Returns the smallest positive integer (greater than 0) that does not occur in Array.

Check array.js

//Sample Arrays
const A = [1, 3, 6, 4, 1, 2];
const B = [-1 - 10, 0, 2, 3, 5];
const C = [1, 2, 3, 500];

function solution(test) {
  //Remove duplicates
  let unique = [...new Set(test)];
  //Sort from lowest value
  let sorted = unique.sort();
  //get minimum value
  let min = sorted[0];
  //Get maximum value
  let max = sorted[sorted.length - 1] + 1;
  //Array for comparison
  let x = [];
  //If the minimum value less than 0 returns 1 as a smallest positive integer
  if (min < 0) {
    return 1;
  }
  //Creating an array for comparison to find the missing smaller positive integer
  for (var i = min; i < max; i++) {
    if (i !== 0) {
      x.push(i);
    }
  }
  //Both array is the same,returns last element's value+1
  if (JSON.stringify(x) == JSON.stringify(sorted)) {
    return max;
  }
  //Finds the smallest missing value
  for (var i = min; i < max; i++) {
    if (sorted.indexOf(i) === -1 && i !== 0) {
      return i;
    }
  }
}

solution(C);