Sum of Two Huge Numbers

/* Write a JavaScript function sumTwoHugeNumbers(value) that accepts as parameter an array containing
the two numbers. The input numbers are represented as strings. You are not allowed to use external libraries.
The result should be printed on the console. */

"use strict";

function sumTwoHugeNumbers(arr) {
    if (!arr[0].length || !arr[1].length ||
        arr[0] != Number(arr[0]) || arr[1] != Number(arr[1])) {
        return undefined + ' - invalid input!';
    }

    var firstNumArr = arr[0].split('').filter(Boolean);
    var secondNumArr = arr[1].split('').filter(Boolean);

    if (firstNumArr.length != secondNumArr.length) {
        var numAddingZeroes = Math.abs(firstNumArr.length - secondNumArr.length);
        if (firstNumArr.length > secondNumArr.length) {
            do {
                secondNumArr.unshift('0');
                numAddingZeroes--;
            } while (numAddingZeroes);
        } else {
            do {
                firstNumArr.unshift('0');
                numAddingZeroes--;
            } while (numAddingZeroes);
        }
    }

    var tempAdd = 0;
    for (var i = firstNumArr.length - 1; i >= 0; i -= 1) {
        var num1 = parseInt(firstNumArr[i]);
        var num2 = parseInt(secondNumArr[i]);
        var sum = num1 + num2 + tempAdd;
        (sum > 9) ? tempAdd = 1 : tempAdd = 0;
        sum = sum.toString(10);
        if (i) {
            sum = sum[sum.length - 1];
        }
        firstNumArr[i] = sum;
    }

    return firstNumArr.join('');
}

console.log(sumTwoHugeNumbers(['155', '65']));
console.log(sumTwoHugeNumbers(['123456789', '123456789']));
console.log(sumTwoHugeNumbers(['887987345974539','4582796427862587']));
console.log(sumTwoHugeNumbers(['347135713985789531798031509832579382573195807',
        '817651358763158761358796358971685973163314321']));