add formData method in superclass rather than monkey-patch

.changeset/tricky-maps-know.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Add formData method in superclass rather than monkey-patch

packages/kit/src/node/index.js

@@ -1,6 +1,4 @@
 import * as set_cookie_parser from 'set-cookie-parser';
-import { Request as NodeFetchRequest } from 'node-fetch';
-import { Readable } from 'stream';
 
 /** @param {import('http').IncomingMessage} req */
 function get_raw_body(req) {
@@ -80,22 +78,11 @@ export async function getRequest(base, req) {
 		delete headers[':scheme'];
 	}
 
-	const request = new Request(base + req.url, {
+	return new Request(base + req.url, {
 		method: req.method,
 		headers,
 		body: get_raw_body(req)
 	});
-
-	request.formData = async () => {
-		return new NodeFetchRequest(request.url, {
-			method: request.method,
-			headers: request.headers,
-			// @ts-expect-error TypeScript doesn't understand that ReadableStream implements Symbol.asyncIterator
-			body: request.body && Readable.from(request.body)
-		}).formData();
-	};
-
-	return request;
 }
 
 /** @type {import('@sveltejs/kit/node').setResponse} */

packages/kit/src/node/polyfills.js

@@ -1,13 +1,26 @@
 import { fetch, Response, Request, Headers } from 'undici';
 import { ReadableStream, TransformStream, WritableStream } from 'stream/web';
+import { Readable } from 'stream';
+import { Request as NodeFetchRequest } from 'node-fetch';
 import { webcrypto as crypto } from 'crypto';
 
 /** @type {Record<string, any>} */
 const globals = {
 	crypto,
 	fetch,
 	Response,
-	Request,
+	// TODO remove the superclass as soon as Undici supports formData
+	// https://github.com/nodejs/undici/issues/974
+	Request: class extends Request {
+		// @ts-expect-error
+		formData() {
+			return new NodeFetchRequest(this.url, {
+				method: this.method,
+				headers: this.headers,
+				body: this.body && Readable.from(this.body)
+			}).formData();
+		}
+	},
 	Headers,
 	ReadableStream,
 	TransformStream,