# Finding the index of the first element that violates the series (first non-consecutive number) in JavaScript

We have to write a function that takes in an array and returns the index of the first nonconsecutive number from it.

Like all the numbers will be in an arithmetic progression of common difference 1. But the number, which violates this rule, we have to return its index. If all the numbers are in perfect order, we should return -1.

## Example

Let’s write the code for this function −

const arr = [1,2,3,4,5,6,8,9,10];
const secondArr = [3,4,5,6,7,8,9,10,11,12,13,14,15];
const findException = (arr) => {
for(let i = 0; i < arr.length-1; i++){
if(arr[i+1] - arr[i] !== 1){
return i+1;
};
};
return -1;
};
console.log(findException(arr));
console.log(findException(secondArr));

## Output

The output in the console −

6
-1