Chapter 8 - JavaScript/JScript: Introduction to Scripting

Report
Chapter 8 - JavaScript/JScript:
Introduction to Scripting
Outline
8.1
8.2
8.3
8.4
8.5
8.6
8.7
Introduction
A Simple Program: Printing a Line of Text in a
Web Page
Another JavaScript Program: Adding Integers
Memory Concepts
Arithmetic
Decision Making: Equality and
Relational Operators
JavaScript Internet and World Wide Web
Resources
 2000 Deitel & Associates, Inc. All rights reserved.
8.1 Introduction
• JavaScript scripting language
– Originally created by Netscape
– Facilitates disciplined approach to designing computer
programs
– Enhances functionality and appearance of Web pages
• Jscript
– Microsoft’s version of JavaScript
 2000 Deitel & Associates, Inc. All rights reserved.
8.2 A Simple Program: Printing a Line of Text in a
Web Page
• Browser includes JavaScript Interpreter
– Processes JavaScript commands
• Whitespace
– Blank lines, space characters, tab characters
– Generally ignored by browser
– Used for readability and clarity
• <SCRIPT>…</SCRIPT> tag:
– Encloses entire script
– Attribute LANGUAGE=“JavaScript”
• Indicates scripting language (JavaScript default in IE5 & Netscape)
– Tag must be closed at the end of the script
 2000 Deitel & Associates, Inc. All rights reserved.
8.2 A Simple Program: Printing a Line of Text in a
Web Page (II)
• Correct method call syntax:
– object.method( “string”, “[additional arguments]” );
• document.writeln( “<H1>argument</H1>” );
– Case-sensitive, like all JavaScript functions
– Uses the writeln method in the browser’s document
object
– Prints the string, which can consist of any text and HTML
tags
– String must be surrounded by quotation marks (“…”)
• Statement terminators
– All statements must end with semi-colons (;)
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<!-- Fig. 8.1: welcome.html -->
<HTML>
<HEAD>
<TITLE>A First Program in JavaScript</TITLE>
<SCRIPT LANGUAGE = "JavaScript">
document.writeln(
"<H1>Welcome to JavaScript Programming!</H1>" );
</SCRIPT>
</HEAD><BODY></BODY>
</HTML>
Outline
1.1 Open scripting area
2.1 Call writeln
method
2.2 Give arguments
2.3 Execute statement
2.4 Close scripting
area
5.1 Close HTML
document
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<!-- Fig. 8.2: welcome.html -->
<HEAD>
<TITLE>Printing a Line with Multiple Statements</TITLE>
<SCRIPT LANGUAGE = "JavaScript">
document.write( "<FONT COLOR='magenta'><H1>Welcome to " );
document.writeln( "JavaScript Programming!</H1></FONT>" );
</SCRIPT>
</HEAD><BODY></BODY>
</HTML>
 2000 Deitel & Associates, Inc. All rights reserved.
Outline
1.1 Call first statement
1.2 Execute statement
1.3 Call second
statement
1.4 Execute statement
8.2 A Simple Program: Printing a Line of Text in a
Web Page (III)
• Object: document methods:
– writeln
• Positions output cursor on next line when finished
– write
• Leaves the output cursor where it is when done executing
– Both begin output where previous statement stopped
– Line breaks inserted in two ways:
• document.writeln( “Have a<br>Nice Day!” )
• document.writeln( “Have a\nNice Day!” )
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<!-- Fig. 8.3: welcome.html -->
<HEAD><TITLE>Printing Multiple Lines</TITLE>
<SCRIPT LANGUAGE = "JavaScript">
document.writeln(
"<H1>Welcome to<BR>JavaScript<BR>Programming!</H1>" );
