jQuery UI
jQuery UI is a curated set of user interface interactions, effects, widgets, and themes built on top of the jQuery JavaScript library. It provides pre-built components like datepickers, sliders, dialogs, and drag-and-drop functionality to enhance web applications with interactive elements. The library is designed to be cross-browser compatible and easily customizable through CSS themes.
Developers should learn jQuery UI when building interactive web interfaces that require common UI components without reinventing the wheel, especially in legacy projects or when working with jQuery-based codebases. It's particularly useful for rapid prototyping, adding rich interactions to content management systems, or when team familiarity with jQuery outweighs the need for modern frameworks. However, for new projects, modern alternatives like React or Vue.js are often preferred due to better performance and component architecture.