code: function checksum$3(number) {
  	var result = 0;
  
  	var i;
  	for (i = 1; i < 11; i += 2) {
  		result += parseInt(number[i]);
  	}
  	for (i = 0; i < 11; i += 2) {
  		result += parseInt(number[i]) * 3;
  	}
  
  	return (10 - result % 10) % 10;
  }
docs: Calulate the checksum digit https://en.wikipedia.org/wiki/International_Article_Number_(EAN)#Calculation_of_checksum_digit