memoize.js

// Return a memoized version of f.
// It only works if arguments to f all have distinct string representations.
function memoize(f) {
    const cache = new Map();  // Value cache stored in the closure.

    return function(...args) {
        // Create a string version of the arguments to use as a cache key.
        let key = args.length + args.join("+");
        if (cache.has(key)) {
            return cache.get(key);
        } else {
            let result = f.apply(this, args);
            cache.set(key, result);
            return result;
        }
    };
}

// Return the Greatest Common Divisor of two integers using the Euclidian
// algorithm: http://en.wikipedia.org/wiki/Euclidean_algorithm
function gcd(a,b) {  // Type checking for a and b has been omitted
    if (a < b) {           // Ensure that a >= b when we start
        [a, b] = [b, a];   // Destructuring assignment to swap variables
    }
    while(b !== 0) {       // This is Euclid's algorithm for GCD
        [a, b] = [b, a%b];
    }
    return a;
}

const gcdmemo = memoize(gcd);
gcdmemo(85, 187)  // => 17

// Note that when we write a recursive function that we will be memoizing,
// we typically want to recurse to the memoized version, not the original.
const factorial = memoize(function(n) {
    return (n <= 1) ? 1 : n * factorial(n-1);
});
factorial(5)      // => 120: also caches values for 4, 3, 2 and 1.