Dynamic API routes in NextJS - Mini Project: Build & Apply

Dynamic API Routes in Next.js

📖 Scenario: You are building a simple Next.js API that returns user information based on a user ID provided in the URL. This is common when you want to fetch data dynamically for different users.

🎯 Goal: Create a dynamic API route in Next.js that reads the userId from the URL and returns a JSON response with the user's details.

📋 What You'll Learn

Create a data object with user details keyed by user IDs

Add a variable to hold the user ID parameter from the request

Write the API handler function that returns user data based on the user ID

Export the API handler as default from the dynamic route file

💡 Why This Matters

🌍 Real World

Dynamic API routes let you build flexible backend endpoints that respond to different inputs in the URL, such as user profiles, product details, or blog posts.

💼 Career

Understanding dynamic API routes is essential for building modern web applications with Next.js, enabling you to create scalable and maintainable server-side logic.

1

Create user data object

Create a constant object called users with these exact entries: '1': { name: 'Alice', age: 25 }, '2': { name: 'Bob', age: 30 }, and '3': { name: 'Charlie', age: 35 }.

NextJS

# Your code here

Hint

Use a JavaScript object with string keys for user IDs and objects for user details.

2

Get userId from request query

Add a constant called userId that extracts userId from req.query inside the API handler function.

NextJS

const users = {
  '1': { name: 'Alice', age: 25 },
  '2': { name: 'Bob', age: 30 },
  '3': { name: 'Charlie', age: 35 }
};

export default function handler(req, res) {
  // Your code here
}

Hint

Use object destructuring to get userId from req.query.

3

Return user data or 404

Inside the handler function, write code that checks if users[userId] exists. If yes, respond with status 200 and JSON of the user data. If not, respond with status 404 and JSON message { error: 'User not found' }.

NextJS

const users = {
  '1': { name: 'Alice', age: 25 },
  '2': { name: 'Bob', age: 30 },
  '3': { name: 'Charlie', age: 35 }
};

export default function handler(req, res) {
  const { userId } = req.query;
  // Add if-else to check user and respond
}

Hint

Use if to check user existence and res.status().json() to send responses.

4

Export the handler as default

Ensure the API handler function is exported as the default export from the file using export default function handler(req, res) { ... }.

NextJS

const users = {
  '1': { name: 'Alice', age: 25 },
  '2': { name: 'Bob', age: 30 },
  '3': { name: 'Charlie', age: 35 }
};

// Confirm export default handler

Hint

The function must be exported as default for Next.js API routing to work.

Practice

1. What is the purpose of using square brackets in Next.js API route filenames like [id].js?

easy

A. To create dynamic API routes that capture parts of the URL

B. To mark the file as a static API route

C. To import external modules dynamically

D. To define middleware for the API route

Solution

1. Step 1: Understand file naming in Next.js API routes

   Square brackets in filenames like [id].js indicate a dynamic segment in the URL path.

2. Step 2: Recognize the effect on routing

   This allows the API route to capture the value in that part of the URL and use it inside the handler.

3. Final Answer:

   To create dynamic API routes that capture parts of the URL -> Option A

4. Quick Check:

   Dynamic routes use brackets = D [OK]

Hint: Square brackets in filenames mean dynamic URL parts [OK]

Common Mistakes:

• Thinking brackets mark static routes
• Confusing with dynamic imports
• Assuming brackets define middleware

2. Which of the following is the correct way to access the dynamic parameter id inside a Next.js API route handler in [id].js?

easy

A. const id = req.params.id;

B. const id = req.body.id;

C. const id = req.query.id;

D. const id = req.route.id;

Solution

1. Step 1: Recall how Next.js passes dynamic route params

   Next.js provides dynamic route parameters inside req.query in API routes.

2. Step 2: Identify the correct syntax

   Accessing id is done by req.query.id, not req.params or others.

3. Final Answer:

   const id = req.query.id; -> Option C

4. Quick Check:

   Dynamic params in API routes = req.query [OK]

Hint: Use req.query to get dynamic route params in Next.js API [OK]

Common Mistakes:

• Using req.params instead of req.query
• Trying to get params from req.body
• Using incorrect property like req.route

3. Given the API route file pages/api/user/[userId].js with this handler:

export default function handler(req, res) {
  const { userId } = req.query;
  res.status(200).json({ message: `User ID is ${userId}` });
}

What will be the JSON response when a client requests /api/user/42?

medium

A. 404 Not Found

B. {"message":"User ID is userId"}

C. {"message":"User ID is undefined"}

D. {"message":"User ID is 42"}

Solution

1. Step 1: Extract dynamic parameter from URL

   The URL /api/user/42 matches the dynamic route [userId].js, so userId is "42".

2. Step 2: Check the handler response

   The handler reads userId from req.query and returns JSON with message including that value.

3. Final Answer:

   {"message":"User ID is 42"} -> Option D

4. Quick Check:

   Dynamic param value used in response = A [OK]

Hint: Dynamic param in URL becomes req.query value [OK]

Common Mistakes:

• Expecting literal string 'userId' instead of value
• Assuming undefined if param missing
• Thinking route returns 404 for dynamic routes

4. Consider this Next.js API route file named pages/api/product/[pid].js with the handler:

export default function handler(req, res) {
  const pid = req.query.pid;
  if (!pid) {
    res.status(400).json({ error: "Product ID missing" });
  }
  res.status(200).json({ productId: pid });
}

What is the bug in this code?

medium

A. It should use res.send instead of res.json

B. It does not return after sending 400 response, causing headers to be sent twice

C. It uses req.query.pid instead of req.params.pid

D. The file name should be [pid].ts instead of .js

Solution

1. Step 1: Analyze the conditional response

   If pid is missing, the code sends a 400 response but does not stop execution.

2. Step 2: Understand HTTP response behavior

   Without a return after sending 400, the code continues and tries to send a 200 response, causing an error.

3. Final Answer:

   It does not return after sending 400 response, causing headers to be sent twice -> Option B

4. Quick Check:

   Return after error response to avoid double send [OK]

Hint: Always return after sending error response in API handlers [OK]

Common Mistakes:

• Ignoring missing return after res.status(400).json()
• Confusing req.query with req.params
• Thinking res.send is required over res.json

5. You want to create a Next.js API route that handles multiple dynamic segments like /api/order/[orderId]/item/[itemId].js. How should you structure the files and access both orderId and itemId inside the handler?

hard

A. Create nested folders: pages/api/order/[orderId]/item/[itemId].js and access via req.query.orderId and req.query.itemId

B. Create a single file pages/api/order-item.js and parse URL manually

C. Use query parameters like /api/order?orderId=1&itemId=2 and access req.query

D. Create a file pages/api/order/[orderId]-[itemId].js and access req.query as an array

Solution

1. Step 1: Understand nested dynamic routes in Next.js

   Next.js supports nested folders with dynamic segments using square brackets for each segment.

2. Step 2: Access multiple dynamic params in handler

   Both orderId and itemId appear in req.query as separate keys.

3. Final Answer:

   Create nested folders: pages/api/order/[orderId]/item/[itemId].js and access via req.query.orderId and req.query.itemId -> Option A

4. Quick Check:

   Nested folders with brackets = multiple params in req.query [OK]

Hint: Use nested folders with brackets for multiple dynamic params [OK]

Common Mistakes:

• Trying to parse multiple params in one filename
• Using query string instead of dynamic routes
• Assuming req.query returns array for multiple params