JavaScript

JavaScript is a scripting language use to enhance or modify the presentation layer of a page by interacting with HTML source code, typically at the client-side rather than service-side level.

JavaScript can be turned off, so it is important that it is 'de-gradable' so design will still work on some level with JavaScript turned off.

Because JavaScript code can run locally in a user's browser (rather
than on a remote server) it can respond to user actions quickly, making
an application feel more responsive. Furthermore, JavaScript code can
detect user actions which HTML alone cannot, such as individual
keystrokes.