how could i run a javascript means with what extension i have to save it

.js is the extension of java script file (if you want to place the script code in a seperate file) and

bellow is the place where you want to place your script code with in a web page

<script type="text/javascript">

     // your script code 

    </script>
    (or)

    <script type="text/javascript" src="path_to_external_javascript_file.js">

     // your script code 

    </script>

this section of code will load automaticaly into browser based on the place where you put (what i mean is you can place this section at header part of the page or at body part ... etc)

please go thrugh the followlink which gives brief description about java script and its usage as well as examples

Click Here

[Click Here](http://www.tutorialspoint.com)

.js extention is used to save JavaScript file. Create a directory named JavaScript and save here all .js extention file (JavaScript Files) which will be beneficial to you to manage multiple files to import (or include) from one place whenever necessary throughout your project.

For more detail information about - What is JavaScript and best resources to learn it CLICK HERE.