Module 11 integration in javascript/nodejs with 18 integers

M11.js

// M11.js  

/*
* Modules 11 control algorithm it's used
* Used by the danish goverment to validate CPR numbers (social security numbers).
* It works by having a number list and giving them each a weight from 2 to 7 then 
* multiplying them to then divide the number with the mod (11 usually) 
* to see if the reminder is 0.
*
* the modules operator (%) in javascript takes bad values.
* where KEY % 11 = 6 but KEY % 12 = 0 but in python KEY % 11 = 0??
*/

// Array with Modules 11 values
// Hardcoded implementation limits input with the length 18. 
const optarray = [2, 3, 4, 5, 6, 7, 2 ,3, 4, 5 ,6 ,7, 2, 3, 4, 5, 6, 7];

class M11 extends Array {
    constructor(carray = [], mod = 11, ...args) {
        super(args);
        this.pop();

        if (carray.length === optarray.length) {
            carray.forEach(v => this.push(v));
        } else {
            let r = M11.gen();
            
            while (!M11.check(r, mod)) {
                r = M11.gen();
            }

            r.forEach(v => this.push(v));
        }
    }

    static gen () {
        var array = [];

        while (array.length !== optarray.length) {
            array.push(Math.floor(Math.random() * (9 - 1 + 1)) + 1);
        }

        return array;
    }

    static check (carray, mod) {
        let o = [];

        for (var i = 0, len = carray.length; i < len; i++) {
            o.push(carray[i] * optarray[i]);
        }

        // Check if it's dividable with mod (11)
        var sum = o.reduce((a, b) => a + b, 0) / mod;

        if(Number(sum) === sum && sum % 1 === 0){
            return false;
        }

        return true;
    }

    // Either takes Number[] and converts it to a String
    // Or it takes a String and converts it to Number[]
    static convert (numbers) {
        let output = undefined;

        if (typeof numbers === 'number' && numbers.toString().length <= 18) numbers = numbers.toString();
        if (typeof numbers === 'string' && !isNaN(numbers)) {
            output = [];
            numbers.split("").forEach(v => output.push(Number(v)) );
        } else if (typeof numbers === 'object' && !isNaN(numbers.length) && numbers.length <= 18) {
            output = "";

            for (let i = 0; i < numbers.length; i++) {
                if (isNaN(numbers[i])) return;
                output += numbers[i].toString();
            }
        }

        return output;
    }
}

module.exports = M11;

Note apparently only used by the danish goverment