findTheFirstNonConsecutiveNumber.js

/* 

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!

*/

function firstNonConsecutive(arr) {
  for (let i = 0; i < arr.length - 1; i++) {
    if (arr[i] + 1 !== arr[i + 1]) {
      return arr[i + 1];
    }
  }
  return null;
}