Adding two numbers #1

Here's my approach:

           
            function sumNum(arr1, arr2) {

                if (arr1.length != arr2.length) {
                    return 'Lengths are not the same';
                }
                let result = []
            
                let addit = 0;
                for (let i = arr1.length - 1; i >= 0; i--) {
                    let sum = 0;
            
                    sum = arr1[i] + arr2[i] + addit;
            
                    if (sum >= 10) {
                        sum = sum % 10;
                        addit = 1;
                    }
                    else {
                        addit = 0;
                    }
            
                    result[i] = sum;
                }
            
                if (addit == 1) {
                    result.unshift(1);
                }
                return result
            }
            
            
            let res = sumNum([5, 8, 9], [1, 2, 1])
            console.log(res);