forgot to memoize false values

cansum.js

// t/f
// use number from numbers as many times as needed to construct targetSum
const canSum = (targetSum, numbers, memo = new Map()) => {
  const key = `${targetSum}`;
  if (targetSum === 0) return true;
  if (memo.has(key)) return memo.get(key);
  for (let currentNum of numbers) {
    const after = targetSum - currentNum;
    if (after < 0) continue;

    const can = canSum(after, numbers, memo);
    memo.set(key, can);
    if (can) return true;
  }
  memo.set(targetSum, false);
  return memo.get(targetSum);
};
console.log(canSum(7, [3, 4, 5, 7]));
console.log(canSum(7, [2, 4]));
console.log(canSum(7, [10,20,3,14,5,60,70, 140, 30, 40, 50])); //false
// canSum(7, [3, 4, 5, 7]) -> true, as 3 + 4 = 7 or 7 = 7
// canSum(7, [2, 4]) -> false