Javascript - Make same function have different inputs

So in Java, you can
do:

void F(int x, int y)
            {
 run(x, y);
}

void F(String a) {

            say(a);
}

but
in Javascript you always have to
do:

function F(x, y) {

            if (typeof x == "string") {
 say(a);
 } else {
 run(x,
            y)

            }
}

I
want to do something like the Java functions in Javascript without checking every single
thing, anyway to do that?