Add createDateTimeFormat to format dates based on locale

app/assets/javascripts/locale/index.js

 import Jed from 'jed';
 import sprintf from './sprintf';
 
-const langAttribute = document.querySelector('html').getAttribute('lang');
-const lang = (langAttribute || 'en').replace(/-/g, '_');
+const languageCode = () => document.querySelector('html').getAttribute('lang') || 'en';
 const locale = new Jed(window.translations || {});
 delete window.translations;
 
@@ -47,9 +46,19 @@ const pgettext = (keyOrContext, key) => {
   return translated[translated.length - 1];
 };
 
-export { lang };
+/**
+  Creates an instance of Intl.DateTimeFormat for the current locale.
+
+  @param formatOptions for available options, please see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DateTimeFormat
+  @returns {Intl.DateTimeFormat}
+*/
+const createDateTimeFormat =
+  formatOptions => Intl.DateTimeFormat(languageCode(), formatOptions);
+
+export { languageCode };
 export { gettext as __ };
 export { ngettext as n__ };
 export { pgettext as s__ };
 export { sprintf };
+export { createDateTimeFormat };
 export default locale;

changelogs/unreleased/winh-translate-contributors-page-dates.yml

+---
+title: Translate date ranges on contributors page
+merge_request: 15846
+author:
+type: changed

doc/development/i18n/externalization.md

@@ -262,6 +262,21 @@ Sometimes you need to add some context to the text that you want to translate
     s__('OpenedNDaysAgo|Opened')
     ```
 
+### Dates / times
+
+- In JavaScript:
+
+```js
+import { createDateTimeFormat } from '.../locale';
+
+const dateFormat = createDateTimeFormat({ year: 'numeric', month: 'long', day: 'numeric' });
+console.log(dateFormat.format(new Date('2063-04-05'))) // April 5, 2063
+```
+
+This makes use of [`Intl.DateTimeFormat`].
+
+[`Intl.DateTimeFormat`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DateTimeFormat
+
 ## Adding a new language
 
 Let's suppose you want to add translations for a new language, let's say French.

spec/javascripts/locale/index_spec.js

+import { createDateTimeFormat, languageCode } from '~/locale';
+
+const setLanguage = (languageCode) => {
+  const htmlElement = document.querySelector('html');
+
+  if (languageCode) {
+    htmlElement.setAttribute('lang', languageCode);
+  } else {
+    htmlElement.removeAttribute('lang');
+  }
+};
+
+describe('locale', () => {
+  afterEach(() => {
+    setLanguage(null);
+  });
+
+  describe('languageCode', () => {
+    it('parses the lang attribute', () => {
+      setLanguage('ja');
+
+      expect(languageCode()).toBe('ja');
+    });
+
+    it('falls back to English', () => {
+      setLanguage(null);
+
+      expect(languageCode()).toBe('en');
+    });
+  });
+
+  describe('createDateTimeFormat', () => {
+    beforeEach(() => {
+      setLanguage('de');
+    });
+
+    it('creates an instance of Intl.DateTimeFormat', () => {
+      const dateFormat = createDateTimeFormat({ year: 'numeric', month: 'long', day: 'numeric' });
+
+      expect(dateFormat.format(new Date(2015, 6, 3))).toBe('3. Juli 2015');
+    });
+  });
+});