For some reason now it works properly.

NOTE: this solution is unstable and can change in the future.

package.json

@@ -9,10 +9,12 @@
     "lint": "next lint"
   },
   "dependencies": {
+    "lowdb": "^7.0.1",
     "match-sorter": "^8.0.0",
     "next": "15.2.4",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
+    "server-only": "^0.0.1",
     "sort-by": "^1.2.0",
     "tiny-invariant": "^1.3.3"
   },

pnpm-lock.yaml

@@ -8,6 +8,9 @@ importers:
 
   .:
     dependencies:
+      lowdb:
+        specifier: ^7.0.1
+        version: 7.0.1
       match-sorter:
         specifier: ^8.0.0
         version: 8.0.0
@@ -20,6 +23,9 @@ importers:
       react-dom:
         specifier: ^19.0.0
         version: 19.1.0(react@19.1.0)
+      server-only:
+        specifier: ^0.0.1
+        version: 0.0.1
       sort-by:
         specifier: ^1.2.0
         version: 1.2.0
@@ -1118,6 +1124,10 @@ packages:
     resolution: {integrity: sha512-lyuxPGr/Wfhrlem2CL/UcnUc1zcqKAImBDzukY7Y5F/yQiNdko6+fRLevlw1HgMySw7f611UIY408EtxRSoK3Q==}
     hasBin: true
 
+  lowdb@7.0.1:
+    resolution: {integrity: sha512-neJAj8GwF0e8EpycYIDFqEPcx9Qz4GUho20jWFR7YiFeXzF1YMLdxB36PypcTSPMA+4+LvgyMacYhlr18Zlymw==}
+    engines: {node: '>=18'}
+
   match-sorter@8.0.0:
     resolution: {integrity: sha512-bGJ6Zb+OhzXe+ptP5d80OLVx7AkqfRbtGEh30vNSfjNwllu+hHI+tcbMIT/fbkx/FKN1PmKuDb65+Oofg+XUxw==}
 
@@ -1348,6 +1358,9 @@ packages:
     engines: {node: '>=10'}
     hasBin: true
 
+  server-only@0.0.1:
+    resolution: {integrity: sha512-qepMx2JxAa5jjfzxG79yPPq+8BuFToHd1hm7kI+Z4zAq1ftQiP7HcxMhDDItrbtwVeLg/cY2JnKnrcFkmiswNA==}
+
   set-function-length@1.2.2:
     resolution: {integrity: sha512-pgRc4hJ4/sNjWCSS9AmnS40x3bNMDTknHgL5UaMBTMyJnU90EgWh1Rz+MC9eFu4BuN/UwZjKQuY/1v3rM7HMfg==}
     engines: {node: '>= 0.4'}
@@ -1401,6 +1414,10 @@ packages:
   stable-hash@0.0.5:
     resolution: {integrity: sha512-+L3ccpzibovGXFK+Ap/f8LOS0ahMrHTf3xu7mMLSpEGU0EO9ucaysSylKo9eRDFNhWve/y275iPmIZ4z39a9iA==}
 
+  steno@4.0.2:
+    resolution: {integrity: sha512-yhPIQXjrlt1xv7dyPQg2P17URmXbuM5pdGkpiMB3RenprfiBlvK415Lctfe0eshk90oA7/tNq7WEiMK8RSP39A==}
+    engines: {node: '>=18'}
+
   streamsearch@1.1.0:
     resolution: {integrity: sha512-Mcc5wHehp9aXz1ax6bZUyY5afg9u2rv5cqQI3mRrYkGC8rW2hM02jWuwjtL++LS5qinSyhj2QfLyNsuc+VsExg==}
     engines: {node: '>=10.0.0'}
@@ -2758,6 +2775,10 @@ snapshots:
     dependencies:
       js-tokens: 4.0.0
 
+  lowdb@7.0.1:
+    dependencies:
+      steno: 4.0.2
+
   match-sorter@8.0.0:
     dependencies:
       '@babel/runtime': 7.27.0
@@ -2996,6 +3017,8 @@ snapshots:
 
   semver@7.7.1: {}
 
+  server-only@0.0.1: {}
+
   set-function-length@1.2.2:
     dependencies:
       define-data-property: 1.1.4
@@ -3092,6 +3115,8 @@ snapshots:
 
   stable-hash@0.0.5: {}
 
+  steno@4.0.2: {}
+
   streamsearch@1.1.0: {}
 
   string.prototype.includes@2.0.1:

src/app/(sidebar)/contacts/[contactId]/Contact.tsx

@@ -1,57 +1,60 @@
 "use client";
 
