It's a JavaScript library providing multiple simultaneous, stable and resumable uploads via the HTML5 File API.

The library is designed to introduce fault-tolerance into the upload of large files through HTTP. This is done by splitting each files into small chunks; whenever the upload of a chunk fails, uploading is retried until the procedure completes. This allows uploads to automatically resume uploading after a network connection is lost either locally or to the server. Additionally, it allows for users to pause and resume uploads without loosing state.

Resumable.js relies on the HTML5 File API and the ability to chunks files into smaller pieces. Currently, this means that support is limited to Firefox 4+ and Chrome 11+.