CountUp.js is a dependency-free, lightweight JavaScript library that animates numerical values by counting to them. It provides a CountUp class that can be initialized with a target element and end value, then animated using requestAnimationFrame with configurable easing, formatting, and duration. The library supports counting in both directions, smart easing for large numbers, auto-animation when elements become visible via IntersectionObserver, and a plugin system for custom rendering animations (like odometer effects). It's implemented in TypeScript with comprehensive tests and produces distributable ES module, UMD, and minified formats.