What is angular.js?

AngularJS is a JavaScript framework for building interactive single-page web applications. The idea behind it was to extend standard HTML with new capabilities so that developers could describe dynamic user interfaces declaratively, meaning they could write HTML that directly expressed what the application should do rather than writing a lot of imperative code to manually update the page when data changed. The framework's most prominent feature was two-way data binding: when data in the application changes, the displayed HTML updates automatically, and when the user changes something in the interface, the underlying data updates in turn, without the developer writing explicit synchronization code. AngularJS introduced concepts like directives, which let you invent new HTML tags or attributes that encapsulate reusable behaviors, controllers, which attached logic and data to sections of the page, and dependency injection, a pattern that made it easier to write testable code by letting the framework supply components with their required dependencies automatically. The framework organized applications according to a pattern called MVC (Model-View-Controller) or a variant of it, separating data, display, and logic into distinct layers. It was built on plain JavaScript and ran directly in browsers without any build step, which was a practical advantage when it launched. The README notes that AngularJS officially reached end of life in January 2022 and is no longer supported. For new projects, the actively maintained successor is simply called Angular (at angular.io) and is a substantially different, TypeScript-based framework. You would encounter AngularJS today only when maintaining older web applications built during its period of popularity in the early-to-mid 2010s.