Recursive Fibonacci - Memoization

function Fibber() {
    this.memo = {};
}

Fibber.prototype.fib = function(n) {

    if (n < 0) {
        throw new Error('Index was negative. No such thing as a negative index in a series.');

    // base cases
    } else if (n === 0 || n === 1) {
        return n;
    }

    // see if we've already calculated this
    if (this.memo.hasOwnProperty(n)) {
        console.log('grabbing memo[' + n + ']');
        return this.memo[n];
    }

    console.log('computing fib(' + n + ')');
    var result = this.fib(n - 1) + this.fib(n - 2);

    // memoize
    this.memo[n] = result;

    return result;
}