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programm-ingovals
Aug 29, 2025

In next.js I have tried this in instrumentation-client.ts

import '@formatjs/intl-datetimeformat/polyfill'
import '@formatjs/intl-datetimeformat/locale-data/is'
import '@formatjs/intl-datetimeformat/add-all-tz'

But this always failed because of

if (Intl.DateTimeFormat && typeof Intl.DateTimeFormat.__addLocaleData === 'function') {
  Intl.DateTimeFormat.__addLocaleData({

__addLocaleData was never present.

If I do this however it works

delete (Intl as any).DateTimeFormat

require('@formatjs/intl-datetimeformat/polyfill')
require('@formatjs/intl-datetimeformat/add-golden-tz')
require('@formatjs/intl-datetimeformat/locale-data/is')

Something obvious I am doing wrong? I am just trying to add is-IS support for browsers like Chromium.
I´m not sure deleting the default DateTimeFormat object is good because then I have to add all locales I use manually.
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longlho
Aug 29, 2025
Maintainer

yes /polyfill has feature detection where it won't polyfill if the API is already there. You should follow this guide https://formatjs.github.io/docs/polyfills/intl-datetimeformat#dynamic-import--capability-detection
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