JSFiddle - React, Tailwind, and code Playground

JavaScript

var number = 123456789012;
var str = number + "";

var result = str.split('').map(function (a, i) { 
    if ((i - str.length) % 3 === 0 && i !== 0) { 
        return ',' + a;
    } else { 
        return a;
    } 
}).join('');
                        
console.log(result);