Intl.NumberFormat.prototype.format()
 The Intl.NumberFormat.prototype.format() method formats a number according to the locale and formatting options of this Intl.NumberFormat object. 
Syntax
format(number)
Parameters
Description
 The format getter function formats a number into a string according to the locale and formatting options of this Intl.NumberFormat object. 
Examples
Using format
 Use the format getter function for formatting a single currency value, here for Russia: 
var options = { style: 'currency', currency: 'RUB' }; var numberFormat = new Intl.NumberFormat('ru-RU', options); console.log(numberFormat.format(654321.987)); // → "654 321,99 руб."
Using format with map
 Use the format getter function for formatting all numbers in an array. Note that the function is bound to the Intl.NumberFormat from which it was obtained, so it can be passed directly to Array.prototype.map. This is considered a historical artefact, as part of a convention which is no longer followed for new features, but is preserved to maintain compatibility with existing programs. 
var a = [123456.789, 987654.321, 456789.123]; var numberFormat = new Intl.NumberFormat('es-ES'); var formatted = a.map(n => numberFormat.format(n)); console.log(formatted.join('; ')); // → "123.456,789; 987.654,321; 456.789,123"
Specifications
Browser compatibility
| Desktop | Mobile | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | WebView Android | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | |
| format | 24 | 12 Before Edge 18, numbers are rounded to 15 decimal digits. For example,  new Intl.NumberFormat('en-US').format(1000000000000005)returns"1,000,000,000,000,010". | 29 | 11 In Internet Explorer 11, numbers are rounded to 15 decimal digits. For example,  new Intl.NumberFormat('en-US').format(1000000000000005)returns"1,000,000,000,000,010". | 15 | 10 | 4.4 | 25 | 56 | 14 | 10 | 1.5 | 
See also
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    https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/NumberFormat/format