Chapter 6 Object-Oriented Java Script JavaScript, Third Edition

Chapter 6

Object-Oriented Java Script

JavaScript, Third Edition

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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

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Introduction to Object-Oriented Programming

• Object-oriented programming allows you to reuse code without having to copy or recreate it

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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

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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

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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

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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

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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

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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

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Encapsulation (Cont.)

• Encapsulation prevents other programmers from:

– Accidentally introducing a bug into a program

– Stealing the code and claiming it as their own

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Encapsulation (Cont.)

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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

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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

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Built-in JavaScript Classes

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Instantiating an Object

• You can use some of the built-in JavaScript objects directly in your code

• Other objects require you to instantiate a new object

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Garbage Collection

• 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

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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

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Array Class Methods

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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;

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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();

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Date Class (Cont.)

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Number Class

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

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Number Class Methods

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Number Class Properties

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Math Class

• The Math class contains methods and properties for performing mathematical calculations in your programs

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Math Class Methods

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Math Class Methods (Cont.)

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Math Class Properties

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Custom JavaScript Objects

• JavaScript is not a true object-oriented programming language:

– Base objects in your programs on built-in JavaScript classes such as the Array and Date objects

– However, you cannot create your own classes in JavaScript

– You can define your own custom objects using a constructor function

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Constructor Function

• A function used as the basis for a custom object

• JavaScript objects inherit all the variables and statements of the constructor function on which they are based

• Any JavaScript function can serve as a constructor

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Properties

• To add a property to a constructor function:

– You must add a statement to the function body that uses the this keyword with the following syntax:

• this.property_name = value;

– In the case of a custom JavaScript object, the this keyword refers to the object that calls the constructor function

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Methods

• You can create a function that will be used as an object method:

– By referring to any object properties using the this reference

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The prototype Property

• The prototype property is a built-in property:

– Specifies the constructor from which an object was

extended

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Chapter Summary

• Object-Oriented programming (OOP):

– Refers to the creation of reusable software objects that can be easily incorporated into another program

• Components:

– Reusable software objects

• Object:

– Programming code and data that can be treated as an individual unit or component

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Chapter Summary (cont.)

• Objects are encapsulated:

– All code and required data are contained within the object itself

• Interface :

– Represents elements required for a source program to communicate with an object

• The principal of information hiding:

– States that any class members that other programmers do not need to access or know about should be hidden

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Chapter Summary (cont.)• In object-oriented programming:

– Code, methods, attributes, and other information that make up an object are organized using classes

• An instance:– Object created from an existing class

• An object inherits, or takes on, the characteristics of the class on which it is based

• The Date class:– Contains methods and properties for manipulating the

date and time

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Chapter Summary (cont.)

• The Number class:

– Contains methods for manipulating numbers and properties that

• The this keyword:

– Refers to the current object

• The prototype property:

– Built-in property that specifies the constructor from which an object was extended