how to use locale from window with datefns format in javascript

First, make sure to install the necessary locale package for date-fns. For example, to use the French locale, install date-fns/locale/fr:

index.tsx
npm install date-fns locale-fr
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Then, import the locale and use it in the format function:

import { format } from 'date-fns'
import { fr } from 'date-fns/locale'

const date = new Date()
const formattedDate = format(date, 'LLL', { locale: fr })
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In this example, the LLL format string corresponds to the "Month abbreviation, day and year" format. The locale option is set to the imported fr locale, which will format the month name in French.

If you want to use the locale from the window (i.e. the user's browser language), you can get the language code and pass it to the corresponding date-fns/locale module:

import { format } from 'date-fns'
import { enUS, fr } from 'date-fns/locale'

const date = new Date()
const locale = window.navigator.userLanguage || window.navigator.language
const formattedDate = format(date, 'LLL', { locale: locale === 'fr' ? fr : enUS })
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In this example, the user's browser language is retrieved from window.navigator, and the corresponding date-fns/locale module is used to format the date. If the user's language is not supported, the default English locale is used.