const digitMod = (value, len) => {
  const mod = len - Math.round(value - Math.floor(value / len) * len);
  return mod >= 10 ? 0 : mod;
};

const mapValuesWithDigitModByKey = (_key, _len) => (value, key) =>
  _key === key ? digitMod(value, _len) : value;

const reduceDigitsByKey = _key => (digits, value, key) => {
  if (key > _key) {
    digits[_key] = digits[_key] + value * (key + (_key === 0 ? 1 : 0));
    digits[key] = value;
  }
  return digits;
};

const genArrayDigits = len => new Array(len).fill(0);

const genRandomArrayDigits = len =>
  genArrayDigits(len).map(() => Math.round(Math.random() * 9));

const reduceArrayDigits = (digits, verifyDigits) =>
  verifyDigits.length
    ? reduceArrayDigits(
        digits
          .reduce(
            reduceDigitsByKey(verifyDigits.length - 1),
            genArrayDigits(verifyDigits.length)
          )
          .map(mapValuesWithDigitModByKey(verifyDigits.length - 1, digits.length)),
        verifyDigits.slice(1)
      )
    : digits;

const calculateDigits = (digits, verifyDigits) =>
  reduceArrayDigits(digits.concat(verifyDigits).reverse(), verifyDigits).reverse();

const generateCpf = () =>
  calculateDigits(genRandomArrayDigits(9), genArrayDigits(2)).join('');

const validateCpf = cpf =>
  calculateDigits(cpf.split('').slice(0, -2), genArrayDigits(2)).join('') === cpf;

// use:
//document.getElementById('generate').innerText = generateCpf();
//document.getElementById('validate').innerText = validateCpf('75314746700');

// Implements your own verified digits:
//const generateAnotherVerifiedDigits = () =>
//  calculateDigits(genRandomArrayDigits(10), genArrayDigits(3)).join('');
//const validateAnotherVerifiedDigits = digits =>
//  calculateDigits(digits.split('').slice(0, -3), genArrayDigits(3)).join('') === digits;