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How about this solution, which – if I'm not mistaken – should fulfill all requirements:

• create a second array
• run through all elements of the input array
• for each number set the respective key in the second array to true
• run through the second array and return the first key which value comes back as undefined
• if no match is found, return 1, so it will work for an empty input array as well

function findNumber(values) {
  let result = [];

  for (let i = 0; i < values.length; ++i) {
    if (0 <= values[i]) {
      result[values[i]] = true;
    }
  }

  for (let i = 1; i <= result.length; ++i) {
    if (undefined === result[i]) {
      return i;
    }
  }

  return 1
}

Try it yourself

^{Patrick and I had a discussion about the real time performance of our solutions (Here's Patrick's elegant solution using Set). We set up a test, containing around 1000 elements in the input array, including lots of negative values. You can try the test yourself.}