JSFiddle - React, Tailwind, and code Playground

HTML

<div id="result"></div>

JavaScript

$("#result").append(kensa_suji("123456789012"));

function kensa_suji(kiso_bango) {
    var sum, n, p, q;
    if(!kiso_bango.match(/^[0-9]{12}$/)) {
        return;
    }
    sum = 0;
    for(n = 1; n <= 12; n++) {
        p = kiso_bango.substr(12 - n, 1);
        q = n % 2 === 0 ? 2 : 1;
        sum += p * q;
    }
    return 9 - sum % 9;
}