While I was reading the book Javascript: The Good Parts. I can not understand the piece of code bellow:

We can generalize this by making a function that helps us make memoized functions. The memoizer function will take an initial memo array and the fundamental function. It returns a shell function that manages the memo store and that calls the fundamental function as needed. We pass the shell function and the function's parameters to the fundamental function:

var memoizer = function (memo, fundamental) {
    var shell = function (n) {
        var result = memo[n];
        if (typeof result !== 'number') {
            result = fundamental(shell, n);
            memo[n] = result;
        }
        return result;
    };
    return shell;
};

We can now define fibonacci with the memoizer, providing the initial memo array and fundamental function:

var fibonacci = memoizer([0, 1], function (test, n) {
    return test(n - 1) + test(n - 2);
});

My question is what is the test function? When does it get defined and invoked? It seems very confusing to me. Also I think this statement: memo[n] = result; is useless. Please correct if I am wrong.