Given an array of numbers, add one to it. Take care of carry overs

arrayaddone.js

/*
  [1,2,3,4] => [1,2,3,5]
  [1,2,9] => [1,3,0]
  [1,2,9,9] => [1,3,0,0]
  [9,9,9] => [1,0,0,0]
*/
function addOne(arr) {
	let sum = 0, carry = 1;
	for(let i = (arr.length - 1); i >= 0; i--) {
		sum = arr[i] + carry;
		arr[i] = sum%10;

		if(sum === 10) {
			carry = 1;
		} else {
			carry = 0;
		}
	}
	//at the end of the loop if carry is one
	if(carry === 1) {
		arr.unshift(carry); //for cases like [9,9,9]
	}
	
	return arr;
}