JavaScript Object Programming - Introduction to Java Script - Lecture Slides, Slides of Javascript programming

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Object-Oriented Java Script

Object-Oriented

JavaScript

Objectives

• Study object-oriented programming
• Learn about the built-in JavaScript objects
• Work with the Array, Date, Math, and Number objects
• Define custom JavaScript objects

Introduction to Object-Oriented Programming

• Object-oriented programming allows you to reuse code without having to copy or recreate it
• You already have some experience with object- oriented programming after working with browser objects(window, Document, Form objects)

Reusable Software

• Object-oriented programming (OOP):
  • Refers to the creation of reusable software objects that can be easily incorporated into multiple programs
• Object:
  • Specifically refers to programming code and data that can be treated as an individual unit or component

Reusable Software (Cont.)

• Data:
  • Refers to information contained within variables or other types of storage structures
• Objects can range from simple controls such as a button, to entire programs such as a database application - In a retail sales program, you could refer to all the code that calculates the sales total as a single object. - You could reuse the object over and over again in the same program by typing the object name.

Reusable Software (Cont.)

• Popular object-oriented programming languages include C++, Java, and Visual Basic
• Using any of these or other object-oriented languages:
  • Programmers can create objects themselves or use objects created by other programmers

Encapsulation

• Objects are encapsulated:
  • All code and required data are contained within the object itself
• Encapsulation:
  • Places code inside what programmers like to call a “black box”
  • When an object is encapsulated, you cannot see “inside” it—all internal workings are hidden

Encapsulation (Cont.)

• The code (methods and statements) and data (variables and constants) contained in an encapsulated object are accessed through an interface
• Interface:
  • Represents elements required for a source program to communicate with an object - Interface elements required to access a Payroll object might be a method named calcNetPay()

Encapsulation (Cont.)

• The principal of information hiding states that:
  • Any class members that other programmers do not need to access or know about should be hidden - Think of a calculator as an object, the interface is the keypad. - You do not need to know or see the inner workings of the calculator to operate it

Encapsulation (Cont.)

• Encapsulation prevents other programmers from:
  • Accidentally introducing a bug into a program
  • Stealing the code and claiming it as their own

Encapsulation (Cont.)

Classes

• In object-oriented programming:
  • Code, methods, attributes, and other information that make up an object are organized into classes
• Class:
  • Template, or blueprint, that serves as the basis for new objects
• Instance :
  • Object that has been created from an existing class
    • Called instantiating the object

Classes (Cont.)

• A particular instance of an object inherits its methods and properties from a class:
  • It takes on the characteristics of the class on which it is based
  • For Example Consider an object called BankAccount that contains methods and properties that you might use to record transactions associated with a checking or savings account - The BankAccount object is created from a BankAccount class - To use the BankAccount class you create an instance of the class - A particular instance of an object inherits its methods and properties from a class - The BankAccout object inherits all its methods and properties from the BankAccout class

Classes (Cont.)

• Class names in traditional object-oriented programming usually begin with an uppercase letter. This convention is also followed in JavaScript

Built-in JavaScript Classes

Instantiating an Object

• You can use some of the built-in JavaScript objects directly in your code - document.write(“The value of PI is “ + Math.PI);
• Other objects require you to instantiate a new object
  • var studentList = new Array()

Garbage Collection

• When you instantiate a new object, you are reserving memory
• Refers to cleaning up, or reclaiming, memory that is reserved by a program
• JavaScript knows when a program no longer needs a variable or object - Automatically cleans up the memory

Array Class

• The Array class creates new array objects
• When creating an array using the Array() constructor, an object from the Array class is instantiated
• In addition to the Array() constructor, the Array class contains: - Methods and properties for manipulating the elements of an array

Array Class Methods

The Length Property

• Returns the number of elements in an array
• You append the length property to the name of the array you want to sort using the following syntax: array_name .length;

Date Class

• Contains methods and properties for manipulating the date and time
• The Date object allows you to use the current date and time in your JavaScript programs
• You create a new instance of the Date class using the syntax var dateObject = new Date();

Date Class (Cont.)

Number Class

• Contains methods for manipulating numbers and properties that contain static values representing some of the numeric limitations in the JavaScript language

Number Class Methods

Number Class Properties