bonus1.js

function kSmallest(A, B, k) {
  // bounds are inclusive so we are subtracting 1
  // we also subtract 1 from k due to 0 based indexing
  return helper(A, B, k - 1, 0, A.length - 1, 0, B.length - 1);
}

function helper(A, B, k, s1, e1, s2, e2) {
  // Base case:
  // If one of the arrays is empty, then it's obviously the kth element of the   
  // other array. 
  if (s1 > e1) { return B[s2 + k] }
  else if (s2 > e2) { return A[s1 + k] }

  const m1 = (s1 + e1) / 2;
  const m2 = (s2 + e2) / 2;
  if ((m1 - s1) + (m2 - s2) < k) {
    // If k is greater than both of the middle values, then we KNOW that k
    // falls into one of the second halves of the arrays
    // We do this check because K COULD be one of the middle values
    // K will be the largest of the two middle values, if it is one
    if (A[m1] < B[m2]) {
      return helper(A, B, k - (m1 - s1 + 1), m1 + 1, e1, s2, e2);
    } else {
      return helper(A, B, k - (m2 - s2 + 1), s1, e1, m2 + 1, e2);
    }
  } else {
    console.log('else', A[m1] < B[m2])
    if (A[m1] < B[m2]) {
      // If the middle value of B is larger than the middle value of A then we can 
      // safely ignore the second half of B
      return helper(A, B, k, s1, e1, s2, m2 - 1);
    } else {
      // Else we can safely ignore the second half of A
      return helper(A, B, k, s1, m1 - 1, s2, e2);
    }
  }
}

// the only modifications I made was removing the types so that it become valid JS
// also line 11 and 12 had bugs (I forgot to add s1 and s2
// other than that this seems to be working (I never actually ran the code before I submitted)
console.log(kSmallest([1, 6, 8], [4, 7, 9], 1));
console.log(kSmallest([1, 6, 8], [4, 7, 9], 2));
console.log(kSmallest([1, 6, 8], [4, 7, 9], 3));
console.log(kSmallest([1, 6, 8], [4, 7, 9], 4));
console.log(kSmallest([1, 6, 8], [4, 7, 9], 5));
console.log(kSmallest([1, 6, 8], [4, 7, 9], 6));