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Javascript
Laboratory of Intelligent Networks(LINK)@KUT
Youn-Hee Han
Scripting Languages
Distinguishing features
not compiled
interpreted
not strongly typed
subprograms can be added at run time
text of program code can be directly executed
programs can be constructed at run time
Scripting Languages (cont.)
Typical additional features
associative arrays
regular expressions
good string processing
objects, classes, inheritance
functionality can be added at run time
JavaScript
Scripting language
associative arrays
DOM (Document Object Model) is directly accessible
Adds functionality to web pages
Runs on the client side
interpreter is embedded in the browser
Makes it possible to change XHTML elements and their
properties
Dynamic HTML (DHTML)
JavaScript = ECMA-Script
Syntax like C
JavaScript vs. Java
JavaScript is quite different from Java
similar syntax
because they both descend from C
Java is strongly typed
Java is compiled (into byte code)
actually hybrid because byte code is mostly interpreted
Java has strong Object Oriented foundation
in JavaScript, OO was an afterthought
now is getting better
The name was primarily a marketing decision
Sun and Netscape against Microsoft
Browser Problems
Before users can run JavaScript scripts in files of local
computer, they may need to change the browser’s
security settings
By default
Internet Explorer (IE) disables JavaScript of local files
FireFox (FF) enables JavaScript of local files
Browsers render web pages differently
DOM programming is cumbersome
each vendor implements DOM differently
legacy of browser wars
JavaScript of local files in IE
Enabling JavaScript in Internet Explorer 7
<script> Element
<script> element
either contains scripts
an entire program can be placed there
or contains reference to external scripts file
this is the preferred way!
language (or type) attribute
specifies the scripting language used
text/javascript for JavaScript
there is also VBScript language
Microsoft's effort to dominate the market
src attribute
URL of a file with an external script
standard file extension .js for JavaScript source files
<script src="login.js" language="text/javascript">
</script>
JavaScript in a Web Page
Typically, <script> is placed within the <head>
element of the XHTML document
the contents inside <head> is interpreted first
this is the preferred way
But <script> elements can be placed in the <body>
element, too
they are executed in the order in which they appear in the
XHTML document
Script fragments occur in the attributes of other
elements
e.g. in elements of a form (<input> button), <div> element
to define the response to events
typically just a function call
Script Inside <script>
Some browsers do not support <script>
we must put the script within an XHTML comment
<script language="text/javascript">
<!–…
//-->
</script>
the JavaScript comment // is needed so that --> isn't
interpreted as JavaScript statement
otherwise the script will appear as web page text
Writing a Web Page
The DOM document object
represents the XHTML document of the web page
currently displayed in the browser
document.write() or document.writeln()
insert XHTML code into the XHTML document
once the page is rendered, document.write()
replaces the entire page
this allows us to specify (portions of) a web page via
scripting!
alert()
displays a dialog with message passed a the
argument string
prompt()
displays a dialog with message passed a the
argument string
and an input field where the user can type a text
Example
<html xmlns = "http://www.w3.org/1999/xhtml">
<head>
<title>Welcome</title>
<link rel="stylesheet" href="sample.css" />
<script language="text/javascript" src="sample.js"></script>
</head>
<body>
<script type = "text/javascript">
<!-var title = "Welcome";
var text = "415 is fun!";
document.writeln("<h1>"+title+"</h1>");
document.writeln("<p>"+text+"</p>");
alert (title+"\n"+text);
//-->
</script>
</body>
</html>
