js每隔3位加一個逗號

function format_number(n) {
    var b = parseInt(n).toString();
    var len = b.length;
    if (len <= 3) { return b; }
    var r = len % 3;
    return r > 0 ? b.slice(0, r) + "," + b.slice(r, len).match(/\d{3}/g).join(",") : b.slice(r, len).match(/\d{3}/g).join(",");
}