Finding the first non-consecutive number in an array in JavaScript

Problem

We are required to write a JavaScript function that takes in an array of numbers. Our function should return that first element from the array which is not the natural successor of its previous element.

It means we should return that element which is not +1 its previous element given that there exists at least one such element in the array.

Example

Following is the code −

const arr = [1, 2, 3, 4, 6, 7, 8];
const findFirstNonConsecutive = (arr = []) => {
   for(let i = 0; i < arr.length - 1; i++){
      const el = arr[i];
      const next = arr[i + 1];
      if(next - el !== 1){
         return next;
      };
   };
   return null;
};
console.log(findFirstNonConsecutive(arr));

Output

Following is the console output −

6