Codility

fibonacci.js

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