It's mainly the Google one: http://google-styleguide.googlecode.com/svn/trunk/javascriptguide.xml

We have some internal additions to it which are not public, but I think the above covers most of the things (including semicolons). To be honest, for Prometheus/PromDash I haven't even followed this to the letter, as some things are probably less relevant to an internal app vs. a user-facing one.