Find the first non-consecutive number.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!


// 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. )

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