gistfile1.js

function classOf(obj) {
    return Object.prototype.toString.call(obj).slice(8, -1)
}

// sanitize the arguments for the `_foo` function
function foo() {
    var args = [].slice.call(arguments)
      , bar, baz

    // get bar if it's a String
    if (classOf(args[0]) == "String") bar = args.shift()

    // baz must be always defined
    baz = args.shift() 

    // calls the `foo` function with the right arguments it expects
    _foo.call(this, bar, baz);
}

function _foo(bar /* String? */, baz /* Anything (required) */) {
    console.log("bar: ", bar)
    console.log(baz)
}

foo("bar");
// >>> bar: bar
// >>> undefined

foo([1,2,3]);
// >>> bar: undefined
// >>> [1,2,3]