jQuery
jQuery is a fast, small, and feature-rich JavaScript library that simplifies HTML document traversal and manipulation, event handling, animation, and Ajax interactions for rapid web development. It provides an easy-to-use API that works across a multitude of browsers, abstracting away many of the complexities of raw JavaScript. While its dominance has waned with the rise of modern frameworks, it remains widely used in legacy systems and for quick prototyping.
Developers should learn jQuery when working on legacy web projects, maintaining older codebases, or needing a lightweight solution for DOM manipulation and Ajax without the overhead of a full framework. It's particularly useful for tasks like adding interactivity to static pages, handling cross-browser compatibility issues, or quickly building simple web applications where modern frameworks like React or Vue might be overkill. Knowledge of jQuery is valuable for understanding historical web development patterns and easing into JavaScript concepts.