React is an open-source JavaScript library, created by Facebook (Meta) in 2013, focused on building user interfaces via a 'view' layer. Unlike full frameworks, it solves early-2010s web complexity issues like inefficient DOM updates and state chaos by introducing a virtual DOM for minimal, performant changes. The core mental model is declarative: you describe the desired UI state, and React handles the updates.

This is achieved through reusable components—modular 'Lego bricks'—and JSX syntax, which blends HTML with JavaScript. As of 2026, powering giants like Netflix and Airbnb, React 18's concurrent rendering and evolving server components maintain its dominance, offering a flexible, unopinionated ecosystem ideal for scalable web and mobile apps.