-import Form from "next/form";
 import { ContactRecord } from "@/app/data";
+import Form from "next/form";
+import { use } from "react";
+import { useRouter } from "next/navigation";
 
-export default function Contact() {
-  const contact = {
-    first: "Your",
-    last: "Name",
-    avatar: "https://placecats.com/200/200",
-    twitter: "your_handle",
-    notes: "Some notes",
-    favorite: true,
-  };
+export default function Contact({
+  contact
+}: {
+  contact: Promise<ContactRecord>
+}) {
+  const c = use(contact);
+  const router = useRouter();
 
   return (
     <div id="contact">
       <div>
         {/* eslint-disable-next-line @next/next/no-img-element */}
         <img
-          alt={`${contact.first} ${contact.last} avatar`}
-          key={contact.avatar}
-          src={contact.avatar}
+          alt={`${c.first} ${c.last} avatar`}
+          key={c.avatar}
+          src={c.avatar}
         />
       </div>
 
       <div>
         <h1>
-          {contact.first || contact.last ? (
+          {c.first || c.last ? (
             <>
-              {contact.first} {contact.last}
+              {c.first} {c.last}
             </>
           ) : (
             <i>No Name</i>
           )}
-          <Favorite contact={contact}/>
+          <Favorite contact={c}/>
         </h1>
 
-        {contact.twitter ? (
+        {c.twitter ? (
           <p>
             <a
-              href={`https://twitter.com/${contact.twitter}`}
+              href={`https://twitter.com/${c.twitter}`}
             >
-              {contact.twitter}
+              {c.twitter}
             </a>
           </p>
         ) : null}
 
-        {contact.notes ? <p>{contact.notes}</p> : null}
+        {c.notes ? <p>{c.notes}</p> : null}
 
         <div>
-          <Form action="edit">
-            <button type="submit">Edit</button>
-          </Form>
+          <button
+            type="button"
+            onClick={() => router.push(`/contacts/${c.id}/edit`)}
+          >
+            Edit
+          </button>
 
           <Form
             action="destroy"

src/app/(sidebar)/contacts/[contactId]/edit/page.tsx

@@ -0,0 +1,87 @@
+import Form from "next/form";
+import { fetchContact } from "@/app/(sidebar)/loaders";
+import { notFound, redirect } from "next/navigation";
+import { updateContact } from "@/app/data";
+import { revalidatePath } from "next/cache";
+
+
+async function editContact(contactId: string, formData: FormData) {
+  "use server";
+  const updates = Object.fromEntries(formData);
+  await updateContact(contactId, updates);
+  revalidatePath('/(sidebar)', 'layout');  // THE NESTED BEHAVIOR IS UNSTABLE AND CAN CHANGE IN FUTURE.
+  redirect(`/contacts/${contactId}`);  // This makes the server component data be reloaded.
+  /*
+   * The server component architecture is a HTMX-like structure where the component rendered is STATE-LESS
+   * and that means that they don't automagically update.
+   * How do you say a server component to "re-render"? YOU REFRESH THE PAGE! THIS IS AS INTENDED!!!
+   * Do you want a component that re-render interactively from a "signal" client-side?
+   * That's a classical Client Component you dummy!! Go fetch clientside library and use that instead of Server Comps.
+   */
+}
+
+export default async function EditContactPage({
+  params
+}: {
+  params: Promise<{ contactId: string }>
+}) {
+  const { contactId } = await params;
+  const contact = await fetchContact(contactId).then((c) => {
+    if (c === null) notFound();
+    return c;
+  });
+  const editThisContact = editContact.bind(null, contactId);
+
+  return (
+    <Form key={contact.id} id="contact-form" action={editThisContact}>
+      <p>
+        <span>Name</span>
+        <input
+          aria-label="First name"
+          defaultValue={contact.first}
+          name="first"
+          placeholder="First"
+          type="text"
+        />
+        <input
+          aria-label="Last name"
+          defaultValue={contact.last}
+          name="last"
+          placeholder="Last"
+          type="text"
+        />
+      </p>
+      <label>
+        <span>Twitter</span>
+        <input
+          defaultValue={contact.twitter}
+          name="twitter"
+          placeholder="@jack"
+          type="text"
+        />
+      </label>
+      <label>
+        <span>Avatar URL</span>
+        <input
+          aria-label="Avatar URL"
+          defaultValue={contact.avatar}
+          name="avatar"
+          placeholder="https://example.com/avatar.jpg"
+          type="text"
+        />
+      </label>
+      <label>
+        <span>Notes</span>
+        <textarea
+          defaultValue={contact.notes}
+          name="notes"
+          rows={6}
+        />
+      </label>
+      <p>
+        <button type="submit">Save</button>
+        <button type="button">Cancel</button>
+      </p>
+    </Form>
+  );
+}

