block5.js

function weakCache(fn) {
  var cache = new WeakMap(); // <-- Behold the Weak!
  return (arg) => {
    if (cache.has(arg)) {
      return cache.get(arg);
    }
    var computed = fn(arg);
    cache.set(arg, computed);
    return computed;
  }
}
cachedCalcArea = weakCache(calcArea);

cachedCalcArea(USSR); // cache miss
cachedCalcArea(USSR); // cache hit