Showdex is a browser extension for Pokémon Showdown that provides a real-time damage calculator (Calcdex) and supplementary tools directly within the battle interface. It integrates deeply with Showdown's client, patching internal classes (Battle, Side, Room) to synchronize all Pokémon, field conditions, and player actions as they occur. The architecture follows a dual-host pattern (legacy Backbone and modern Preact clients) with abstract bootstrappers, React/Redux for UI state, and RTK Query for fetching Pokémon presets from Smogon APIs. It also includes a standalone calculator (Honkdex), a note-taking tool (Notedex), a settings dashboard (Hellodex), and a debug panel (Devdex). The codebase demonstrates strong engineering with well-structured utility modules, custom hooks, drag-and-drop layout components, internationalization (English/French), and a complex hydration/dehydration system for data persistence. While primarily an application, it incorporates significant data processing for Pokémon battle mechanics (stat calculations, legacy DV conversion, preset matching).