</SCRIPT>
</HEAD><BODY></BODY>
</HTML>
 2000 Deitel & Associates, Inc. All rights reserved.
Outline
1.1 Call writeln
method
1.2 Format text inside
argument as desired
1.3 Execute statement
8.2 A Simple Program: Printing a Line of Text in a
Web Page (IV)
• Methods in window object
– Call on-screen windows
– window.alert( “argument” );
• Method calls alert window with window text "argument"
• Outputs button with text and ‘OK’ button
– window.prompt(“”);
• Prompts user for string (discussed later)
• Scripts restart when page reloaded/refreshed
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<!-- Fig. 8.4: welcome.html -->
<!-- Printing multiple lines in a dialog box -->
<HEAD>
<SCRIPT LANGUAGE = "JavaScript">
window.alert( "Welcome to\nJavaScript\nProgramming!" );
</SCRIPT>
</HEAD>
<BODY>
<P>Click Refresh (or Reload) to run this script again.</P>
</BODY>
</HTML>
 2000 Deitel & Associates, Inc. All rights reserved.
Outline
1.1 Call
window.alert();
method
2.1 Give instructions
for script restart
8.2 A Simple Program: Printing a Line of Text in a
Web Page (IV)
Common Escape Sequences
Esc a p e se q u e n c e
D e sc rip t io n
\n
N ew lin e. P o sitio n th e screen cu rso r to th e b eg in n in g o f th e n ex t lin e.
\t
H o rizo n tal tab . M o v e th e screen cu rso r to th e n ex t tab sto p .
\r
C arriag e retu rn . P o sitio n th e screen cu rso r to th e b eg in n in g o f th e
cu rren t lin e; d o n o t ad v an ce to th e n ex t lin e. A n y ch aracters o u tp u t
after th e carriag e retu rn o v erw rite th e p rev io u s ch aracters o u tp u t o n
th at lin e.
\\
B ack slash . U sed to rep resen t a b ack slash ch aracter in a strin g .
\"
D o u b le q u o te. U sed to rep resen t a d o u b le q u o te ch a racter in a strin g
co n tain ed in d o u b le q u o tes. F o r ex am p le,
window.alert( "\"in quotes\"" );
d isp lay s " i n q u o t e s " in an a l e r t d ialo g .
\’
S in g le q u o te. U sed to rep resen t a sin g le q u o te ch aracter in a strin g .
F o r ex am p le,
window.alert( ’\’in quotes\’’ );
d isp lay s ’ i n q u o t e s ’ in an a l e r t d ialo g .
 2000 Deitel & Associates, Inc. All rights reserved.
8.3 Another JavaScript Program: Adding Integers
• Variables
– Location in memory where values are stored
– Variable name can be any valid identifier
• Identifier = series of characters
– Letters, digits, underscores (‘_’) and dollar signs (‘$’)
– Cannot begin with a digit
• Valid identifiers:
Welcome, $value, _value, m_inputField1, C3PO and R2D2
• Invalid identifiers: 7button, Say\Hello and field#5
– Identifiers are case-sensitive
 2000 Deitel & Associates, Inc. All rights reserved.
8.3 Another JavaScript Program: Adding Integers
(II)
• Variable name convention
– Begin with lowercase first letter
– Every following word has first letter capitalized
• goRedSox, bostonUniversityRules
• Declarations
– var name1, name2
– Indicate that name1 and name2 are program variables
 2000 Deitel & Associates, Inc. All rights reserved.
8.3 Another JavaScript Program: Adding Integers
(III)
• Method window.prompt( “arg1”, “arg2” )
– Calls window that allows user to enter value to use in the
script
– arg1 : text that will appear in window
– arg2 : text that will initially appear in input line
• firstNumber = window.prompt();
– Assigns value entered by the user in prompt window to
variable first
– "=" a binary operator
• Assigns value of right operand to left operand
 2000 Deitel & Associates, Inc. All rights reserved.
8.3 Another JavaScript Program: Adding Integers
(IV)
• Good programmers write many comments
– Helps other programmers decode script
– Aids debugging
– Comment Syntax:
• One-line comment: // [text]
• Multi-line comment: /* [text] */
• parseInt();
– Function accepts a string and returns an integer value
• Not a method because we do not refer to an object name
number1 = parseInt( firstNumber );
– Operates right-to-left (due to the "=" sign)
 2000 Deitel & Associates, Inc. All rights reserved.
8.3 Another JavaScript Program: Adding Integers (V)
• sum = number1 + number2;
– Adds number1 and number2
– Assigns result to variable sum
• String concatenation:
– Combines string and another data type
• Other data type can be another string
– Example:
• If age = 20,
document.writeln( “I am ” + age + “years old!” );
Prints: I am 20 years old!
