Vergi Kimlik Numarası (VKN) doğrulama.

verifyVkn.js

// Examples:
// Valid: "3973535717", "2037637860", "2823097943", "2012460224"
// Not Valid: "3973535711", "123", "9999999999", 

function verifyVkn(vkn) {
  if (typeof vkn !== 'string') throw new TypeError('vkn should be a string');
  if (vkn.length !== 10) throw new TypeError('invalid length');

  const digits = vkn.split('');
  const control = digits.pop(); // eslint-disable-line

  const r = [];

  digits.forEach((el, i) => {
    const x = Number(el) + 10 - (i + 1);
    const mod = x % 10;
    if (mod === 9) {
      r.push(mod);
    } else {
      const pow = 2 ** (10 - (i + 1));
      r.push((mod * pow) % 9);
    }
  });

  const total = r.reduce((acc, cur) => acc + cur);
  const c = (10 - ((total % 10) % 10)) % 10;

  return `${digits.join('')}${c}` === vkn;
}