JavaScript is a
lightweight, interpreted programming language. It is designed for creating
network-centric applications. It is complimentary to and integrated with Java.
JavaScript is very easy to implement because it is integrated with HTML. It is
open and cross-platform.
<html>
<body>
<script language="javascript" type="text/javascript">
<!--
document.write("Hello
World!")
//-->
</script>
</body>
</html>
What is JavaScript ?
Javascript is a dynamic computer programming language. It is
lightweight and most commonly used as a part of web pages, whose
implementations allow client-side script to interact with the user and make
dynamic pages. It is an interpreted programming language with object-oriented
capabilities.
JavaScript
was first known as LiveScript, but
Netscape changed its name to JavaScript, possibly because of the excitement
being generated by Java. JavaScript made its first appearance in Netscape 2.0
in 1995 with the name LiveScript.
The general-purpose core of the language has been embedded in Netscape,
Internet Explorer, and other web browsers.
The ECMA-262 Specification defined
a standard version of the core JavaScript language.
·
JavaScript is a lightweight, interpreted programming language.
·
Designed for creating network-centric applications.
·
Complementary to and integrated with Java.
·
Complementary to and integrated with HTML.
·
Open and cross-platform
Client-side JavaScript
Client-side
JavaScript is the most common form of the language. The script should be
included in or referenced by an HTML document for the code to be interpreted by
the browser.
It
means that a web page need not be a static HTML, but can include programs that
interact with the user, control the browser, and dynamically create HTML
content.
The
JavaScript client-side mechanism provides many advantages over traditional CGI
server-side scripts. For example, you might use JavaScript to check if the user
has entered a valid e-mail address in a form field.
The
JavaScript code is executed when the user submits the form, and only if all the
entries are valid, they would be submitted to the Web Server.
JavaScript
can be used to trap user-initiated events such as button clicks, link
navigation, and other actions that the user initiates explicitly or implicitly.
Advantages of JavaScript
The
merits of using JavaScript are −
·
Less
server interaction − You can validate user input before sending the page off to
the server. This saves server traffic, which means less load on your server.
·
Immediate
feedback to the visitors − They don't have to wait for a page
reload to see if they have forgotten to enter something.
·
Increased
interactivity − You can create interfaces that react when the user hovers
over them with a mouse or activates them via the keyboard.
·
Richer
interfaces − You can use JavaScript to include such items as
drag-and-drop components and sliders to give a Rich Interface to your site
visitors.
Limitations of JavaScript
We
cannot treat JavaScript as a full-fledged programming language. It lacks the
following important features −
·
Client-side JavaScript does not allow the reading or writing of
files. This has been kept for security reason.
·
JavaScript cannot be used for networking applications because
there is no such support available.
·
JavaScript doesn't have any multithreading or multiprocessor
capabilities.
Once
again, JavaScript is a lightweight, interpreted programming language that
allows you to build interactivity into otherwise static HTML pages.
JavaScript Development Tools
One
of major strengths of JavaScript is that it does not require expensive development
tools. You can start with a simple text editor such as Notepad. Since it is an
interpreted language inside the context of a web browser, you don't even need
to buy a compiler.
To
make our life simpler, various vendors have come up with very nice JavaScript
editing tools. Some of them are listed here −
·
Microsoft
FrontPage − Microsoft has developed a popular HTML editor called
FrontPage. FrontPage also provides web developers with a number of JavaScript
tools to assist in the creation of interactive websites.
·
Macromedia
Dreamweaver MX − Macromedia Dreamweaver MX is a very popular HTML and
JavaScript editor in the professional web development crowd. It provides
several handy prebuilt JavaScript components, integrates well with databases,
and conforms to new standards such as XHTML and XML.
·
Macromedia
HomeSite 5 − HomeSite 5 is a well-liked HTML and JavaScript editor from
Macromedia that can be used to manage personal websites effectively.
Where is JavaScript Today ?
The
ECMAScript Edition 5 standard will be the first update to be released in over
four years. JavaScript 2.0 conforms to Edition 5 of the ECMAScript standard,
and the difference between the two is extremely minor.
Today,
Netscape's JavaScript and Microsoft's JScript conform to the ECMAScript
standard, although both the languages still support the features that are not a
part of the standard.
JavaScript can be implemented using JavaScript statements that are
placed within the <script>...
</script>.
You
can place the <script> tags,
containing your JavaScript, anywhere within your web page, but it is normally
recommended that you should keep it within the <head> tags.
The
<script> tag alerts the browser program to start interpreting all the
text between these tags as a script. A simple syntax of your JavaScript will
appear as follows.
<script ...>
JavaScript code
</script>
The
script tag takes two important attributes −
·
Language −
This attribute specifies what scripting language you are using. Typically, its
value will be javascript. Although recent versions of HTML (and XHTML, its
successor) have phased out the use of this attribute.
·
Type −
This attribute is what is now recommended to indicate the scripting language in
use and its value should be set to "text/javascript".
So
your JavaScript segment will look like −
<script language="javascript" type="text/javascript">
JavaScript code
</script>
Your First JavaScript Script
Let
us take a sample example to print out "Hello World". We added an
optional HTML comment that surrounds our JavaScript code. This is to save our
code from a browser that does not support JavaScript. The comment ends with a
"//-->". Here "//" signifies a comment in JavaScript, so
we add that to prevent a browser from reading the end of the HTML comment as a
piece of JavaScript code. Next, we call a function document.write which
writes a string into our HTML document.
This
function can be used to write text, HTML, or both. Take a look at the following
code.
<html>
<body>
<script language="javascript" type="text/javascript">
<!--
document.write("Hello World!")
//-->
</script>
</body>
</html>
This
code will produce the following result −
Hello World!
Whitespace and Line Breaks
JavaScript
ignores spaces, tabs, and newlines that appear in JavaScript programs. You can
use spaces, tabs, and newlines freely in your program and you are free to
format and indent your programs in a neat and consistent way that makes the
code easy to read and understand.
Semicolons are Optional
Simple
statements in JavaScript are generally followed by a semicolon character, just
as they are in C, C++, and Java. JavaScript, however, allows you to omit this
semicolon if each of your statements are placed on a separate line. For
example, the following code could be written without semicolons.
<script language="javascript" type="text/javascript">
<!--
var1 = 10
var2 = 20
//-->
</script>
But
when formatted in a single line as follows, you must use semicolons −
<script language="javascript" type="text/javascript">
<!--
var1 = 10; var2 = 20;
//-->
</script>
Note − It is a good programming practice
to use semicolons.
Case Sensitivity
JavaScript
is a case-sensitive language. This means that the language keywords, variables,
function names, and any other identifiers must always be typed with a consistent
capitalization of letters.
So
the identifiers Time and TIME will convey different meanings in
JavaScript.
NOTE − Care should be taken while writing
variable and function names in JavaScript.
Comments in JavaScript
JavaScript
supports both C-style and C++-style comments, Thus −
·
Any text between a // and the end of a line is treated as a
comment and is ignored by JavaScript.
·
Any text between the characters /* and */ is treated as a comment.
This may span multiple lines.
·
JavaScript also recognizes the HTML comment opening sequence
<!--. JavaScript treats this as a single-line comment, just as it does the
// comment.
·
The HTML comment closing sequence --> is not recognized by
JavaScript so it should be written as //-->.
Example
The
following example shows how to use comments in JavaScript.
<script language="javascript" type="text/javascript">
<!--
// This is a comment. It is similar to comments in C++
/*
* This is a multiline comment in JavaScript
* It is very similar to comments in C Programming
*/
//-->
</script>
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