1. Create a dynamic folder: app/[id]/page.tsx
2. Access the parameter: const { id } = await params;
3. Fetch data using that identifier
4. Render the requested information

Using app/page.tsx for this scenario prevents access to per-path identifiers.

// app/[id]/page.tsx

// IMPORTANT: Server component (NO 'use client' needed!)
export default async function ProductPage({
  params,
}: {
  params: Promise<{ id: string }>;
}) {
  // Next.js 15+: params must be awaited
  const { id } = await params;

  // Fetch data using the ID from the URL
  const response = await fetch(`https://api.example.com/products/${id}`);
  const product = await response.json();

  // Return JSX with the fetched data
  return (
    <div>
      <h1>{product.name}</h1>
      <p>{product.description}</p>
      <p>Price: ${product.price}</p>
    </div>
  );
}

app/
└── [id]/                 ← Dynamic route folder with brackets
    └── page.tsx          ← Server component page

✅ app/[id]/page.tsx
✅ app/[productId]/page.tsx
✅ app/[slug]/page.tsx
❌ app/id/page.tsx        (no brackets = static route)
❌ app/page.tsx            (can't access params here)

// ✅ CORRECT - No 'use client' needed
export default async function Page({ params }) {
  const { id } = await params;
  const data = await fetch(`/api/${id}`);
  return <div>{data.name}</div>;
}

// ❌ WRONG - Don't add 'use client' for server components
'use client';  // ← Remove this!
export default async function Page({ params }) { ... }

// ✅ CORRECT - Next.js 15+
export default async function Page({
  params,
}: {
  params: Promise<{ id: string }>;
}) {
  const { id } = await params;  // Must await
  // ...
}

// ⚠️ OLD (Next.js 14 and earlier - deprecated)
export default async function Page({
  params,
}: {
  params: { id: string };
}) {
  const { id } = params;  // No await needed in old versions
  // ...
}

✅ app/[id]/page.tsx              (simple, clean)
❌ app/products/[id]/page.tsx     (only if explicitly required!)

// ❌ WRONG
function processProduct(product: any) { ... }

// ✅ CORRECT - Define proper types
interface Product {
  id: string;
  name: string;
  price: number;
}

function processProduct(product: Product) { ... }

// ✅ ALSO CORRECT - Use unknown if type truly unknown
function processData(data: unknown) {
  // Type guard required before using
  if (typeof data === 'object' && data !== null) {
    // ...
  }
}

// app/[productId]/page.tsx
export default async function Page({
  params,
}: {
  params: Promise<{ productId: string }>;
}) {
  const { productId } = await params;
  // ...
}

// app/[slug]/page.tsx
export default async function Page({
  params,
}: {
  params: Promise<{ slug: string }>;
}) {
  const { slug } = await params;
  // ...
}

// app/[category]/[id]/page.tsx
export default async function Page({
  params,
}: {
  params: Promise<{ category: string; id: string }>;
}) {
  const { category, id } = await params;
  const data = await fetch(`/api/${category}/${id}`);
  // ...
}

// app/[id]/page.tsx - Product detail page

interface Product {
  id: string;
  name: string;
  description: string;
  price: number;
  inStock: boolean;
}

export default async function ProductPage({
  params,
}: {
  params: Promise<{ id: string }>;
}) {
  // Get the ID from the URL
  const { id } = await params;

  // Fetch product data using the ID
  const response = await fetch(
    `https://api.example.com/products/${id}`,
    { cache: 'no-store' } // Always fresh data
  );

  if (!response.ok) {
    throw new Error('Failed to fetch product');
  }

  const product: Product = await response.json();

  // Render the product
  return (
    <div>
      <h1>{product.name}</h1>
      <p>{product.description}</p>

      <div>
        <strong>Price:</strong> ${product.price}
      </div>

      <div>
        <strong>Availability:</strong>{' '}
        {product.inStock ? 'In Stock' : 'Out of Stock'}
      </div>
    </div>
  );
}