how to restrict javascript function to only accept one argument

I've created a simple javascript function that determines the nth fibonacci number. It works, but I am trying to figure out why it still works when it is passed two arguments.

function fib(n){
    if (n <= 1){
        return n; 
    }
    else if(n > 1){
        return fib(n-1) + fib(n-2); 

    }
}

So when I run fib(10) I get the correct output of 55. But if I were to do something like

fib(1,2) 

I would expect to get Undefined, but the method is outputting 1.

My question is how can I keep a javascript method to only accept a certain number of arguments? In my case I only want fib to accept one argument.