src/app/(sidebar)/contacts/[contactId]/loading.tsx

@@ -0,0 +1,5 @@
+export default function ContactLoading() {
+  return (
+    <div>Loading Contact...</div>
+  );
+}

src/app/(sidebar)/contacts/[contactId]/not-found.tsx

@@ -0,0 +1,8 @@
+export default function NotFound() {
+  return (
+    <main id="error-page">
+      <h1>404</h1>
+      <p>The requested contact could not be found.</p>
+    </main>
+  );
+}

src/app/(sidebar)/contacts/[contactId]/page.tsx

@@ -0,0 +1,22 @@
+import { notFound } from "next/navigation";
+import Contact from "@/app/(sidebar)/contacts/[contactId]/Contact";
+import { fetchContact } from "@/app/(sidebar)/loaders";
+import React, { Suspense } from "react";
+
+export default async function ContactPage({
+  params
+}: {
+  params: Promise<{ contactId: string }>
+}) {
+  const { contactId } = await params;
+  const contact = fetchContact(contactId).then((c) => {
+    if (c === null) notFound();
+    return c;
+  });
+
+  return (
+    <Suspense fallback={<div>Loading contact...</div>}>
+      <Contact contact={contact}/>
+    </Suspense>
+  );
+}

src/app/(sidebar)/layout.tsx

@@ -0,0 +1,56 @@
+import React, { Suspense } from "react";
+import Link from "next/link";
+import Form from "next/form";
+import ContactList from "./sidebar-contacts";
+import { createEmptyContact } from "@/app/data";
+import { revalidatePath } from "next/cache";
+import { fetchContacts } from "@/app/(sidebar)/loaders";
+
+async function newContact() {
+  "use server";
+  await createEmptyContact();
+  revalidatePath('/(sidebar)', 'layout');
+}
+
+export default function SidebarRootLayout({
+  children,
+}: Readonly<{
+  children: React.ReactNode;
+}>) {
+  const contacts = fetchContacts();
+
+  return (
+    <>
+      <div id="sidebar">
+        <h1>
+          <Link href="/about">React Router Contacts</Link>
+        </h1>
+        <div>
+          <Form id="search-form" role="search" action="." formMethod="get">
+            <input
+              aria-label="Search contacts"
+              id="q"
+              name="q"
+              placeholder="Search"
+              type="search"/>
+            <div
+              aria-hidden
+              hidden={true}
+              id="search-spinner"/>
+          </Form>
+          <Form action={newContact}>
+            <button type="submit">New</button>
+          </Form>
+        </div>
+        <nav>
+          <Suspense fallback={<div>Loading...</div>}>
+            <ContactList contacts={contacts}/>
+          </Suspense>
+        </nav>
+      </div>
+      <div id={'detail'}>
+        {children}
+      </div>
+    </>
+  );
+}

src/app/(sidebar)/loaders.ts

@@ -0,0 +1,13 @@
+import { getContact, getContacts } from "@/app/data";
+
+// This files is a non-needed interface file. I can directly call getContact/s into Server Components.
+
+export const fetchContacts = async () => {
+  console.log("fetch contacts");
+  return await getContacts();
+};
+
+export const fetchContact = async (contactId: string) => {
+  console.log(`fetch contact ${contactId}`);
+  return await getContact(contactId);
+};

src/app/(sidebar)/page.tsx

@@ -0,0 +1,15 @@
+import React from "react";
+
+export default function Home() {
+  return (
+    <p id="index-page">
+      This is a demo for React Router.
+      <br/>
+      Check out{" "}
+      <a href="https://reactrouter.com">
+        the docs at reactrouter.com
+      </a>
+      .
+    </p>
+  );
+}

