App directory (App Router) in NextJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the app directory in Next.js App Router?

The app directory organizes your application’s UI and routing using React Server Components. It replaces the older pages directory and allows for nested layouts, server rendering, and improved routing control.

beginner

How do you define a route in the Next.js app directory?

Each folder inside the app directory corresponds to a route segment. A file named page.jsx or page.tsx inside a folder defines the component rendered for that route.

intermediate

What is a layout in the Next.js App Router and how is it used?

A layout is a React component named layout.jsx or layout.tsx inside a folder in app. It wraps pages and nested routes, allowing shared UI like headers or footers to persist across pages.

intermediate

Explain the difference between Server Components and Client Components in the Next.js App Router.

Server Components run on the server and can fetch data directly, improving performance. Client Components run in the browser and handle interactivity. You mark Client Components with "use client" at the top of the file.

intermediate

How does Next.js handle loading states in the App Router?

You can create a loading.jsx or loading.tsx file inside a route folder to show a loading UI while the page or data is loading. This improves user experience by showing feedback during navigation.

In Next.js App Router, which file defines the UI for a route?

A route.js

B page.jsx

C app.js

D index.js

The page.jsx file inside a folder in the app directory defines the UI for that route.

What file do you add to share UI like headers across multiple pages in Next.js App Router?

A shared.jsx

B header.jsx

C layout.jsx

D app.jsx

The layout.jsx file wraps pages and nested routes to share UI elements like headers or footers.

How do you mark a component as a Client Component in Next.js App Router?

A Use a special ClientComponent wrapper

B Name the file client.jsx

C Import React from 'client'

D Add "use client" at the top of the file

Adding "use client" at the top of the file marks it as a Client Component.

Where do you place a loading.jsx file in Next.js App Router?

A Inside the route folder to show loading UI for that route

B In the root <code>app</code> directory only

C Next to <code>page.jsx</code> but outside any folder

D In the <code>public</code> folder

Placing loading.jsx inside a route folder shows a loading UI while that route is loading.

What is the main advantage of Server Components in Next.js App Router?

A They run on the server and can fetch data directly

B They run faster in the browser

C They allow client-side routing

D They enable CSS styling

Server Components run on the server and can fetch data directly, improving performance and reducing client bundle size.

Describe how routing works in the Next.js App Router using the app directory structure.

Think about how folders and files map to URLs and UI.

Explain the difference between Server Components and Client Components in Next.js App Router and when to use each.

Consider where the code runs and what it does.

Practice

1. In Next.js App Router, what does a folder containing a page.js file represent?

easy

A. A route in the application

B. A layout wrapper for pages

C. A server API endpoint

D. A static asset folder

Solution

1. Step 1: Understand the role of page.js in App Router

   In Next.js App Router, each folder with a page.js file defines a route that users can visit.

2. Step 2: Differentiate from other files

   Files like layout.js define layouts, not routes. API routes are in api/ folders, and static assets are in public/.

3. Final Answer:

   A route in the application -> Option A

4. Quick Check:

   Folder + page.js = route [OK]

Hint: Folder with page.js always means a route [OK]

Common Mistakes:

• Confusing layout.js with page.js
• Thinking page.js is for API routes
• Assuming static assets are routes

2. Which of the following is the correct way to define a layout in the Next.js App Router?

easy

A. Create a layout.js file inside the route folder

B. Create a page.js file inside the route folder

C. Create a _app.js file in the root folder

D. Create a server.js file in the route folder

Solution

1. Step 1: Identify layout file naming in App Router

   Layouts are defined by creating a layout.js file inside the folder representing the route.

2. Step 2: Exclude old or unrelated files

   The _app.js file is used in the old pages router, not App Router. page.js defines pages, not layouts. server.js is unrelated.

3. Final Answer:

   Create a layout.js file inside the route folder -> Option A

4. Quick Check:

   layout.js = layout file [OK]

Hint: Layouts use layout.js, not page.js or _app.js [OK]

Common Mistakes:

• Using _app.js instead of layout.js
• Confusing page.js with layout.js
• Trying to define layout in server.js

3. Given this folder structure in the app/ directory:

app/
  layout.js
  page.js
  dashboard/
    layout.js
    page.js

What happens when a user visits /dashboard?

medium

A. Only the root layout wraps the dashboard page, dashboard layout is ignored

B. Only the dashboard layout wraps the dashboard page, root layout is ignored

C. The root layout wraps the dashboard layout, which wraps the dashboard page

D. No layout is applied, only the dashboard page renders

Solution

1. Step 1: Understand nested layouts in App Router

   Layouts in parent folders wrap layouts and pages in child folders. So root layout wraps dashboard layout, which wraps dashboard page.

2. Step 2: Confirm layout application order

   Visiting /dashboard applies root layout first, then dashboard layout, then renders dashboard page inside.

3. Final Answer:

   The root layout wraps the dashboard layout, which wraps the dashboard page -> Option C

4. Quick Check:

   Layouts nest from root to child [OK]

Hint: Layouts nest from parent to child folders [OK]

Common Mistakes:

• Thinking only one layout applies
• Ignoring root layout when inside subfolder
• Assuming layouts do not nest

4. You wrote this code in app/dashboard/page.js:

export default function DashboardPage() {
  return 
Dashboard
}

But when you visit /dashboard, you get a blank page. What is the most likely cause?

medium

A. The page.js file must be named index.js

B. The DashboardPage component must be a class component

C. You forgot to export the component as default

D. Missing layout.js in app/ folder

Solution

1. Step 1: Verify root layout presence

   In Next.js App Router, a root layout.js file in the app/ directory is required to render any pages, including nested ones.

2. Step 2: Confirm naming and export correctness

   The page.js file name is correct, and the component is exported as default. Class components are not required.

3. Final Answer:

   Missing layout.js in app/ folder -> Option D

4. Quick Check:

   Root layout required for all pages [OK]

Hint: Root layout.js required for nested pages [OK]

Common Mistakes:

• Renaming page.js to index.js incorrectly
• Forgetting default export
• Thinking class components are required

5. You want to create a nested route /blog/[slug] using the App Router. Which folder and file structure is correct?

hard

A. app/blog/_slug/page.js

B. app/blog/[slug]/page.js

C. app/blog/slug/page.js

D. app/blog/[slug].js

Solution

1. Step 1: Understand dynamic route syntax in App Router

   Dynamic routes use square brackets around the parameter name as folder names, e.g., [slug].

2. Step 2: Confirm correct file placement

   The dynamic folder [slug] contains a page.js file to render the dynamic route. Other options use incorrect naming or file placement.

3. Final Answer:

   app/blog/[slug]/page.js -> Option B

4. Quick Check:

   Dynamic routes use [param] folder with page.js [OK]

Hint: Dynamic routes use [param] folders with page.js inside [OK]

Common Mistakes:

• Using _slug instead of [slug]
• Placing dynamic param in filename instead of folder
• Omitting page.js inside dynamic folder