Math for testing numbers divisible by 13

isDivisibleBy13.js

var isDivisibleBy13 = function(test) {
    if (Math.abs(test) === 13 || test % 13 === 0) {
        return true;
    }
    var lastDigit = +String(number).slice(-1),
        remainingDigits = +String(test).slice(0, -1);

    if (test >= 13 && (remainingDigits - (lastDigit * 9)) % 13 === 0) {
        return isDivisibleBy13(remainingDigits);
    }
    return false;
};