[dancsi]

I would guess that it is very unsuitable, since for anything but the most trivial models, even those 'Python' libraries resort to highly-optimized (compiled) C subroutines, especially for the training part. Even then you require a highly nontrivial amount of time to train the network, especially if you are working with medium to large datasets.

Later on, when the speed is not so critical, you can do inference with those pretrained weights and your model implemented in Javascript.