Calculate/verify check digit (luhn) of credit cards
// Is the card number valid? var card = '6123451234567893'; luhn(true, pan); // true // Calculate check digit // MII + IIN + "Individual account number" var incomplete = '612345123456789'; luhn(incomplete); // "3"
Implemented according to ISO/IEC 7812-1:2015(E) which goes:
Double the value of alternate digits beginning with the first right-hand digit (low order).
Add the individual digits comprising the products obtained in Step 1 to each of the unaffected digits in the original number.
Subtract the total obtained in Step 2 from the next higher number ending in 0 [this is the equivalent of calculating the “tens complement” of the low-order digit (unit digit) of the total]. If the total obtained in Step 2 is a number ending in zero (30, 40, etc.), the check digit is 0.