UPC check digit function that validates the check digit on a UPC

check-UPC-digit.js

function reverse(string) {
    return string
        .split('')
        .reverse()
        .join('');
}

// function to calculate EAN / UPC checkdigit
function eanCheckDigit(s) {
    var result = 0;
    var rs     = reverse(s);
    for (var counter = 0; counter < rs.length; counter++) {
        result = result + parseInt(rs.charAt(counter)) * Math.pow(3, ((counter + 1) % 2));
    }
    return (10 - (result % 10)) % 10;
}

function checksum(s) {
    return s ? eanCheckDigit(s.slice(0, -1)) === Number(s.slice(-1)) : false;
}