protobuf.js is a JavaScript library for efficient protocol buffer encoding and decoding. It provides a pure JavaScript alternative to native implementations, suitable for browsers and Node.js. Key features include static code generation from .proto files, reflection-based usage, and comprehensive support for protocol buffer features like enums, messages, and extensions. protobuf.js balances usability and performance, making it a valuable tool for data serialization and communication in JavaScript environments. It's widely used in projects that require efficient and structured data handling.