regex format string number with commas and 2 decimals in javascript

Question

I have a string n which is a number. The following adds the commas where needed, but I also want to change the number of decimals. It should also round where appropriate.

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

This turns a number like 1234.567 in to 1,234.567. But I need my number to look like: 1,234.57

I tried taking parts[1] and converting to a Number, then rounding, then concatenating it back; but that is error prone.

How can I get this result by altering my regex? Thanks.

Answer 1

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