Four, install a CLI interpreter with a REPL if you don't have one already. `jsc` is standard on Mac OS X but there are a lot to choose from, including `rhino`, `spidermonkey`… Your browser's dev tools has a REPL, too, if you like it more. Either way, a REPL is very handy when you need to try out a few things without messing anything.
Five, you might want to force your code to comply with loosely defined and not even widely accepted code standards. Installing a `jslint` variant on your system may help.