AngularJS (Angular 1.x) is a JavaScript-based structural framework developed by Google for building dynamic single-page applications (SPAs). It extends HTML's syntax with directives and enables two-way data binding between models and views, allowing the DOM to update automatically when data changes. While AngularJS reached end-of-life on December 31, 2021, it remains widely encountered in legacy codebases. Understanding AngularJS's declarative approach — where you describe what the UI should do rather than how to manipulate the DOM — is crucial for maintaining existing applications and recognizing the patterns that influenced modern frameworks.