He's not helming javascript in any way or form or even contributing to the standard to my knowledge.
Brendan may not be helming JavaScript anymore. But he was very active during critical standardization period. For example:
2009: https://www.youtube.com/watch?v=eUtsgUrF-ec
2011: https://brendaneich.com/2011/08/my-txjs-talk-twitter-remix/
2012: https://brendaneich.com/2012/10/harmony-of-dreams-come-true/
Also note that Asm.js (2013 precursor to WebAssembly) was developed during his term at Mozilla.