A client-side language is run directly through the client being used by the viewer. In the case of JavaScript, the client is a Web browser. Therefore, JavaScript is run directly in the Web browser and doesn't need to go through the extra step of sending and retrieving information from the Web server.
With a client-side language, the browser reads and interprets the code, and the results can be given to the viewer without getting information from the server fIrst. This process can make certain tasks run more quickly.

A client-side language can also access special features of a browser window that may not be accessible with a server-side language. However, a client-side language lacks the ability to save files or updates to files on a Web server like a server-side language can.
A client-side language is most useful for tasks that deal with parts of the browser that allow information to be validated before it is sent to a server-side program or script. For instance, JavaScript can open a new window with specific dimensions, specific features (such as a toolbar or status bar), and a specific point of placement on the screen.
JavaScript can also be used to check the information entered into a form before the form is sent to a server-side program to be processed. This information check can prevent strain on the Web server by preventing submissions' with inaccurate or incomplete information. Rather than runl1ing the program on the server until the information is coirect, that data can be sent to the server just once with correct information.