"15000000".split("").reverse().join("").replace(/(\d{3})/g, "$1,").split("").reverse().join("");

"115000000".split("").reverse().join("").replace(/(\d{3})(?=[^$])/g, "$1,").split("").reverse().join("");

var str = '123123211312.333123'.replace(/(?=(?!^)(?:\d{3})+(?:\.|$))(\d{3}(\.\d+$)?)/g,',$1');
console.log(str)

'1500000000000'.replace(/\B(?=(\d{3})+$)/g,',')

function formatNumber(value) {
    value = value.toString();
    if (value.length <= 3) {
        return value;
    } else {
        return formatNumber(value.substr(0, value.length - 3)) + ',' + value.substr(value.length - 3);
    }
}

15000000..toLocaleString();