validateEan.js

const GTIN_SSCC_LENGTH = 16;
const WEIGHT_EVEN = 3;
const WEIGHT_ODD = 1;

function ceil(n, base) {
  // TODO: 0.1, 0.01, 0.001, etc.
  return Math.ceil(n / base) * base;
}

function validateEan(str = '') {
  const digits = str.padStart(GTIN_SSCC_LENGTH, '0').split('');
  const validationCode = parseInt(digits.pop());
  const checksum = digits.reduce((acc, cur, i) => 
    acc + (i % 2 === 0
        ? WEIGHT_EVEN * cur
        : WEIGHT_ODD * cur),
    0
  );
  const nearestChecksum = ceil(checksum, 10);
  return nearestChecksum - checksum === validationCode;
}

console.log(validateEan('1337133713379')); // false
console.log(validateEan('1337133713378')); // true