputs the book title 
book.title = 'new title'; //updates the value of the book title 
console.log(book.title); //outputs the updated value 

A class can also contain functions. We can declare and use a function as the following code demonstrates:

Book.prototype.printTitle = function(){ 
  console.log(this.title); 
}; 
book.printTitle(); 

We can declare functions directly inside the class definition as well:

function Book(title, pages, isbn){ 
  this.title = title; 
  this.pages = pages; 
  this.isbn = isbn; 
  this.printIsbn = function(){ 
  console.log(this.isbn); 
  } 
} 
book.printIsbn(); 
 

Note

In the prototype example, the printTitle function will be shared between all the instances and only one copy will be created. When we use a class-based definition, as in the previous example, each instance will have its own copy of the functions. Using the prototype method saves memory and processing cost with regard to assigning the functions to the instance. However, you can only declare public functions and properties using the prototype method. With a class-based definition, you can declare private functions and properties and the other methods inside the class can also access them. ECMAScript 6 introduces a simplified syntax very similar to the class-based example and it is prototype based. We will discuss more on this later in this chapter.