use platform agnostic feature test

packages/astro/src/runtime/server/render/page.ts

@@ -5,7 +5,7 @@ import type { AstroComponentFactory } from './index.js';
 import { isAstroComponentFactory } from './astro/index.js';
 import { renderToAsyncIterable, renderToReadableStream, renderToString } from './astro/render.js';
 import { encoder } from './common.js';
-import { isNode, isDeno } from './util.js';
+import { supportsResponseBodyAsyncIterator } from './util.js';
 
 export async function renderPage(
 	result: SSRResult,
@@ -48,9 +48,7 @@ export async function renderPage(
 
 	let body: BodyInit | Response;
 	if (streaming) {
-		// isNode is true in Deno node-compat mode but response construction from
-		// async iterables is not supported, so we fallback to ReadableStream if isDeno is true.
-		if (isNode && !isDeno) {
+		if (supportsResponseBodyAsyncIterator) {
 			const nodeBody = await renderToAsyncIterable(
 				result,
 				componentFactory,

packages/astro/src/runtime/server/render/util.ts

@@ -205,10 +205,17 @@ export function renderToBufferDestination(bufferRenderFunction: RenderFunction):
 	};
 }
 
-export const isNode =
-	typeof process !== 'undefined' && Object.prototype.toString.call(process) === '[object process]';
-// @ts-expect-error: Deno is not part of the types.
-export const isDeno = typeof Deno !== 'undefined';
+export const supportsResponseBodyAsyncIterator = (() => {
+	let supported = false;
+	new Response({
+		// @ts-expect-error: Response types don't expect the property.
+		get [Symbol.asyncIterator]() {
+			supported = true;
+			return undefined;
+		},
+	});
+	return supported;
+})();
 
 // We can get rid of this when Promise.withResolvers() is ready
 export type PromiseWithResolvers<T> = {