Ratchet-JS is a Javascript implementation of the Signal double-ratchet in Javascript using TweetNaCl-JS (a Javascript port of TweetNaCl) as its cryptographic primitives.
While TweetNaCl-JS has been audited (and passed with flying colors) the rest of the Ratchet-JS code has NOT been audited. I am actively soliciting code reviews. Until the code has been reviewed you SHOULD NOT USE IT for anything mission-critical.
Ratchet-JS is based on an earlier Common-Lisp implementation which also has not yet been reviewed.