codility cache fibonacci to improve performance

fibonacci-cached.js

let cache = {};

var yourself = {
    fibonacci : function(n) {
        if (n === 0) {
            return 0;
        } else if (n === 1) {
            return 1;
        } else {
            let r = cache[n] = cache[n] ? cache[n] : this.fibonacci(n - 1) +
                this.fibonacci(n - 2);
            return r;
        }
    }
}