Quiz: next fibonacci number

fibonacci.js

var getNextFibonacciAfter = (function() {
    var fibonacciNumbers = [0, 1];

    function nextFibonnacciIn(numbers) {
        return numbers[numbers.length] + numbers[numbers.length - 1];
    }

    return function(number) {
        var nextFibonacci;

        if (number <= 1) {
            throw new Error("Your number must be greater than 1");
        }

        while (fibonacciNumbers[fibonacciNumbers.length-1] <= number) {
            fibonacciNumbers.push(nextFibonnacciIn(fibonacciNumbers));
        }

        nextFibonacci = fibonacciNumbers[fibonacciNumbers.indexOf(number) + 1];

        if (nextFibonacci) {
            return nextFibonacci;
        } else {
            throw new Error(number + " is not a fibonacci number.");
        }
    };
}());