Intl.NumberFormat

JavaScript has a number formatter (part of the Internationalization API).

// Create our number formatter.
const formatter = new Intl.NumberFormat('en-US', {
  style: 'currency',
  currency: 'USD',

  // These options can be used to round to whole numbers.
  trailingZeroDisplay: 'stripIfInteger'   // This is probably what most people
                                          // want. It will only stop printing
                                          // the fraction when the input
                                          // amount is a round number (int)
                                          // already. If that's not what you
                                          // need, have a look at the options
                                          // below.
  //minimumFractionDigits: 0, // This suffices for whole numbers, but will
                              // print 2500.10 as $2,500.1
  //maximumFractionDigits: 0, // Causes 2500.99 to be printed as $2,501
});

// Use the formatter with the value of an input.
let input = document.getElementById('amount');
input.addEventListener('keyup', e => {
    document.getElementById('result').innerText = formatter.format(e.target.value);
});
input.dispatchEvent(new Event('keyup'));

<label>
    Amount
    <input id="amount" value="2500">
</label>
Result:
<span id="result"></span>

Use undefined in place of the first argument ('en-US' in the example) to use the system locale (the user locale in case the code is running in a browser). Further explanation of the locale code.

Here's a list of the currency codes.

Intl.NumberFormat vs Number.prototype.toLocaleString

A final note comparing this to the older .toLocaleString. They both offer essentially the same functionality. However, toLocaleString in its older incarnations (pre-Intl) does not actually support locales: it uses the system locale. So when debugging old browsers, be sure that you're using the correct version (MDN suggests to check for the existence of Intl). There isn't any need to worry about this at all if you don't care about old browsers or just use the shim.

Also, the performance of both is the same for a single item, but if you have a lot of numbers to format, using Intl.NumberFormat is ~70 times faster. Therefore, it's usually best to use Intl.NumberFormat and instantiate only once per page load. Anyway, here's the equivalent usage of toLocaleString:

console.log((2500).toLocaleString('en-US', {
  style: 'currency',
  currency: 'USD',
})); /* $2,500.00 */

Some notes on browser support and Node.js

• Browser support is no longer an issue nowadays with 99+% support globally
• There is a shim to support it on fossilized browsers (like Internet Explorer 8), should you really need to
• Node.js before v13 only supports en-US out of the box. One solution is to install full-icu, see here for more information
• Have a look at CanIUse for more information

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/toLocaleString#using_options

var number = 1000;

// request a currency format
console.log(number.toLocaleString('en-US', { style: 'currency', currency: 'USD' }));

Without methods and just formatting with commas

        function addComma(value) {
            return value.split('').map((v, i) => {
                if ( i === value.length - 2) {
                    return ',' + v
                }
                return v
            }).join('')
        }
        console.log(addComma('1000'))
        console.log(addComma('10000'))
        console.log(addComma('100000'))

Integers.

There is no need to use floating-point for currency. Use fixed-point, where the number of decimal points is 0.

You count in pennies (or possibly in tenths of pennies).