Fibonacci using recursive JavaScript

fibonacci.js

function fib(n,undefined){
    if(fib.cache[n] === undefined)  
        fib.cache[n] = fib(n-1) + fib(n-2);
    return fib.cache[n];
}
fib.cache = [0,1,1];

fib(10);
console.log(fib.cache);