 2000 Deitel & Associates, Inc. All rights reserved.
1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
2 <HTML>
3 <!-- Fig. 8.6: Addition.html -->
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5 <HEAD>
6 <TITLE>An Addition Program</TITLE>
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8 <SCRIPT LANGUAGE = "JavaScript">
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var firstNumber,
// first string entered by user
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secondNumber, // second string entered by user
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number1,
// first number to add
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number2,
// second number to add
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sum;
// sum of number1 and number2
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// read in first number from user as a string
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firstNumber = window.prompt( "Enter first integer", "0" );
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// read in second number from user as a string
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secondNumber = window.prompt( "Enter second integer", "0" );
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// convert numbers from strings to integers
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number1 = parseInt( firstNumber );
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number2 = parseInt( secondNumber );
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// add the numbers
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sum = number1 + number2;
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// display the results
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document.writeln( "<H1>The sum is " + sum + "</H1>" );
30 </SCRIPT>
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 2000
Deitel & Associates, Inc. All rights reserved.
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</HEAD>
Outline
1.1 Declare strings
1.2 Insert comments
2.1 Prompt for strings
& store values
3.1 Convert strings to
integers
3.2 Calculate sum of
variables
4.1 Display result
(concatenate strings)
User Input:
Script Output:
 2000 Deitel & Associates, Inc. All rights reserved.
8.4 Memory Concepts
• Variables:
– Name corresponds to location in memory
– Have 3 attributes:
• Name
• Type
• Value
• Memory
– When a value assigned to a variable, it overwrites any
previous value
– Reading values is non-destructive
• sum = number1 + number2
• Does not change number1 or number2
 2000 Deitel & Associates, Inc. All rights reserved.
8.5 Arithmetic
• Binary Operators
– Used in arithmetic operations
• Modulus operator (%)
– Yields remainder after division
– Examples:
43 % 5 = 3
8.7 % 3.4 = 1.9
24 % 6 = 0
 2000 Deitel & Associates, Inc. All rights reserved.
8.5 Arithmetic (II)
Ja v a Sc rip t
o p e ra t io n
A rit h m e t ic
o p e ra t o r
A lg e b ra ic Ja v a Sc rip t
e xp re ssio n e xp re ssio n
A d d itio n
+
f+ 7
f+ 7
S u b tractio n
-
p – c
p -c
M u ltip licatio n
*
bm
b*m
D iv isio n
/
x / y or x
M o d u lu s
%
r m od s
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
y
x/y
r% s
8.5 Arithmetic (III)
• Arithmetic operations
– Operate right to left (like the ‘=’ sign)
• Rules of operator precedence
– Operations execute in a specific order
O p e ra t o r( s)
O p e ra t io n ( s)
O rd e r o f e v a lu a t io n ( p re c e d e n c e )
()
P aren th eses
1 ) If th e p aren th eses n ested , ex p ressio n in in n erm o st p air ev alu ated first. If
sev eral p airs o f p aren th eses “o n th e sam e lev el” (n o t n ested ), ev alu ated left
to rig h t.
*, / or %
M u ltip licatio n , D iv isio n , 2 ) If m o re th en o n e, th en ev alu ated left to rig h t.
M o d u lu s
A d d itio n , S u b tractio n
3 ) If m o re th an o n e, th en ev alu ated left to rig h t.
+ or -
 2000 Deitel & Associates, Inc. All rights reserved.
8.5 Arithmetic (IV)
Order of evaluation
Step 1.
y = 2 * 5 * 5 + 3 * 5 + 7;
2 * 5 is 10
Step 2.
y = 10 * 5 + 3 * 5 + 7;
10 * 5 is 50
Step 3.
