Say you have a number like 10, and it represents the price of something.

You want to transform it to $10,00.

If the number has more than 3 digits however it should be displayed differently, for example 1000 should be displayed as $1,000.00

This is in USD, however.

Different countries have different conventions to display values.

JavaScript makes it very easy for us with the ECMAScript Internationalization API, a relatively recent browser API that provides a lot of internationalization features, like dates and time formatting.

It is very well supported:

const formatter = new Intl.NumberFormat('en-US', {
  style: 'currency',
  currency: 'USD',
  minimumFractionDigits: 2
})

formatter.format(1000) // "$1,000.00"
formatter.format(10) // "$10.00"
formatter.format(123233000) // "$123,233,000.00"

The minimumFractionDigits property sets the fraction part to be always at least 2 digits. You can check which other parameters you can use in the NumberFormat MDN page.

This example creates a number formatter for the Euro currency, for the Italian country:

const formatter = new Intl.NumberFormat('it-IT', {
  style: 'currency',
  currency: 'EUR'
})