missingInteger

const missingInteger = (A) => {
  // remove dups and filter by integers
  A = Array.from(new Set(A)).filter((i) => i > 0);
  if (!A.length) return 1;

  const max = Math.max.apply(Math, A);
  const items = new Array(max);

  A.forEach((x) => (items[x - 1] = x));

  let idx = 0;
  while (items[idx]) {
    idx += 1;
  }
  return idx + 1;
};

test.each`
  input                 | expected
  ${[1, 3, 6, 4, 1, 2]} | ${5}
  ${[1, 2, 3]}          | ${4}
  ${[-1, -3]}           | ${1}
`(`should return $expected when inputs $input`, ({ input, expected }) => {
  expect(missingInteger2(input)).toEqual(expected);
});