(L eftm ost addition )
y = 65 + 7;
65 + 7 is 72
Step 6.
(M u ltiplication before addition )
y = 50 + 15 + 7;
50 + 15 is 65
Step 5.
(L eftm ost m u ltiplication )
y = 50 + 3 * 5 + 7;
3 * 5 is 15
Step 4.
(L eftm ost m u ltiplication )
y = 72;
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(L ast addition )
(L ast operation — assign m en t )
8.5 Decision Making: Equality and Relational
Operators
• if structure:
– Program makes decision based on truth or falsity of condition
• If condition met (true)
– Statement(s) in body of structure executed
• If condition not met (false)
– Statement(s) in body of structure skipped
• Format:
if (condition) {
statement;
(additional statements);
}
• Semi-colon (‘;’)
– Do not place after condition
– Place after every statement in body of structure
 2000 Deitel & Associates, Inc. All rights reserved.
8.5 Decision Making: Equality and Relational
Operators (II)
Equality and Relational Operators:
St a n d a rd a lg e b ra ic
e q u a lit y o p e ra t o r
or
re la t io n a l o p e ra t o r
Ja v a Sc rip t e q u a lit y
or
re la t io n a l o p e ra t o r
Sa m p le
Ja v a Sc rip t
c o n d it io n
M e a n in g o f
Ja v a Sc rip t c o n d it io n
E q u a lity O p era to rs
=
==
x == y
x is eq u al to y
?
!=
x != y
x is n o t eq u al to y
R ela tio n a l O p era to rs
>
>
x > y
x is g reater th an y
<
<
x < y
x is less th an y
=
>=
x >= y
x is g reater th an o r eq u al to y
=
<=
x <= y
x is less th an o r eq u al to y
 2000 Deitel & Associates, Inc. All rights reserved.
1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
2 <HTML>
3 <!-- Fig. 8.14: comparison.html -->
4 <!-- Using if statements, relational operators, -->
5 <!-- and equality operators -->
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7 <HEAD>
8 <TITLE>Performing Comparisons</TITLE>
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10 <SCRIPT LANGUAGE = "JavaScript">
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var first,
// first string entered by user
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second; // second string entered by user
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// read first number from user as a string
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first = window.prompt( "Enter first integer:", "0" );
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// read second number from user as a string
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second = window.prompt( "Enter second integer:", "0" );
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document.writeln( "<H1>Comparison Results</H1>" );
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document.writeln( "<TABLE BORDER = '1' WIDTH = '100%'>" );
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if ( first == second )
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document.writeln( "<TR><TD>" + first + " == " + second +
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"</TD></TR>" );
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if ( first != second )
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document.writeln( "<TR><TD>" + first + " != " + second +
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"</TD></TR>" );
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if ( first < second )
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2000
Deitel
& Associates, Inc. All rights
reserved. + first + " < " + second +
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document.writeln(
"<TR><TD>"
Outline
1.1 Initialize variables
2.1 Prompt for values
2.2 Initialize table
3.1 Execute if
structures
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"</TD></TR>" );
if ( first > second )
document.writeln( "<TR><TD>" + first + " > " + second +
"</TD></TR>" );
if ( first <= second )
document.writeln( "<TR><TD>" + first + " <= " + second +
"</TD></TR>" );
if ( first >= second )
document.writeln( "<TR><TD>" + first + " >= " + second +
"</TD></TR>" );
// Display results
document.writeln( "</TABLE>" );
</SCRIPT>
</HEAD>
<BODY>
<P>Click Refresh (or Reload) to run the script again</P>
</BODY>
</HTML>
 2000 Deitel & Associates, Inc. All rights reserved.
Outline
3.2 Complete if
structures
4.1 Display results
If: First Integer = 123
Second Integer = 123
If: First Integer = 100
Second Integer = 200
If: First Integer = 200
Second Integer = 100
 2000 Deitel & Associates, Inc. All rights reserved.

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