Intl.NumberFormat

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

// Create our number formatter.var formatter = new Intl.NumberFormat('en-US', {  style: 'currency',  currency: 'USD',  // These options are needed to round to whole numbers if that's what you want.  //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)});formatter.format(2500); /* $2,500.00 */

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:

(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 98% support globally, 99% in the US and 99+% in the EU
• 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

Number.prototype.toFixed

This solution is compatible with every single major browser:

  const profits = 2489.8237;  profits.toFixed(3) // Returns 2489.824 (rounds up)  profits.toFixed(2) // Returns 2489.82  profits.toFixed(7) // Returns 2489.8237000 (pads the decimals)

All you need is to add the currency symbol (e.g. "$" + profits.toFixed(2)) and you will have your amount in dollars.

Custom function

If you require the use of , between each digit, you can use this function:

function formatMoney(number, decPlaces, decSep, thouSep) {    decPlaces = isNaN(decPlaces = Math.abs(decPlaces)) ? 2 : decPlaces,    decSep = typeof decSep === "undefined" ? "." : decSep;    thouSep = typeof thouSep === "undefined" ? "," : thouSep;    var sign = number < 0 ? "-" : "";    var i = String(parseInt(number = Math.abs(Number(number) || 0).toFixed(decPlaces)));    var j = (j = i.length) > 3 ? j % 3 : 0;    return sign +        (j ? i.substr(0, j) + thouSep : "") +        i.substr(j).replace(/(\decSep{3})(?=\decSep)/g, "$1" + thouSep) +        (decPlaces ? decSep + Math.abs(number - i).toFixed(decPlaces).slice(2) : "");}document.getElementById("b").addEventListener("click", event => {  document.getElementById("x").innerText = "Result was: " + formatMoney(document.getElementById("d").value);});

<label>Insert your amount: <input id="d" type="text" placeholder="Cash amount" /></label><br /><button id="b">Get Output</button><p id="x">(press button to get output)</p>