12-length-of-missing-array.js

/* You get an array of arrays.
   If you sort the arrays by their length, you will see, that their length-values are consecutive.
   But one array is missing!
   You have to write a method, that return the length of the missing array.
   Example:
   [[1, 2], [4, 5, 1, 1], [1], [5, 6, 7, 8, 9]] --> 3
   If the array of arrays is null/nil or empty, the method should return 0.
   When an array in the array is null or empty, the method should return 0 too!
   There will always be a missing element and its length will be always between the given arrays. */

export default (arrs) => {
  if (arrs === null || arrs.includes(null)) {
    return 0;
  }
  const arr = arrs.reduce((acc, n) => acc.concat(n.length), []).sort((a, b) => a - b);
  let result;
  for (let i = 0; i < arr.length - 1; i += 1) {
    if (arr[i] + 1 !== arr[i + 1]) {
      result = arr[i] + 1;
    }
  }
  return !result || arr[0] === 0 ? 0 : result;
};