Solid.js is a declarative JavaScript library for creating efficient, fine-grained reactive user interfaces. It uses a JSX syntax, similar to React, but unlike React, it does not use a virtual DOM and instead updates the DOM directly. Solid's fine-grained reactivity system re-renders only the minimal amount of DOM necessary when the state changes. It offers signal primitives to handle state and side effects. Moreover, Solid optimizes at compile-time, converting declarative JSX into highly efficient imperative code, and it's impressively small in size.