Files with the .JS extension are text files containing source code for scripts written in the JavaScript programming language, and are used for executing instructions on webpages ran by servers.

The functionality enabled by this script language encompasses, inter alia, opening new windows, verifying information in formula boxes, as well as creating unfolding menus and interactive photo galleries. JS files are made to allow the use of the same code or its core by multiple parts of a website that supports JavaScript; they are imported in the HTML code of the website.