Sum of Large Number Strings

sumstrings.js

//Source: https://codereview.stackexchange.com/questions/92966/multiplying-and-adding-big-numbers-represented-with-strings
function add(a, b) {
    if ((a | 0) == 0 && (b | 0) == 0) {
        return '0';
    }

    a = a.split('').reverse();
    b = b.split('').reverse();
    var result = [];

    for (var i = 0; (a[i] >= 0) || (b[i] >= 0); i++) {
        var sum = (parseInt(a[i]) || 0) + (parseInt(b[i]) || 0);

        if (!result[i]) {
            result[i] = 0;
        }

        var next = ((result[i] + sum) / 10) | 0;
        result[i] = (result[i] + sum) % 10;

        if (next) {
            result[i + 1] = next;
        }
    }

    return result.reverse().join('');
}