src/app/(sidebar)/sidebar-contacts.tsx

@@ -0,0 +1,27 @@
+import { ContactRecord } from "@/app/data";
+import React, { use } from "react";
+import Link from "next/link";
+
+export default function ContactList({ contacts }: { contacts: Promise<ContactRecord[]> }) {
+  const allContacts = use(contacts);
+  return (
+    <ul>
+      {allContacts.map((c) => (
+        <li key={c.id}>
+          <Link href={`/contacts/${c.id}`}>
+            {c.first || c.last ? (
+              <>
+                {c.first} {c.last}
+              </>
+            ) : (
+              <i>No Name</i>
+            )}
+            {c.favorite ? (
+              <span>★</span>
+            ) : null}
+          </Link>
+        </li>
+      ))}
+    </ul>
+  );
+}

src/app/about/page.tsx

@@ -0,0 +1,45 @@
+import Link from "next/link";
+
+export default function About() {
+  return (
+    <div id="about">
+      <Link href="/">← Go to demo</Link>
+      <h1>About React Router Contacts</h1>
+
+      <div>
+        <p>
+          This is a demo application showing off some of the
+          powerful features of React Router, including
+          dynamic routing, nested routes, loaders, actions,
+          and more.
+        </p>
+
+        <h2>Features</h2>
+        <p>
+          Explore the demo to see how React Router handles:
+        </p>
+        <ul>
+          <li>
+            Data loading and mutations with loaders and
+            actions
+          </li>
+          <li>
+            Nested routing with parent/child relationships
+          </li>
+          <li>URL-based routing with dynamic segments</li>
+          <li>Pending and optimistic UI</li>
+        </ul>
+
+        <h2>Learn More</h2>
+        <p>
+          Check out the official documentation at{" "}
+          <a href="https://reactrouter.com">
+            reactrouter.com
+          </a>{" "}
+          to learn more about building great web
+          applications with React Router.
+        </p>
+      </div>
+    </div>
+  );
+}

src/app/data.ts

@@ -2,11 +2,16 @@
 // 🛑 Nothing in here has anything to do with React Router, it's just a fake database
 ////////////////////////////////////////////////////////////////////////////////
 // noinspection UnnecessaryLocalVariableJS,JSUnusedGlobalSymbols
+import "server-only";
 
 import { matchSorter } from "match-sorter";
 // @ts-expect-error - no types, but it's a tiny function
 import sortBy from "sort-by";
 import invariant from "tiny-invariant";
+import { Low } from "lowdb";
+import { JSONFile } from "lowdb/node";
+import { join } from "path";
+import { mkdir } from "fs/promises";
 
 type ContactMutation = {
   id?: string;
@@ -23,42 +28,97 @@ export type ContactRecord = ContactMutation & {
   createdAt: string;
 };
 
-////////////////////////////////////////////////////////////////////////////////
-// This is just a fake DB table. In a real app you'd be talking to a real db or
-// fetching from an existing API.
-const fakeContacts = {
-  records: {} as Record<string, ContactRecord>,
+// Define the database schema
+type Schema = {
+  records: Record<string, ContactRecord>;
+};
 
+// Set up lowdb
+const initDb = async (): Promise<Low<Schema>> => {
+  // Ensure the db directory exists
+  const dbDir = join(process.cwd(), '.db');
+  await mkdir(dbDir, { recursive: true });
+
+  const file = join(dbDir, 'contacts.json');
+  const adapter = new JSONFile<Schema>(file);
+  const defaultData: Schema = { records: {} };
+  const db = new Low<Schema>(adapter, defaultData);
+
+  // Load existing data
+  await db.read();
+
+  // Initialize if needed
+  if (!db.data) {
+    db.data = defaultData;
+  }
+
+  return db;
+};
+
+// Create a singleton instance of the database
+let dbPromise: Promise<Low<Schema>> | null = null;
+
+const getDb = () => {
+  if (!dbPromise) {
+    dbPromise = initDb();
+  }
+  return dbPromise;
+};
+
+////////////////////////////////////////////////////////////////////////////////
+// This is a DB wrapper using lowdb for persistence
+const fakeContacts = {
   async getAll(): Promise<ContactRecord[]> {
-    return Object.keys(fakeContacts.records)
-      .map((key) => fakeContacts.records[key])
+    const db = await getDb();
+    return Object.keys(db.data.records)
+      .map((key) => db.data.records[key])
       .sort(sortBy("-createdAt", "last"));
   },
 
