docs: initial migration guide for v2

MIGRATION.md

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+---
+icon: icons8:up-round
+---
+
+# Migration guide for v1 to v2
+
+h3 v2 includes some behavior and API changes that you need to consider applying when migrating.
+
+> [!NOTE]
+> Currently v2 is in beta stage You can try with [`h3-nightly@2x`](https://www.npmjs.com/package/h3-nightly?activeTab=versions)
+
+> [!NOTE]
+> This is an undergoing migration guide and is not finished yet.
+
+## Fully decoupled from Node.js
+
+We started migrating h3 towards Web standards since [v1.8](https://unjs.io/blog/2023-08-15-h3-towards-the-edge-of-the-web). h3 apps are now fully decoupled from Node.js using an adapter-based abstraction layer to support Web and Node.js runtime features and performances natively. 
+
+This migration significantly reduces your bundle sizes and overhead in Web-native runtimes such as [Bun](https://bun.sh/), [Deno](https://deno.com) and [Cloudflare Workers](https://workers.cloudflare.com/).
+
+Since v2, Event properties `event.node.{req,res}` and `event.web` is not available anymore, instead, you can use `getNodeContext(event)` and `getWebContext(event)` to access raw objects for each runtime.
+
+## Response handling
+
+You should always explicitly `return` or `throw` responses and errors from event handlers.
+
+Previously h3 had `send*` utils that could interop the response handling lifecycle **anywhere** in any utility or middleware causing unpredictable application state control. To mitigate edge cases of this, previously h3 added `event.handler` property which is now gone!
+
+If you were previously using these methods, you can replace them with `return` statements returning a text, JSON value, stream, or web `Response` (h3 smartly detects and handles them):
+
+- `send(event, value)`: Use `return <value>`
+- `sendNoContent(event)`: Use `return null`
+- `sendError(event, error)`: Use `throw createError()`
+- `sendStream(event, stream)`: Use `return stream`
+- `sendWebResponse(event, response)`: Use `return response`
+
+Other send utils that are renamed and need explicit `return`:
+
+- `sendIterable(event, value)`: Use `return iterable()`
+- `sendRedirect(event, location, code)`: Use `return redirect(event, location, code)`
+- `sendProxy(event, target)`: Use `return proxy(event, target)`
+- `handleCors(event)`: Check return value (boolean) and early `return` if handled.
+- `serveStatic(event, content)`: Make sure to add `return before.
+
+## Body utils
+
+The legacy `readBody` and `readRawBody` utils are replaced with a new set of body utils that can leverage native runtime primitives better.
+
+- `readRawBody`: Returns body as [`Uint8Array`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array) which is similar to Node.js `Buffer`.
+- `readTextBody`: Returns body as text (`utf8`).
+- `readJSONBody` / `readValidatedJSONBody`: Returns JSON value of the body using `JSON.parse` or `URLSearchParams` for `form-urlencoded`
+- `readFormDataBody`: Returns body parsed as [FormData](https://developer.mozilla.org/en-US/docs/Web/API/FormData).
+- `getBodyStream`: Returns body as a [`ReadableStream`](https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream), replacing older experimental `getRequestWebStream`: Util.
+
+**Behavior changes:**
+
+- Body utils won't throw an error if the incoming request has no body (or is a `GET` method for example) but instead, returns `undefined`
+- `readJSONBody` does not use [unjs/destr](https://destr.unjs.io) anymore. You should always filter and sanitize data coming from user to avoid [prototype-poisoning](https://medium.com/intrinsic-blog/javascript-prototype-poisoning-vulnerabilities-in-the-wild-7bc15347c96)
+
+## Router
+
+h3 migrated to a brand new route-matching engine [unjs/rou3](https://rou3.unjs.io/).
+
+You might experience slight (and more intuitive) behavior changes for matching patterns.
+
+- `router.use(path, handler)` is deprecated. Use `router.all(path, handler)` instead.
+- `router.add(path, method: Method | Method[]` signature is changed to `router.add(method: Method, path)` (**important**)
+- `app.resolve` and `handler.__resolve` signature changed from `(path)` to `(method, path)`.
+
+## Cookie and Headers
+
+h3 migrated to leverage standard web [`Headers`](https://developer.mozilla.org/en-US/docs/Web/API/Headers) for all utils.
+
+Header values are always a plain `string` now (no `null` or `undefined` or `number` or `string[]`).
+
+For the [`Set-Cookie`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie) header, you can use [`headers.getSetCookie`](https://developer.mozilla.org/en-US/docs/Web/API/Headers/getSetCookie) that always returns a string array.
+
+### Other deprecations
+
+h3 v2 deprecated some legacy and aliased utilities.
+
+**Handler:**
+
+- `eventHandler`: Use `defineEventHandler`
+- `toEventHandler`: (it is not required anymore)
+- `lazyEventHandler`: Use `defineLazyEventHandler`
+
+**Request:**
+
+- `getHeader`: Use `getRequestHeader`
+- `getHeaders`: Use `getRequestHeaders`
+- `getRequestPath`: Use `event.path`
+
+**Response:**
+
+- `appendHeader`: Use `appendResponseHeader`
+- `appendHeaders`: Use `appendResponseHeaders`
+- `setHeader`: Use `setResponseHeader`
+- `setHeaders` => Use `setResponseHeaders`
+
+**Node.js:**
+
+- `defineNodeListener`: Use `defineNodeHandler`
+- `fromNodeMiddleware`: Use `fromNodeHandler`
+- `createEvent`: Use `fromNodeRequest`
+- `toNodeListener`: Use `toNodeHandler`
+- `callNodeListener`: Use `callNodeHandler`
+- `promisifyNodeListener` (removed)
+- `callNodeHandler`: (internal)
+
+**Web:**
+
+- `callWithWebRequest`: (removed)
+
+**Body:**
+
+- `readBody`: Use `readJSONBody`
+- `readFormData`: Use `readFormDataBody`
+- `readValidatedBody`: Use `readValidatedJSONBody`
+- `getRequestWebStream`: Use `getBodyStream`
+- `readMultipartFormData`: Migrate to `readFormDataBody`
+
+**Types:**
+
+- `_RequestMiddleware`: Use `RequestMiddleware`
+- `_ResponseMiddleware`: Use `ResponseMiddleware`
+- `NodeListener`: Use `NodeHandler`
+- `TypedHeaders`: Use `RequestHeaders` and `ResponseHeaders`
+- `HTTPHeaderName`: Use `RequestHeaderName` and `ResponseHeaderName`
+- `H3Headers`: Use native `Headers`
+- `H3Response`: Use native `Response`
+- `WebEventContext`
+- `NodeEventContext`
+- `NodePromisifiedHandler`
+- `MultiPartData`: Use `FormData`
+- `RouteNode`: Use `RouterEntry`
+  `CreateRouterOptions`: use `RouterOptions`
+
+- **Utils:**
+
+- `isStream`: Use `instanceof ReadableStream` and `.pipe` properties for detecting Node.js `ReadableStream`
+- `isWebResponse`: Use `use instanceof Response`
+- `MIMES`: Removed internal map.

docs/5.migration/0.index.md

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+../../MIGRATION.md