Dynamic API routes in NextJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a dynamic API route file name for a user ID.

NextJS

export default function handler(req, res) {
  const { [1] } = req.query;
  res.status(200).json({ id: [1] });
}

Drag options to blanks, or click blank then click option'

Aid

Bslug

Cparam

DuserId

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different key than the file name in req.query.

Forgetting to destructure the parameter from req.query.

The dynamic route parameter is accessed via req.query.id when the file is named [id].js.

2fill in blank

medium

Complete the code to create a dynamic API route file that captures a post slug.

NextJS

export default function handler(req, res) {
  const { [1] } = req.query;
  res.status(200).json({ slug: [1] });
}

Drag options to blanks, or click blank then click option'

Apost

BpostSlug

Cid

Dslug

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different parameter name than the file name.

Not matching the dynamic segment name exactly.

The file should be named [slug].js to capture the slug parameter, accessed as req.query.slug.

3fill in blank

hard

Fix the error in accessing the dynamic route parameter in this API handler.

NextJS

export default function handler(req, res) {
  const id = req.[1].id;
  res.status(200).json({ id });
}

Drag options to blanks, or click blank then click option'

Aquery

Bslug

Cparam

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using req.params which is undefined in Next.js API routes.

Not destructuring the parameter from req.query.

In Next.js API routes, dynamic parameters are accessed via req.query, not req.params.

4fill in blank

hard

Fill both blanks to correctly extract and respond with a dynamic user ID and post ID from the API route.

NextJS

export default function handler(req, res) {
  const { [1], [2] } = req.query;
  res.status(200).json({ userId: [1], postId: [2] });
}

Drag options to blanks, or click blank then click option'

AuserId

BpostId

Cid

Dslug

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up parameter names that don't match the file names.

Using id or slug instead of the correct keys.

The dynamic route file might be named [userId]/[postId].js, so the parameters are userId and postId in req.query.

5fill in blank

hard

Fill all three blanks to create a dynamic API route handler that returns the category, subcategory, and item ID.

NextJS

export default function handler(req, res) {
  const { [1], [2], [3] } = req.query;
  res.status(200).json({ category: [1], subcategory: [2], itemId: [3] });
}

Drag options to blanks, or click blank then click option'

Acategory

Bsubcategory

CitemId

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using generic names like id instead of specific dynamic segment names.

Not matching the order of destructured variables to the file path segments.

For a dynamic route like [category]/[subcategory]/[itemId].js, the parameters are category, subcategory, and itemId in req.query.

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