Working with JavaScript

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Objectives

• Introducing JavaScript• Inserting JavaScript into a Web Page File• Writing Output to the Web Page• Working with Variables and Data• Working with Dates• Working with Expressions and Operators• Creating JavaScript Functions• Working with Conditional Statements• Using Arrays• Working with Program Loops• Debugging Your JavaScript Programs

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Introduction to JavaScript

• Server-side Programs pose problems• Client-side Programs were developed to

run programs and scripts on the client side of a Web browser

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The Development of JavaScript

• JavaScript is a subset of Java

• Differences between Java and JavaScript:– Java is a compiled language– JavaScript is an interpreted language

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Comparing Java and JavaScript

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The Development of JavaScript

• Jscript is a version of JavaScript supported by Internet Explorer

• The European Computer Manufacturers Association (ECMA) develops scripting standards– The standard is called ECMAScript but

browsers still generally call is JavaScript

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Inserting JavaScript into a Web Page File

• Outline the main tasks you want the program to perform first

• A JavaScript program can either be placed directly in a Web page file or saved in an external text file

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Inserting JavaScript into a Web Page File

• Insert a client-side script in a Web page when using the script element

• Comments are useful for hiding scripts from older browsers

• Specify alternative content using the nonscript element for browsers that don’t support scripts (or have their script support disabled)

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Writing Output to the Web Page

• An object-oriented programming language writes the output by manipulating tasks

• An action you perform on an object is called a method

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Writing Output to the Web Page

• To write text to a Web page, use the following JavaScript commands:document.write(“text”);

or

document.writeln(“text”)’

Where text is the content to be written to the page. The doucment.write() and document.writeln() methods are identical, except that the document.writeln() method preserves any line

breaks in the text string.

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Working with Variables and Data

• A variable is a named item in a program that stores information

• Variable names have restrictions.

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Working with Variables and Data

• JavaScript variable types:– Numeric variables– String variables– Boolean variables– Null variables

• You must declare a variable before using it

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Working with Variables and Data

• Numeric variable- any number, such as 13, 22.5, etc– Can also be expressed in scientific notation

• String variable- any group of text characters, such as “Hello” or “Happy Holidays!”– Must be enclosed within either double or single

quotations (but not both)

• Boolean variable- accepts only true and false values

• Null variable- has no value at all

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Declaring a JavaScript Variable

• You can declare variables with any of the following JavaScript commands:var variable;

var variable = value;

variable = value;

Where variable is the name of the variable and value is the initial value of the variable. The first command creates the variable without assigning it a value; the second and third commands both create the variable and assign it a value.

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Working with Dates

• Create a date object to store date information

Date Methods

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Working with Expressions and Operators

• Expressions are JavaScript commands that assign values and variables

• Operators are elements that perform actions within expressions– Arithmetic operators: perform simple mathematical

calculations– Binary operators: work on two elements in an expression– Unary operators: work on only one variable– Increment operators: can be used to increase the value of a

variable by 1– Assignment operators: used to assign values in expressions

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Working with Expressions and Operators

• The Math object is a JavaScript object used for calculations other than simple math

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Creating JavaScript Functions

• A function is a series of commands that perform an action or calculates a value

• A function name identifies a function

• Parameters are values used by the function

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Creating JavaScript Functions

• A group of commands set off by curly braces is called a command block. Command blocks exist for other JavaScript structures in addition to functions.

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Working with Conditional Statements

• Conditional statements are commands that run only when specific conditions are met

• Conditional statements require a Boolean expression– you need one of the following operators to create

a Boolean expression:• Comparison operator• Logical operator• Conditional operator

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Using Arrays

• An array is an ordered collection of values referenced by a single variable name

var variable = new Array (size);

Where variable is the name of the array variable and

size is the number of elements in the array

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Working with Program Loops

• A program loop is a set of instructions that is executed repeatedly– Use program loops to configure a group of

commands to be executed a set number of times

– The loop uses a counter to track the number of times the command block has been run

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Working with Program Loops

Creating a For loop

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Working with Program Loops

Nesting a For loop

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Working with Program Loops

Creating a While loop

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Working with Program Loops

Nesting a While loop

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Debugging Your JavaScript Programs

• Three types of errors:– Load-time errors (occurs when the script is

loading)– Run-time errors (occurs when the being

executed)– Logical errors (free from syntax and structural

mistakes, but result in incorrect results)

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Common Mistakes

• You need to debug your program to fix the mistakes

• Common mistakes include:– Misspelling a variable name– Mismatched parentheses or braces– Mismatched quotes– Missing quotes– Using ( instead of [– Using = in place of ==

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Debugging Tools and Techniques

• To avoid making mistakes and quickly located those you do make:– Write modular code– Use the Microsoft Script Debugger to debug

(for use with Internet Explorer)– Use the Netscape JavaScript Console to

debug (for use with Netscape)

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Tips for Writing Good JavaScript Code

• Use good layout to make your code more readable. Indent command blocks to make them easier to read and to set them off from other code

• Use descriptive variable names to indicate the purpose of your variables

• Be careful how you use uppercase and lowercase letters in your code, because JavaScript commands and names are case-sensitive

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Tips for Writing Good JavaScript Code

• Add comments to your code to document the purpose of each script

• Initialize all variables at the top of your script and insert comments describing the purpose and nature of your variables

• Create customized functions that can be reused in different scripts. Place your customized functions in external files to make them available to your entire Web site