   async get(id: string): Promise<ContactRecord | null> {
-    return fakeContacts.records[id] || null;
+    const db = await getDb();
+    return db.data.records[id] || null;
   },
 
   async create(values: ContactMutation): Promise<ContactRecord> {
+    const db = await getDb();
     const id = values.id || Math.random().toString(36).substring(2, 9);
     const createdAt = new Date().toISOString();
     const newContact = { id, createdAt, ...values };
-    fakeContacts.records[id] = newContact;
+    db.data.records[id] = newContact;
+    await db.write();
     return newContact;
   },
 
   async set(id: string, values: ContactMutation): Promise<ContactRecord> {
-    const contact = await fakeContacts.get(id);
+    const db = await getDb();
+    const contact = await this.get(id);
     invariant(contact, `No contact found for ${id}`);
     const updatedContact = { ...contact, ...values };
-    fakeContacts.records[id] = updatedContact;
+    db.data.records[id] = updatedContact;
+    await db.write();
     return updatedContact;
   },
 
-  destroy(id: string): null {
-    delete fakeContacts.records[id];
+  async destroy(id: string): Promise<null> {
+    const db = await getDb();
+    delete db.data.records[id];
+    await db.write();
     return null;
   },
+
+  // New reset function to restore the initial data
+  async reset(): Promise<void> {
+    const db = await getDb();
+    db.data.records = {};
+    await db.write();
+
+    // Restore initial data
+    const initialContacts = getInitialContacts();
+    for (const contact of initialContacts) {
+      await this.create(contact);
+    }
+  }
 };
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -93,10 +153,18 @@ export async function updateContact(id: string, updates: ContactMutation) {
 }
 
 export async function deleteContact(id: string) {
-  fakeContacts.destroy(id);
+  await fakeContacts.destroy(id);
 }
 
-[
+// New function to reset the database to initial state
+export async function resetDatabase() {
+  console.log("RESET DATABASE");
+  await fakeContacts.reset();
+}
+
+// Helper function to get initial contacts data
+function getInitialContacts() {
+  return [
     {
       avatar:
         "https://sessionize.com/image/124e-400o400o2-wHVdAuNaxi8KJrgtN3ZKci.jpg",
@@ -309,12 +377,19 @@ export async function deleteContact(id: string) {
       last: "Jensen",
       twitter: "@jenseng",
     },
-].forEach((contact) => {
-  fakeContacts.create({
+  ].map(contact => ({
     ...contact,
     id: `${contact.first
       .toLowerCase()
       .split(" ")
-      .join("_")}-${contact.last.toLocaleLowerCase()}`,
-  });
-});
+      .join("_")}-${contact.last.toLowerCase()}`,
+  }));
+}
+
+// Initialize with seed data on first load
+(async () => {
+  const db = await getDb();
+  if (Object.keys(db.data.records).length === 0) {
+    await resetDatabase();
+  }
+})();

src/app/layout.tsx

@@ -1,9 +1,7 @@
-import type { Metadata } from "next";
 import React from "react";
+import type { Metadata } from "next";
 
 import "./globals.css";
-import Form from "next/form";
-import Link from "next/link";
 
 export const metadata: Metadata = {
   title: "Next.js Address Book",
@@ -18,41 +16,7 @@ export default function RootLayout({
   return (
     <html lang="en">
     <body>
-    <div id="sidebar">
-      <h1>React Router Contacts</h1>
-      <div>
-        <Form id="search-form" role="search" action="." formMethod="get">
-          <input
-            aria-label="Search contacts"
-            id="q"
-            name="q"
-            placeholder="Search"
-            type="search"
-          />
-          <div
-            aria-hidden
-            hidden={true}
-            id="search-spinner"
-          />
-        </Form>
-        <Form action="." formMethod="post">
-          <button type="submit">New</button>
-        </Form>
-      </div>
-      <nav>
-        <ul>
-          <li>
-            <Link href={`/contacts/1`}>Your Name</Link>
-          </li>
-          <li>
-            <Link href={`/contacts/2`}>Your Friend</Link>
-          </li>
-        </ul>
-      </nav>
-    </div>
-    <div id={'detail'}>
     {children}
-    </div>
     </body>
     </html>
   );

src/app/page.tsx

@@ -1,8 +0,0 @@
-import Form from "next/form";
-import React from "react";
-
-export default function Home() {
-  return (
-    <div></div>
-  );
-}