Fibonacci generator

fibonacci.js

(function (window, undefined) {

    var Math = window.Math,
        a = [0, 1];

    if (!Math.fibonacci) {
        Math.fibonacci = function (n) {
            var i;

            if (a[n] === undefined) {

                if (typeof n !== 'number' || n < 0 || Math.floor(n) < n) {
                    throw new RangeError("'" + n + "' is not a valid Fibonacci index.");
                }

                for (i = a.length - 1; i < n; i++) {
                    a.push(a[i] + a[i - 1]);
                }
            }

            return a[n];
        };
    }

}(this));