Is there a formal (ideally BNF) typescript js language grammar (or only typescript subset)?

The formal grammar is in Appendix A of the TypeScript Language Reference, version 1.8 if which is found here or here (for current version)

Since typescript is a strict superset of Ecmascript 5 (http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf/), you could exend any ecmascript bnf with syntax definitions from the Typescript spec.

A starting point could be: http://www.antlr3.org/grammar/1153976512034/ecmascriptA3.g