Generate check digit for UPC-A

checkDigit_UPC-A.js

function checkDigit(value) {
    value = value.toString();
    
    var checkDigit = 0,
        evens = 0,
        odds = 0,
        checkDigit = 0;

    for (var i = 0; i < value.length; i++) {
        // For zero-based arrays, odd digits are even indexes
        if (i % 2) {
            evens += parseInt(value[i], 10);
        } else {
            odds += parseInt(value[i], 10);
        }
    }

    // Multiply odds by 3, add to evens.
    // Subtract the modulo of that sum from 10
    checkDigit = 10 - (((odds * 3) + evens) % 10);

    // If checksum is 10, just use zero
    checkDigit = (checkDigit == 10) ? 0 : checkDigit;

    return checkDigit
}