1function numberWithCommas(x) {
2  return x.toString().replace(/\B(?=(\d{3})+(?!\d))/g, ',');
3}

1function numberWithCommas(x) {
2    return x.toString().replace(/\B(?=(\d{3})+(?!\d))/g, ",");
3}

1//ES6 Way
2const numberWithCommas = (x) => {
3  return x.toString().replace(/\B(?=(\d{3})+(?!\d))/g, ',');
4}

1x.toString().toLocaleString(); // x must be a number