JavaScript: Control Structures

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JavaScript Control Structures

• September 27, 2005
• Slides modified fromInternet World Wide Web
  How to Program. 2004 (3rd) edition. By Deitel,
  Deitel, and Goldberg. Published by Prentice Hall.
  ISBN 0-13-145091-3

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Chapter 7 - JavaScript Introduction to Scripting
Outline 7.1 Introduction 7.2 Simple Program
Printing a Line of Text in a Web Page 7.3
Obtaining User Input with prompt
Dialogs 7.3.1 Dynamic Welcome Page 7.3.2 Adding
Integers 7.4 Memory Concepts 7.5 Arithmetic 7.6
Decision Making Equality and Relational
Operators 7.7 Web Resources
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Objectives

• In this lesson, you will learn
• To be able to write simple JavaScript programs.
• To be able to use input and output statements.
• To understand basic memory concepts.
• To be able to use arithmetic operators.
• To understand the precedence of arithmetic
  operators.
• To be able to write decision-making statements.
• To be able to use relational and equality
  operators.

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Introduction to JavaScripting
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7.1 Introduction

• JavaScript scripting language
• Enhances functionality and appearance
• Client-side scripting
• Makes pages more dynamic and interactive
• Foundation for complex server-side scripting
• Program development
• Program control

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What can JavaScript programs do?From
http//www.javascripter.net/faq/javascri.htm

• Giving the user more control over the browser
• Detecting the user's browser, OS, screen size,
  etc.
• Performing simple computations on the client side
• Validating the user's input
• Handling dates and time
• Generating HTML pages on-the-fly without
  accessing the Web server.

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What cant JavaScript programs do?From
http//www.javascripter.net/faq/javascr2.htm

• Use printers or other devices on the user's
  system or the client-side LAN
• Directly access files on the user's system or the
  client-side LAN the only exception is the
  access to the browser's cookie files.
• Directly access files on the Web server.
• Implement multiprocessing or multithreading.

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Programming context

• Interpreted language
• Web browser as the interpreter
• Client-side only
• Compatibility depends on browser version
• But virtually every browser supports JavaScript
• Follow ECMAScript standard!
• Security issues
• You cannot access the clients system beyond
  their web browser
• Thus, no file access or direct printing

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Notes on compatibility

• Browser versions
• 80-90 of people user Internet Explorer
• Of which 99 use IE 6 or IE 5
• 10-15 use Gecko-based browsers
• Firefox, Netscape, Mozilla
• JavaScript and cookies
• Only 85-90 have JavaScript enabled
• Cookies usually enabled by defaultI dont have
  statistics for how many leave this on
• Screen resolutions
• 25 use 800x600
• 70 use 1024x768 or higher
• Recommendations
• Test all pages with IE 5/6 and Firefox
• Dont assume that users have JavaScript or
  cookies enabled
• Your site should still be functional without
  client-side scripting or cookies
• Design to be comfortably viewable with 800x600
  resolution
• Sources
• http//www.upsdell.com/BrowserNews/stat_trends.htm
  
• http//www.w3schools.com/browsers/browsers_stats.a
  sp

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Object orientation in JavaScript

• JavaScript is an object-oriented programming
  language
• Based on objects with properties and methods
• JavaScript uses prototyping to replicate
  behaviour, rather than classification (classes)
• There are two types of variables
• Simple variables (also called primitives)
• Objects

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Objects in JavaScript

• An object is a complex type of variable that
  contains its own variables (properties), and has
  its own functions (methods)
• Properties
• The variables of an object
• Can also be other objects
• Methods
• The functions that belong to an object
• e.g. document.bgColor, window.location
• JavaScript has a rich set of built-in objects
• http//www.devguru.com/Technologies/ecmascript/qui
  ckref/js_objects.html
• You can also define your own objects

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How to specify JavaScript code

