I guess it's still work in progress, but they are probably aiming to achieve something in the direction of the scientific Python stack (i.e. Jupyter, TensorFlow and the like). Maybe starting with a subset of methods useful in ML.

It will be interesting to see how this scientific JS toolchain evolves. Would be fun to get a more feature-complete Propel ML working with Mike Bostock's Observable project:

https://beta.observablehq.com/