SolidJS is a declarative JavaScript library for building user interfaces with fine-grained reactivity. Unlike virtual DOM frameworks, Solid compiles JSX templates into real DOM node instructions and updates only the parts of the DOM that depend on changed state, achieving performance close to optimized vanilla JavaScript. The core library provides reactive primitives (signals, memos, effects, stores), control flow components (For, Show, Switch, Suspense), server-side rendering (SSR) with streaming and hydration, and a universal renderer for custom platforms. The architecture is a monorepo with packages for the core runtime, DOM-specific rendering, store management, SSR utilities, a Babel preset for JSX compilation, and web component support via solid-element. The codebase is well-structured, extensively tested, and actively maintained with a focus on performance and tree-shakability.