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Here's the short answer: JavaScript is the leading client-side
scripting language for use on Web pages.
So, what does that mean?
 JavaScript is a scripting
language, and scripting languages are easier to learn and use
than traditional programming languages.
JavaScript works on
the client side, which means that it runs on the
user's computer and not on the Web server. The browsers interprets
JavaScript statements embedded directly in an HTML page.
JavaScript is
designed for use on Web pages and is closely integrated with
HTML.
JavaScript statements
embedded in an HTML page can recognize and respond to
User Events such as Mouse Clicks, Form Input, and Page
Navigation.
For example, you can write a JavaScript function to verify that users
enter valid information into a form requesting a telephone number or zip
code. Without any network transmission, an HTML page with embedded
JavaScript can interpret the entered text and alert the user with a
message dialog if the input is invalid. Or you can use JavaScript to
perform an action (such as play an audio file, execute an applet, or
communicate with a plug-in) in response to the user opening or exiting a
page.
Here's the long answer:
Scripting languages have been in use long before the Web came around.
In Unix environment, scripts have been used to perform repetitive system
administration tasks and to automate many tasks for less
computer-literate users. In addition, scripting languages are the basis
of much of the cgi-bin programming that is currently
used to add limited form of interactivity to Web pages.
Of course, this still doesn’t tell what the main advantages of
scripting languages are. Like all scripting languages, JavaScript is
interpreted, which provides an easy development process; it contains a
limited and easy-to-learn command set and syntax; and it is designed for
performing a well-defined set of tasks.
- Designed for Simple, Small Programs
Because JavaScript is a scripting language, it is well suited to
implementing simple, small programs. For instance, JavaScript would
ideally be suited to developing a unit conversion calculator between
miles and kilometers or pounds and kilograms. These tasks can be
easily written and performed at acceptable speeds with JavaScript and
would be easily integrated into a Web page. A more robust language
such as Java would be less suitable for the quick development and easy
maintenance of these types of applications.
By contrast, JavaScript would not be well suited to implementing a
distributed CAD document display and manipulation environment. While
eventually JavaScript will be a tool for integrating this type of Java
applet or plug-in into a Web page, to attempt to develop the actual
applet in JavaScript would at best, difficult and inefficient and,
more likely, would be impossible.
- Performs Repetitive Tasks
Just as JavaScript is suited to producing small programs, it is
especially well designed for repetitive, event-invoked tasks. For
example, JavaScript is ideal for calculating the content of on field
in a form based on changes to the data in another field. Each time the
data changes, the JavaScript program to handle the event is invoked,
and the new data for the other field is calculated and displayed.
- Designed for Programming User Events
Because of the way in which JavaScript is integrated into the
browser and can interact directly with the HTML pages, JavaScript
makes it possible to program responses to user events such as mouse
clicks and data entry in forms.
For instance, a JavaScript script could be used to implement a
simple help system. Whenever the user points a button or a link on the
page, a helpful and informative message can be displayed in the bar at
the bottom of the browser window.
This adds interactivity to Web pages, makes forms dynamic, and can
decrease the bandwidth requirements and server load incurred by using
forms and CGI programming.
- Easy Debugging and Testing
Like other scripting languages, JavaScript eases development and
trouble-shooting because it is not compiled. It is easy to test
program code, look at the results, make changes, and test it again
without the overhead and delay of compiling.
LiveConnect enables communication between JavaScript and Java
applets in a page and between JavaScript and Plug-ins loaded on a
page. JavaScript can trigger events in Java applets, and Java applets
can call JavaScript methods and functions.
 Summary
of JavaScript Strengths
JavaScript offers several strengths to the programmer including a
short development cycle, ease of learning, and small size scripts. These
strengths mean that JavaScript can be easily and quickly used to extend
HTML pages already on the web.
- Quick Development
– because of JavaScript does not require
time-consuming compilation, scripts can be developed in a relatively
short period of time. This is enhanced by the fact that most of the
interface features, such as dialog boxes, forms, and other GUI
elements, are handled by the browser and HTML code. JavaScript
programmers don’t have to worry about creating or handling these
elements of their applications.
- Easy to Learn
– while JavaScript may share many similarities
with Java, it doesn’t include the complex syntax and rules of Java.
By learning just a few commands and simple rules of syntax, along with
understanding the way objects are used in JavaScript, it is possible
to begin creating fairly sophisticated programs.
- Platform Independence
– Because the WWW, by its very nature,
is platform-independent, JavaScript programs created for the browsers
are not tied to any specific hardware platform or operating system.
The same program code can be used on any platform for which a browser
is available.
- Small Overhead
– JavaScript programs tend to be fairly
compact and are quite small, compared to the binary applets produced
by Java. This minimizes storage requirements on the server and
download times for the user. In addition, because JavaScript programs
usually are included in the same file as the HTML code for a page,
they require fewer network accesses.
Weakness
of JavaScript
As would be expected, JavaScript has it own unique weaknesses. These
include limited set of built-in methods, the inability to protect source
code from prying eyes.
- Limited Range of Built-in Methods - however, more and more Methods
are being introduced in the newer JavaScript versions.
- Generally, only Client-Side script
- No Code
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