block3.js

function cachify(fn) {
  // create a cache, which only accessible inside the closure
  var cache = new Map();
  return arg => {
    if (cache.has(arg)) {
      return cache.get(arg);
    }
    var computed = fn(arg);
    cache.set(arg, computed);
    return computed;
  };
}

cachedCalcArea = cachify(calcArea);

cachedCalcArea(USSR); // goes and computes the area
cachedCalcArea(USSR); // already computed, returns the cached area