Encapsulation

Encapsulation

The purpose of Encapsulation is data access control. For data integrity, some data should be accessible for some certain methods not others. In traditional object-oriented programming languages, there are keywords private, public, protect to define the visuality of data and methods. However, Javascript has no such thing.

Here we define a class called Book with basic private, public access. If you are not able to understand the code, try to read Basic of Javascript. We apply closure to emulate the privacy of property. You can check code in this repository to see the whole reasoning process. (How do we end up here).

Interface = require('./interface')
validate_isbn = require('./validation_isbn')

/* Explicitely inform all programmers to use the methods defined in the interface */
var Publication = new Interface('Publication', 
    ['getIsbn', 'setIsbn', 'getTitle', 'setTitle', 'getAuthor', 'setAuthor', 'display']);


var Book = (function() {
    // Private static attributes
    var numOfBooks = 0;

    // Private static method
    function checkIsbn(isbn) {
        return validate_isbn(isbn);
    };

    var constructor_fn = function(newIsbn, newTitle, newAuthor) {
        // Private attributes
        var isbn, title, author;

        // Privileged methods
        this.getIsbn = function() {
            return isbn;
        };

        this.setIsbn = function(newIsbn) {
            if(!checkIsbn(newIsbn)) throw new Error("Book: Invalid ISBN");
            isbn = newIsbn;
        };

        this.getTitle = function() {
            return title;
        };

        this.setTitle = function(newTitle) {
            title = newTitle || "No Title Specified";
        };

        this.getAuthor = function() {
            return author;
        };

        this.setAuthor = function(newAuthor) {
            author = newAuthor || "No Author Specified";
        };

        // constructor code
        numOfBooks++;

        if(numOfBooks > 50) throw new Error('Book: Only 50 instances of Book can be created.');

        this.setIsbn(newIsbn);
        this.setTitle(newTitle);
        this.setAuthor(newAuthor);
    };

    // Public static method
    constructor_fn.NumOfBooks = function() {
        return numOfBooks;
    };
    
    // return the constructor
    return constructor_fn;
})();

// Public static method
Book.convertToTitleCase = function(inputString) {
    console.log(inputString.toUpperCase());
};

// Public, non-privileged methods
Book.prototype = {
    display: function() {
        console.log("------------Book Info-------------");
        console.log("ISBN: " + this.getIsbn());
        console.log("Title: " + this.getTitle());
        console.log("Author: " + this.getAuthor());
        console.log("----------------------------------");
    }
};

/* Usage */
console.log("----------------- The number of Books -------------------");
console.log(Book.NumOfBooks());  // 0

var book = new Book('978-1-59059-908-2', "Pro Javascript Design Patterns", "Ross Harmes");
book.display();
/*
------------Book Info-------------
ISBN: 978-1-59059-908-2
Title: Pro Javascript Design Patterns
Author: Ross Harmes
----------------------------------
*/
console.log("----------------- The number of Books -------------------");
console.log(Book.NumOfBooks());  // 1

console.log("******** Good Case by using setIsbn which is defined in Interface ***********");
book.setIsbn('1-59059-908-X');
book.display();
/*
------------Book Info-------------
ISBN: 1-59059-908-X
Title: Pro Javascript Design Patterns
Author: Ross Harmes
----------------------------------
*/
console.log("----------------- The number of Books -------------------");
console.log(Book.NumOfBooks());  // 1

console.log('******** Good Case by modifying isbn directly ************** ');
book.isbn = "hello world"; // Programmer can not access isbn directly, which is safe.
book.display();
/*
------------Book Info-------------
ISBN: 1-59059-908-X
Title: Pro Javascript Design Patterns
Author: Ross Harmes
----------------------------------
*/
console.log("----------------- The number of Books -------------------");
console.log(Book.NumOfBooks());  // 1

var book1 = new Book('978-1-59059-908-2', "Pro Javascript Design Patterns", "Ross Harmes");
book1.display();

/*
------------Book Info-------------
ISBN: 978-1-59059-908-2
Title: Pro Javascript Design Patterns
Author: Ross Harmes
----------------------------------
*/
console.log("----------------- The number of Books -------------------");
console.log(Book.NumOfBooks());  // 2