The resolution of the screen cast was not high enough for me to read the text he was typing.

As for usefulness, "my favorite language -> Javascript" compilers are cute, but I am skeptical of adding another leaky abstraction layer. Javascript is a pretty feature-ful, modern language with closures, first class function, collection literals, and some pretty good APIs. Not sure if bridging to other languages will give much of a productivity boost over just learning Javascript well, and you will probably need to learn Javascript well, anyways, whenever the translation from your favorite language to Javascript inevitably leaks.