i18next 26.0.0

v26.0.0

on Node.js NPM

This is a major breaking release:

Breaking Changes

• Remove deprecated initImmediate option — the backward-compatibility mapping from initImmediate to initAsync (introduced in v24) has been removed. Use initAsync instead.
• Remove legacy interpolation.format function — the old monolithic format function (interpolation: { format: (value, format, lng) => ... }) is no longer supported. The built-in Formatter (or a custom Formatter module via .use()) is now always used. Migrate to the new formatting approach using i18next.services.formatter.add() or .addCached() for custom formatters.
• Remove console support notice — the console support notice introduced in v25.8.0 has been removed, along with the showSupportNotice option and all related internal suppression logic (globalThis.__i18next_supportNoticeShown, I18NEXT_NO_SUPPORT_NOTICE env var). See our blog post for the full story.
• Remove simplifyPluralSuffix option — this option was unused by the core PluralResolver (which relies entirely on Intl.PluralRules). It only had an effect in the old v1/v2/v3 compatibility layer. The v4 test compatibility layer now defaults to true internally.
• Remove deprecated @babel/polyfill from devDependencies.

Improvements

• Code modernization across all source files:
  • Replace indexOf() > -1 / indexOf() < 0 with .includes() (~40+ occurrences)
  • Replace indexOf() === 0 with .startsWith() where appropriate
  • Replace var with const, '' + object with String(object), .substring() with .slice()
  • Replace .apply(observer, [event, ...args]) with direct call observer(event, ...args)
  • Remove unnecessary .call(this, ...) in BackendConnector retry logic
  • Fix array-callback-return in LanguageUtils getBestMatchFromCodes
  • Clean up all stale eslint-disable comments from source files
• EventEmitter: add once() method for one-time event subscriptions
• Memory leak fix: move module-level checkedLoadedFor cache to Translator instance, preventing cross-instance state leakage
• TypeScript: fix BackendModule generic parameter naming inconsistency between CJS and ESM type definitions
• TypeScript: add once() method to i18n and ResourceStore type interfaces
• ESLint 9: migrate from ESLint 8 (airbnb-base) to ESLint 9 flat config with neostandard
• Vitest 4: upgrade from vitest 3 to vitest 4, migrate workspace files to test.projects config