JavaScript (JS) is a programming language that adds interactivity to web pages. It's a core technology of the World Wide Web and is used to create:

Image carousels

Click-to-show dropdown menus

Dynamically changing element colors

Interactive maps

Animated graphics

Scrolling video jukeboxes 

JS is a lightweight, dynamic language that supports object-oriented, imperative, and declarative styles. It's based on the Java and C languages, and many structures from those languages apply to JS as well. 

JS is used in many non-browser environments, such as: Node.js, Apache CouchDB, and Adobe Acrobat. 

JS can also be used to: 

Build games with HTML5 and an Application Programming Interface (API) like WebGL 

Build native mobile apps and desktop applications with frameworks like React Native and Electron 

Some disadvantages of JS include:

Lack of security on the client-side

Lack of support for older browsers

Difficult to debug the code