Q.

Your task is to find the first element of an array that is not consecutive.
By not consecutive we mean not exactly 1 larger than the previous element of the array.
E.g. If we have an array [1,2,3,4,6,7,8] then 1 then 2 then 3 then 4 are all consecutive but 6 is not, so that's the first non-consecutive number.
If the whole array is consecutive then return null2.
The array will always have at least 2 elements1 and all elements will be numbers. The numbers will also all be unique and in ascending order. The numbers could be positive or negative and the first non-consecutive could be either too!
If you like this Kata, maybe try this one next: https://www.codewars.com/kata/represent-array-of-numbers-as-ranges
1 Can you write a solution that will return null2 for both [] and [ x ] though? (This is an empty array and one with a single number and is not tested for, but you can write your own example test. )
2
Swift, Ruby and Crystal: nil
Haskell: Nothing
Python, Rust, Scala: None
Julia: nothing
Nim: none(int) (See options)

A)

function firstNonConsecutive (arr) {
  for (let i = 1; i < arr.length; i++) {
    if (arr[i] - 1 !== arr[i - 1])
      return arr[i];
  }
  return null;
}