SOSML [0] came out recently, written in TypeScript and sporting an online interpreter. Then there's this prominent SML-er's JavaScript backend for Moscow ML [1]. And Andreas Rossberg's HaMLet [2] also has a JavaScript backend.

[0] https://github.com/SOSML/SOSML

[1] https://github.com/kfl/mosml-js

[2] https://people.mpi-sws.org/~rossberg/hamlet/