Other commenters (or their links) haven't mentioned Theano, which is excellent for deep learning, so I'll contribute that to the discussion.
I am no expert in neither Python nor JS. But, the AI community really seems to be obsessed with Python and moreover many specific libraries [1] are available in Python that concentrates on AI alone.
On the contrary, AI in JS is mainly being used in games and like.
[1] - https://wiki.python.org/moin/PythonForArtificialIntelligence
1. Numpy
2. Scipy
3. Pandas
4. Orange
5. Gensim
6. Scikit-Learn
7. NLTK
What does JavaScript have that even comes close?