regex format string number with commas and 2 decimals in javascript

Edit: Sept 18, 2019

Since new APIs are available, thought it would be helpful to point out that you can do this much more simply using toLocaleString options:

const numbers = [1, 1000, 2345.67, 21589.334719999995];
const options = { 
  minimumFractionDigits: 2,
  maximumFractionDigits: 2 
};
numbers.forEach(num => {
  const formatted = Number(num).toLocaleString('en', options);
  console.log(formatted);
});

original answer

To add the commas, you could use:

n = parseFloat(n).toFixed(2)
var withCommas = Number(n).toLocaleString('en');

Here is a fiddle

var val = Math.round(Number(n) *100) / 100;
var parts = val.toString().split(".");
var num = parts[0].replace(/\B(?=(\d{3})+(?!\d))/g, ",") + (parts[1] ? "." + parts[1] : "");