Plus One

Problem statement

You are given a large integer represented as an integer array digits, where each digits[i] is the ith digit of the integer. The digits are ordered from most significant to least significant in left-to-right order. The large integer does not contain any leading 0's.

Increment the large integer by one and return the resulting array of digits.

Example 1:

Input: digits = [1,2,3]Output: [1,2,4]Explanation: The array represents the integer 123.Incrementing by one gives 123 + 1 = 124.Thus, the result should be [1,2,4].

Example 2:

Input: digits = [4,3,2,1]Output: [4,3,2,2]Explanation: The array represents the integer 4321.Incrementing by one gives 4321 + 1 = 4322.Thus, the result should be [4,3,2,2].

Example 3:

Input: digits = [9]Output: [1,0]Explanation: The array represents the integer 9.Incrementing by one gives 9 + 1 = 10.Thus, the result should be [1,0].

Constraints:

• 1 <= digits.length <= 100
• 0 <= digits[i] <= 9
• digits does not contain any leading 0's.

My solution

/**
 * @param {number[]} digits
 * @return {number[]}
 */
var plusOne = function(digits) {
    let carry;
    function addToIndex(index) {
        const act_sum = digits[index] + 1;
        let sum = Math.floor(act_sum % 10);
        // console.log(`sum ${sum}`);
        carry = Math.floor(act_sum / 10);
        // console.log(`carry ${carry}`);
        
        digits[index] = sum;
        if (digits.length === 1 && carry === 1) {
            digits.unshift(carry)
        } else if (carry === 1 && digits[index - 1]) {
            return addToIndex(index - 1)
        } else if (carry === 1) {
            digits.unshift(carry)
        }
        
        return digits;
    }
    
    return addToIndex(digits.length - 1)
};