Add decision doc for react-router.config.ts (#12330)

Author: markdalgleish

decisions/0013-react-router-config-ts.md

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+# `react-router.config.ts`
+
+Date: 2024-11-21
+
+Status: accepted
+
+## Context
+
+Previously in Remix and earlier pre-releases of React Router, framework config was passed directly to the Vite plugin as an options object. While this has worked well so far when limited to Vite-specific use cases or simple CLI commands, we've started to run into some limitations as our `react-router` CLI has become more advanced.
+
+Some key issues with the current approach:
+
+1. **Tight coupling with Vite**
+
+   Our CLI commands (`react-router routes` and `react-router typegen`) need access to framework config but have nothing to do with Vite. We previously worked around this by using Vite to resolve `vite.config.ts` and then extracting our React Router config from the resolved Vite config object, but this approach proved to be difficult as we added more features to our CLI.
+
+2. **Limited config watching capabilities**
+
+   The introduction of `react-router typegen --watch` in particular highlighted the limitations of our Vite-coupled approach. We needed to not only resolve our config but also watch for changes. Having this tied to the Vite config made implementing this functionality unnecessarily complex.
+
+3. **Heavy-handed config updates**
+
+   Changes to Vite plugin options are treated like any other change to the Vite config, triggering a full reload of the dev server. This takes away any ability for us to handle config updates more gracefully.
+
+4. **Difficulty with config documentation**
+
+   Documentation of our config options was difficult since we either had to show a complete Vite config file with a lot of extra noise, or only show a call to the `reactRouter` plugin which looked a bit confusing since it was labelled as a `vite.config.ts` file. Neither approach was ideal for clearly explaining our config options while keeping code snippets to a minimum.
+
+## Goals
+
+1. Decouple framework config from Vite
+2. Enable granular config watching for tools like `react-router typegen --watch`
+3. Avoid unnecessary dev server reloads when config changes
+4. Improve documentation by separating framework config from Vite config
+
+## Decisions
+
+### Introduce dedicated `react-router.config.ts` in the root of the project
+
+We will introduce a dedicated config file, `react-router.config.ts/js`.
+
+### Config is provided via a default export
+
+To maintain consistency with other JS build tool configuration patterns, we will export the config object as the default export of the `react-router.config.ts` file.
+
+### Change `app/routes.ts` API to use a default export rather than a named `routes` export
+
+Now that we have multiple config files (`react-router.config.ts` and `app/routes.ts`), we should be internally consistent and use default exports for all of our config files. Now is a good time to make this change since the `routes.ts` API hasn't yet had a stable release.
+
+### Any config APIs should be exported from `@react-router/dev/config`
+
+The exported config object should satisfy the `Config` type from `@react-router/dev/config`. This follows our established pattern of using `@react-router/dev/*` namespaces for dev-time APIs that are scoped to particular files, e.g. `@react-router/dev/routes` and `@react-router/dev/vite`.
+
+### Config file is optional but recommended
+
+While the lack of a config file won't be treated as an error, we should include a blank config file in all official templates to make the config options more discoverable and self-documenting.
+
+### Remove options from Vite plugin
+
+The Vite plugin will no longer accept config options. All framework options will be handled through the dedicated config file.
+
+### Improved config update handling
+
+Config changes should no longer trigger full dev server reloads. We may re-introduce this behavior in certain cases where it makes sense.