Find two prices that sum to a coupon target

 1function twoSum(prices, target) {
 2  const seen = new Map();
 3  for (let i = 0; i < prices.length; i++) {
 4    const need = target - prices[i];
 5    if (seen.has(need)) return [seen.get(need), i];
 6    seen.set(prices[i], i);
 7  }
 8  return null;
 9}