I would still stick to python. Or java. Or anything else which has a clear syntax and can run at a useful speed (I'm not mentioning C++ because of the coding overhead and dirty tricks which makes it a bit unfriendly for learning an algorithm)
Implying that JavaScript can't "run at a useful speed" is wrong, using modern implementations. This is especially true for code that runs through lots of repetition as the just-in-time compilers in the JS engines do a remarkable job.
Not to mention that viewing JS as a UI-oriented language seems a bit out of date given the 40k or so packages for Node.js that are in npm.
JavaScript of today is pretty different than JS of 2007, and there are more changes coming with generators, iterators, destructuring, class syntax, arrow functions, promises, etc.
I'm actually adding multi-threading to classifier training in node-natural as we speak [0] so it's something I'm recently familiar with. Multi-threading in JS isn't new or particularly exciting (even less so is multithreading in ML/NLP applications) but the marriage of the two has led to a few interesting problems in JS's asynchronous/event based view of the world!
[0]: https://github.com/NaturalNode/natural/issues/124
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Edited for clarity