sumExists.js

function existsSum(arr, n, lIndex = 0, hIndex = null, iter = 0) {
  const low = arr[lIndex];
  const high = arr[hIndex || arr.length - 1];
  const predicate = low + high;
  if (iter === arr.length - 1) return false;
  if (predicate === n) {
    return true;
  }
  if (predicate < n) {
    // increment lower index
    return existsSum(arr, n, lIndex + 1, hIndex, iter + 1);
  } else {
    // decrement higher index
    return existsSum(arr, n, lIndex, hIndex - 1, iter + 1);
  }
}