You don't need to make use of the argument list in order to be able to make arguments optional in JavaScript. When fewer parameters are passed to a function than the function has arguments defined then the excess arguments will be undefined. It is therefore just a simple matter of testing for undefined arguments where you want them to be optional.
In this example we create an add function that will add two or optionally three numbers together.
add = function(a, b, c) {
var tot = a + b;
if (c !== undefined) tot += c;
return tot;
}