FingerprintJS is a client-side browser fingerprinting library written in TypeScript. It collects over 30 entropy signals (canvas, audio, fonts, WebGL, etc.) from the browser environment and hashes them into a stable visitor identifier. The library operates in two phases: a load phase where entropy sources are prepared asynchronously, and a get phase where values are collected and combined. It includes stabilization logic for browsers that apply anti-fingerprinting measures (e.g., Safari 17, Firefox 120+) and provides a confidence score for the identifier. The architecture is modular: each entropy source is a separate function with a consistent interface, the agent orchestrates collection, and the hashing uses a custom MurmurHash3 x64 128-bit implementation. The library has no runtime dependencies and is tree-shakable. It is used for fraud detection and visitor identification without cookies.