Swarm.js is a JavaScript library that enables interaction with the Swarm network, allowing users to upload and download files and directories to and from Swarm gateways. The library supports multiple data formats including raw data, directories, and files, and works in both Node.js and browser environments, with functionality for computing SwarmHash values. Built for JavaScript and usable via npm, the project is no longer actively maintained by its original author, who recommends Erebos as an alternative for continued development.