Client-side scripting

Client-side scripting generally refers to the class of computer programs on the web that

are executed client-side, by the user's web browser, instead of server-side (on the web

server). This type of computer programming is an important part of the Dynamic HTML

(DHTML) concept, enabling web pages to be scripted; that is, to have different and

changing content depending on user input, environmental conditions (such as the time

of day), or other variables.

Web authors write client-side scripts in languages such as JavaScript (Client-side

JavaScript) and VBScript.

Client-side scripts are often embedded within an HTML document, but they may also be

contained in a separate file, which is referenced by the document (or documents) that

use it. Upon request, the necessary files are sent to the user's computer by the web

server (or servers) on which they reside. The user's web browser executes the script,

then displays the document, including any visible output from the script. Client-side

scripts may also contain instructions for the browser to follow if the user interacts with

the document in a certain way, e.g., clicks a certain button. These instructions can be

followed without further communication with the server, though they may require such

communication.

By viewing the file that contains the script, users may be able to see its source code.

Many web authors learn how to write client-side scripts partly by examining the source

code for other authors' scripts.

In contrast, server-side scripts, written in languages such as Perl, PHP, and server-side

VBScript, are executed by the web server when the user requests a document. They

produce output in a format understandable by web browsers (usually HTML), which is

then sent to the user's computer. The user cannot see the script's source code (unless

the author publishes the code separately), and may not even be aware that a script

was executed. The documents produced by server-side scripts may, of course, contain

client-side scripts.

Client-side scripts have greater access to the information and functions available on

the user's browser, whereas server-side scripts have greater access to the information

and functions available on the server. Server-side scripts require that their language's

interpreter is installed on the server, and produce the same output regardless of the

client's browser, operating system, or other system details. Client-side scripts do not

require additional software on the server (making them popular with authors who lack

administrative access to their servers); however, they do require that the user's web

browser understands the scripting language in which they are written. It is therefore

impractical for an author to write scripts in a language that is not supported by the web

browsers used by a majority of his or her audience.

Due to security restrictions, client-side scripts may not be allowed to access the users

computer beyond the browser application. Techniques like ActiveX controls can be used

to sidestep this restriction.

Unfortunately, even languages that are supported by a wide variety of browsers may

not be implemented in precisely the same way across all browsers and operating

systems. Authors are well-advised to review the behavior of their client-side scripts on

a variety of platforms before they put them into use.


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