return x.toString().replace(/\B(?=(\d{3})+(?!\d))/g, ","); code example

Example 1: number with commas js

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

Example 2: js format number thousands separator

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