From the title and domain I though they either had ported TF to Javascript(!) or we connecting to a server.
As far as I can see the API[3] isn't much like TensorFlow.
[1] https://github.com/tensorflow/playground
[2] http://cs.stanford.edu/people/karpathy/convnetjs/
[3] https://github.com/tensorflow/playground/blob/master/nn.ts