Why? For CoffeeScript you already need to know and understand how JavaScript work.
And for a experienced JS programmer, it will be very easy to code in CoffeeScript.
Yeah this point needs to be stressed. CS is just javascript. You're not suppose to use CS unless you know JS well. Enough said.
People say this all the time but it's not true. There are a bunch of things in JS that just don't work (are compile errors) in CS. Like conditional expressions.