Checksum algorythm

i have this luhn-mod n checksum algorythm in javascript code which generates a 15 alphanumeric string and calculates a checksum at the end. i want to apply this formula to my records in a field. can u help me out?

this is the code:

(function(){
var possible = ‘0123456789abcdefghijklmnopqrstuvwxyz’;

function randomString(length) {
    var text = '';

    for( var i=0; i < length; i++) {
        text += possible.charAt(Math.floor(Math.random() * possible.length));
    }

    return text;   
}

function generateCheckCharacter(input) {

    var factor = 2;
    var sum = 0;
    var n = possible.length;

    // Starting from the right and working leftwards is easier since 
    // the initial "factor" will always be "2" 
    for (var i = input.length - 1; i >= 0; i--) {
            var codePoint = possible.indexOf(input.charAt(i));
            var addend = factor * codePoint;

            // Alternate the "factor" that each "codePoint" is multiplied by
            factor = (factor == 2) ? 1 : 2;

            // Sum the digits of the "addend" as expressed in base "n"
            addend = Math.floor(addend / n) + (addend % n);
            sum += addend;
    }

    // Calculate the number that must be added to the "sum" 
    // to make it divisible by "n"
    var remainder = sum % n;
    var checkCodePoint = (n - remainder) % n;

    return possible.charAt(checkCodePoint);
}

function isValid(input) {
    
    var factor = 1;
    var sum = 0;
    var n = possible.length;

    // Starting from the right, work leftwards
    // Now, the initial "factor" will always be "1" 
    // since the last character is the check character
    for (var i = input.length - 1; i >= 0; i--) {
            var codePoint = possible.indexOf(input.charAt(i));
            var addend = factor * codePoint;
        

            // Alternate the "factor" that each "codePoint" is multiplied by
            factor = (factor == 2) ? 1 : 2;

            // Sum the digits of the "addend" as expressed in base "n"
            addend = Math.floor(addend / n) + (addend % n);
            sum += addend;
    }

    var remainder = sum % n;
    
    return (remainder == 0);
}

$('#validate').click(function() {
   $('#isValid').text(isValid($('#result').val())); 
});

$('#generate').click(function() {
    var prefix = randomString(15);
    var check = generateCheckCharacter(prefix);

    $('#result').val(prefix + "-" + check);
    $('#isValid').text(isValid($('#result').val()));
});

})();

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