IBAN validation checksum

Question

How to get a string of an IBAN number. I need to calculate the check digit, but there are too many characters.

This is an example IBAN: BR0012345678901234567890123

Answer 1

IBAN validation is described on the Wikipedia page under "Validating the IBAN". Just follow the algorithm as described.

Follow the code below to see how validation happens step-by-step.

Note: BigInt is required to calculate the modulo.

const validate = (iban) => {
  const
    [, head, tail] = iban.split(/(^\w{4})(\w+)$/),
    rearrange = `${tail}${head}`,
    replace = rearrange
      .split('')
      .map(c =>
        /[a-z]/i.test(c)
          ? c.toLowerCase().charCodeAt(0) - 87
          : parseInt(c, 10)
        )
        .join('') 
  return BigInt(replace) % 97n === 1n;
};

// This IBAN is invalid, since the remainder is not 1
console.log(validate('BR0012345678901234567890123'));

// Valid, via https://www.iban.com/iban-checker example
console.log(validate('GB33BUKB20201555555555'));

This can be shortened to simply:

const validate = (iban) =>
  BigInt([...iban.slice(4), ...iban.slice(0, 4)]
    .map(c => /[a-z]/i.test(c) ? c.toLowerCase().charCodeAt(0) - 87 : c)
    .join('')) % 97n === 1n;

// This IBAN is invalid, since the remainder is not 1
console.log(validate('BR0012345678901234567890123'));

// Valid, via https://www.iban.com/iban-checker example
console.log(validate('GB33BUKB20201555555555'));