[JS] IBAN check digits calculator

iban_check_digits_calculator.js

/**
 * Calculate IBAN check digits.
 *
 * Inspired by https://github.com/arhs/iban.js/blob/master/iban.js
 *
 * @param string countryCode ISO 3166 2-letter country code, e.g. "FR"
 * @param string bban IBAN without country code and without check digits, e.g. "20041010050500013M02606"
 *
 * @return string e.g. "14"
 */
function calculateCheckDigits(countryCode, bban) {
  
    // Fake IBAN: IBAN with fake check digits, on which real check digits are calculated
    var fakeIban = countryCode + '00' + bban;
    
    // Standardize fake IBAN, according to ISO 13616
    fakeIban = function (iban) {
        iban = iban.toUpperCase();
        iban = iban.substr(4) + iban.substr(0,4);
        return iban
          .split('')
            .map(function(n) {
                var code = n.charCodeAt(0);
                if (code >= 'A'.charCodeAt(0) && code <= 'Z'.charCodeAt(0)){
                    // A = 10, B = 11, ... Z = 35
                    return code - 'A'.charCodeAt(0) + 10;
                } else {
                    return n;
                }
            }).join('');
    }(fakeIban);

    // Calculate check digits
    var checkDigits = function (iban) {
        var remainder = iban;
        var block;
        while (remainder.length > 2){
            block = remainder.slice(0, 9);
            remainder = parseInt(block, 10) % 97 + remainder.slice(block.length);
        }
        return parseInt(remainder, 10) % 97;
    }(fakeIban);
    
    // Format check digits
    var checkDigitsAsAtring = ('0' + (98 - checkDigits)).slice(-2);

    return checkDigitsAsAtring;
    
}