fixup! Upgrade to Next.js@15 and React@19

src/emails/react-dom-server.edge.d.ts

@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-// This is needed because `renderEmail.ts` needs to import from server.edge
+// This is needed because `StorybookEmailRenderer.tsx` needs to import from server.edge
 // directly — see the comment in that file.
 declare module "react-dom/server.edge" {
   export * from "react-dom/server";

src/emails/renderEmail.ts

@@ -4,28 +4,46 @@
 
 import "../app/functions/server/notInClientComponent";
 import mjml2html from "mjml";
+import { Transform } from "node:stream";
 import { ReactNode } from "react";
 
 export async function renderEmail(emailTemplate: ReactNode): Promise<string> {
+  return mjml2html(await renderToString(emailTemplate), {
+    validationLevel: "strict",
+    beautify: false,
+    minify: false,
+    ignoreIncludes: true,
+  }).html;
+}
+
+async function renderToString(node: ReactNode): Promise<string> {
   // Importing react-dom/server dynamically here is a workaround for the following error:
   //
   //    Error: react-dom/server is not supported in React Server Components.
   //
-  // It's a bit of a pain though, as it's the only reason this needs to be an
-  // async function.
   // https://github.com/vercel/next.js/issues/43810#issuecomment-2437931415
+  const { renderToPipeableStream } = await import("react-dom/server");
+  // It looks like we can't use `renderToStaticMarkup`, as that results in the following error:
   //
-  // Also, react-dom/server.edge is apparently needed instead of react-dom/server
-  // to avoid this error:
+  //    Internal Error: do not use legacy react-dom/server APIs. If you encountered this error, please open an issue on the Next.js repo.
   //
-  //     Uncaught ReferenceError: MessageChannel is not defined
-  //
-  // See https://github.com/facebook/react/issues/31827#issuecomment-2563094822
-  const { renderToStaticMarkup } = await import("react-dom/server.edge");
-  return mjml2html(renderToStaticMarkup(emailTemplate), {
-    validationLevel: "strict",
-    beautify: false,
-    minify: false,
-    ignoreIncludes: true,
-  }).html;
+  // Hence this `renderToPipeableStream` mess.
+  const { pipe } = renderToPipeableStream(node);
+  return new Promise((resolve, reject) => {
+    const parts: string[] = [];
+    // It's not clear to me if there's a better to `pipe` a rendered `node` to a string,
+    // but this works and at this point I'm fed up, so leaving it at this.
+    // Feel free to replace by a better way.
+    const transformer = new Transform({
+      transform: (chunk, encoding, callback) => {
+        callback(null, chunk.toString());
+      },
+    });
+    transformer.on("data", (part) => parts.push(part));
+    transformer.on("error", reject);
+    transformer.on("end", () => {
+      return resolve(parts.join(""));
+    });
+    pipe(transformer);
+  });
 }