Higher-Order Functions — Series 3

function memoize(fn) {
  const cache = new Map();
  return (...args) => {
    const key = args.join(',');
    if (cache.has(key)) return cache.get(key);
    const value = fn(...args);
    cache.set(key, value);
    return value;
  };
}

let calls = 0;
const countParts = memoize((...parts) => {
  calls++;
  return parts.length;
});

console.log(countParts('a,b'), countParts('a', 'b'), calls);

function once(fn) {
  let called = false;
  let result;
  return (...args) => {
    if (!called) {
      called = true;
      result = fn(...args);
    }
    return result;
  };
}

let n = 0;
const init = once((x) => {
  n++;
  return x * 2;
});

console.log(init(5), init(10), n);

const compose = (...fns) => (x) => fns.reduceRight((acc, f) => f(acc), x);

const inc = (n) => n + 1;
const double = (n) => n * 2;

console.log(compose(inc, double)(5), compose(double, inc)(5));