katakana.js

Copy and paste this into your JavaScript console:

([,ウ,,,,ア]=[]+{},[ネ,ホ,ヌ,セ,,ミ,ハ,ヘ,,,
ナ]=[!!ウ]+!ウ+ウ.ウ)[ア+=ウ+ナ+ヘ+ネ+ホ+ヌ+
ア+ネ+ウ+ホ][ア](ミ+ハ+セ+ホ+ネ+'(-~ウ)')()

The minified source is exactly 99 chars.

Many thanks to @centime, @subzey, @xem, @nitram509 and @drzhbe for their great help!

Check out the annotated.js to see whats going on.