i18next is a mature, full-featured internationalization (i18n) library for JavaScript and TypeScript, used in production by thousands of projects across web (browser), Node.js, and Deno environments. It provides a complete translation pipeline: key-based lookup with namespaces, interpolation of variables ({{var}}), nesting of translations ($t(...)), pluralization via Intl.PluralRules (with ordinal support), context variants, and rich formatting (number, currency, datetime, relative time, list) using Intl APIs. The library is backend-agnostic: resources can be loaded synchronously or asynchronously from any source (files, HTTP, databases) via a plugin system, with built-in retry logic and parallel-read throttling. It also includes language detection, fallback chains, post-processing, and an event system for lifecycle hooks. The architecture is modular and event-driven. The core I18n class orchestrates initialization, plugin registration (via .use()), and delegates translation to the Translator. The Translator resolves keys using LanguageUtils (hierarchy/fallback), PluralResolver, and Interpolator, then looks up strings in the in-memory ResourceStore. The BackendConnector manages async loading from backends. The library exposes a selector API (keyFromSelector) that uses a revocable Proxy to collect property paths, enabling type-safe key construction in TypeScript. TypeScript support is extensive: users can augment CustomTypeOptions or ResourceNamespaceMap to get compile-time validation of translation keys, interpolation variables, and return types. Key entry points are the default instance (i18next.t(), i18next.init()) and factory methods (createInstance, getFixedT). The library has zero runtime dependencies; all external integrations (React, filesystem backends, language detectors) are separate packages. The codebase demonstrates high craftsmanship: performance-critical paths are optimized (caching of plural rules, language hierarchies, compiled regexps), security issues (ReDoS, prototype pollution, log injection) have been systematically addressed, and the type system is one of the most sophisticated in the JavaScript i18n ecosystem.

This repository provides the official browser language detection plugin for i18next. It implements a modular strategy pattern where each detector (cookie, querystring, localStorage, sessionStorage, navigator, htmlTag, path, subdomain, hash) is a self-contained module with a consistent interface. The main `Browser` class orchestrates detection by iterating through a configurable order of detectors, filters out XSS vectors, and optionally converts detected language codes. The code is well-structured, tested, and production-grade; the cookie detector in particular includes a robust RFC-compliant serialization function. Everything is browser-native with no heavy dependencies beyond i18next itself.