foo (a) ->
	if x? then y
# generates
# foo(function(a) {
#  return (typeof x !== "undefined" && x !== null) ? y : undefined;
# });

# However, if you are checking for the existence of a param to the function, then no  undefined check
foo (x) ->
	if x? then y

# foo(function(x) {
#  return (x != null) ? y : undefined;
# });