• Inline Code
• Add dynamic functionality to an individual
  element
• ltimg onMouseOverdoThisFunction(this)gt
• Write JavaScript code directly in appropriate
  event attributes
• Embedded scripts
• Embed JavaScript code in an XHTML documenteither
  in the head or in the body
• ltscript typetext/javascriptgt x3 y9
  document.writeln( x y ) lt/scriptgt
• Inserting external scripts
• Insert a file with .js extension that contains
  JavaScript code
• ltscript typetext/javascript srcexternal.js
  /gt
• JavaScript libraries can be shared by multiple
  XHTML documents
• A note on CDATA
• lt!CDATA gt
• Necessary for legal XML to ignore and lt symbols
  (which are common in scripts)
• However, most browsers work fine without them
• http//www.w3schools.com/xml/xml_cdata.asp
• The textbook says to use lt!-- comments to hide
  your code --gt
• But CDATA is proper XML syntax
• Comments should validate, but not guaranteed

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First scripts
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7.2 Simple Program Printing a Line of Text in a
Web Page

• Inline scripting
• Written in the ltbodygt of a document
• ltscriptgt tag
• Indicate that the text is part of a script
• type attribute
• Specifies the type of file and the scripting
  language use
• writeln method
• Write a line in the document
• Escape character ( \ )
• Indicates special character is used in the
  string
• alert method
• Dialog box

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welcome.html(1 of 1)
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welcome2.html(1 of 1)
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welcome3.html1 of 1
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welcome4.html1 of 1
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Illegal characters inXHTML vs. JavaScript

• XHTML
• HTML entities
• amp lt gt
• Others http//www.w3schools.com/tags/ref_entities
  .asp
• JavaScript
• Escape sequences
• \ \ \n \t \\
• Others http//www.devguru.com/Technologies/ecmasc
  ript/quickref/escaped_characters.html
• To convert between the two,use an XHTML ?
  JavaScript converter
• Google HTML JavaScript converter
• Best that Ive found
• http//javascript.about.com/library/blscr02.htm

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7.2 Common escape sequences
For full list, see http//www.devguru.com/Technolo
gies/ecmascript/quickref/escaped_characters.html
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7.3.1 Dynamic Welcome Page

• A script can adapt the content based on input
  from the user or other variables

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welcome5.html(1 of 2)
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7.3.1 Dynamic Welcome Page
This is the text field in which the user types
the value.
Fig. 7.7 Prompt dialog displayed by the window
objects prompt method.
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JavaScript data types

• In JavaScript, you can use the var statement to
  create a variable, but it is not required
• You do not declare any data type (loosely typed)
• For scripting languages, data types are
  automatically determined
• Automatically converts between values of
  different types
• JavaScript data types
• Numbers
• Numbers are numbers, whether integer or floating
  point
• Boolean values
• true or false
• Strings
• Anything within quotes
• Anything within quotes
• Null
• "special key word denoting a null value"
• Means no value
• From http//www.ryerson.ca/JavaScript/lectures/bas
  icDataTypes/basicTypes.html

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7.3.2 Adding Integers

• Prompt user for two integers and calculate the
  sum (Fig. 7.8)
• NaN (not a number)
• parseInt
• Converts its string argument to an integer

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Addition.html(1 of 2)
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JavaScript native functions

• See http//www.devguru.com/Technologies/ecmascript
  /quickref/js_functions.html
• parseInt()
• Converts a string into an integer, if possible
• parseFloat ()
• Converts a string into a floating-point number,
  if possible
• string ()
• Converts any object into a string
• number ()
• Converts any object into a number, if possible
• isNAN ()
• true if the object is Not A Number
• escape() and unescape()
• Converts and deconverts strings to safe
  characters

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Operators in JavaScript

• http//www.devguru.com/Technologies/ecmascript/qui
  ckref/js_operators.html

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7.5 Arithmetic operators
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7.6 Decision Making Equality and Relational
Operators
?

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welcome6.html(1 of 3)
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welcome6.html(2 of 3)
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7.6 Decision Making Equality and Relational
Operators