Why API routes serve backend logic in NextJS

Introduction

API routes let your Next.js app run backend code like talking to databases or handling user data. They act like helpers that do tasks behind the scenes.

When you need to save or get data from a database without showing it directly on the page.

When you want to handle user login or signup securely.

When you need to process form submissions or send emails.

When you want to keep secret keys safe and not expose them to the browser.

When you want to create a small backend service inside your Next.js app without a separate server.

Syntax

NextJS

export default function handler(req, res) {
  if (req.method === 'GET') {
    res.status(200).json({ message: 'Hello from API route!' })
  } else {
    res.status(405).end() // Method Not Allowed
  }
}

API routes are files inside the pages/api folder in Next.js.

The handler function receives req (request) and res (response) objects to handle HTTP methods.

Examples

This example always sends a JSON message when the API route is called.

NextJS

export default function handler(req, res) {
  res.status(200).json({ message: 'Simple response' })
}

This example only accepts POST requests and sends back the data it received.

NextJS

export default function handler(req, res) {
  if (req.method === 'POST') {
    const data = req.body
    res.status(200).json({ received: data })
  } else {
    res.status(405).end()
  }
}

Sample Program

This API route responds with a friendly greeting when you send a GET request. For other methods, it tells you only GET is allowed.

NextJS

export default function handler(req, res) {
  if (req.method === 'GET') {
    res.status(200).json({ greeting: 'Hello from backend!' })
  } else {
    res.status(405).json({ error: 'Only GET allowed' })
  }
}

Important Notes

API routes run only on the server, so you can safely use secrets like API keys here.

Remember to handle different HTTP methods to avoid errors.

You can test API routes by visiting their URL in the browser or using tools like Postman.

Summary

API routes let Next.js apps run backend code inside the same project.

They handle tasks like data fetching, form processing, and secure operations.

API routes use req and res to manage requests and responses.

Practice

1. Why do Next.js API routes serve backend logic in a Next.js project?

easy

A. They are used only to style the frontend components.

B. They replace the need for React components in the app.

C. They allow running server-side code like data fetching and secure operations within the same project.

D. They automatically generate static HTML pages.

Solution

1. Step 1: Understand the role of API routes

   API routes in Next.js let you write backend code inside your project to handle tasks like fetching data or processing forms.

2. Step 2: Identify what backend logic means

   Backend logic means code that runs on the server, such as secure operations or database access, which API routes enable.

3. Final Answer:

   They allow running server-side code like data fetching and secure operations within the same project. -> Option C

4. Quick Check:

   API routes = backend logic handler [OK]

Hint: API routes run server code inside Next.js projects [OK]

Common Mistakes:

• Thinking API routes style frontend
• Confusing API routes with React components
• Believing API routes generate static pages

2. Which of the following is the correct way to define a Next.js API route handler?

easy

A. const handler = () => Hello

B. function handler() { return Hello }

C. export function handler() { console.log('Hello') }

D. export default function handler(req, res) { res.status(200).json({ message: 'Hello' }) }

Solution

1. Step 1: Recognize API route syntax

   Next.js API routes export a default function that receives req and res to handle requests and responses.

2. Step 2: Check each option

   export default function handler(req, res) { res.status(200).json({ message: 'Hello' }) } correctly exports a default function with req and res and sends a JSON response. Others either lack export default or return JSX, which is incorrect for API routes.

3. Final Answer:

   export default function handler(req, res) { res.status(200).json({ message: 'Hello' }) } -> Option D

4. Quick Check:

   API route = export default function(req, res) [OK]

Hint: API routes export default function with req, res [OK]

Common Mistakes:

• Returning JSX instead of JSON
• Not exporting default function
• Missing req and res parameters

3. Given this Next.js API route code, what will be the JSON response when a GET request is made?

export default function handler(req, res) {
  if (req.method === 'GET') {
    res.status(200).json({ success: true, data: 'Hello World' })
  } else {
    res.status(405).json({ error: 'Method Not Allowed' })
  }
}

medium

A. {"error":"Method Not Allowed"}

B. {"success":true,"data":"Hello World"}

C. An HTML page with 'Hello World'

D. A runtime error occurs

Solution

1. Step 1: Check request method handling

   The handler checks if the request method is 'GET'. If yes, it sends a 200 status with JSON containing success and data.

2. Step 2: Determine response for GET request

   Since the request is GET, the response will be JSON: {"success":true,"data":"Hello World"} with status 200.

3. Final Answer:

   {"success":true,"data":"Hello World"} -> Option B

4. Quick Check:

   GET request returns success JSON [OK]

Hint: GET method returns success JSON, others error [OK]

Common Mistakes:

• Assuming all methods return success
• Expecting HTML instead of JSON
• Confusing status codes

4. Identify the error in this Next.js API route code:

export default function handler(req, res) {
  if (req.method === 'POST') {
    res.json({ message: 'Data received' })
  } else {
    res.status(405).json({ error: 'Method Not Allowed' })
  }
}

medium

A. No error, code is correct

B. Using res.json instead of res.send

C. Missing status code in the POST response

D. Missing req parameter

Solution

1. Step 1: Review response methods

   In Next.js API routes, res.json() sends a JSON response with a default status code of 200, which is correct and functional.

2. Step 2: Check each option

   The code properly handles POST with res.json() (200 OK implied) and other methods with 405 error. There are no syntax errors, runtime issues, or missing parameters. res.json is the right method for JSON responses.

3. Final Answer:

   No error, code is correct -> Option A

4. Quick Check:

   res.json defaults to 200, code runs correctly [OK]

Hint: res.json() defaults to status 200 [OK]

Common Mistakes:

• Thinking missing explicit status code is an error
• Confusing res.json and res.send
• Forgetting req or res parameters

5. You want to create a Next.js API route that securely processes a form submission only if the request method is POST and the request body contains a non-empty 'email' field. Which code snippet correctly implements this logic?

hard

A. export default function handler(req, res) {
if (req.method === 'POST' && req.body.email) {
res.status(200).json({ message: `Email ${req.body.email} received` })
} else {
res.status(400).json({ error: 'Invalid request' })
}
}

B. export default function handler(req, res) {
if (req.method === 'GET' || !req.body.email) {
res.status(200).json({ message: 'Form processed' })
} else {
res.status(400).json({ error: 'Invalid request' })
}
}

C. export default function handler(req, res) {
if (req.method === 'POST') {
res.status(200).json({ message: 'Form processed' })
} else {
res.status(400).json({ error: 'Invalid request' })
}
}

D. export default function handler(req, res) {
res.status(200).json({ message: 'Form processed' })
}

Solution

1. Step 1: Check method and body content

   The handler must verify the request method is POST and that the 'email' field exists and is not empty in the request body.

2. Step 2: Validate each option

   export default function handler(req, res) { if (req.method === 'POST' && req.body.email) { res.status(200).json({ message: `Email ${req.body.email} received` }) } else { res.status(400).json({ error: 'Invalid request' }) } } correctly checks both conditions and returns a success message with the email. Others either check wrong methods, ignore the email field, or always respond without validation.

3. Final Answer:

   export default function handler(req, res) { if (req.method === 'POST' && req.body.email) { res.status(200).json({ message: `Email ${req.body.email} received` }) } else { res.status(400).json({ error: 'Invalid request' }) } } -> Option A

4. Quick Check:

   POST + email check = export default function handler(req, res) { if (req.method === 'POST' && req.body.email) { res.status(200).json({ message: `Email ${req.body.email} received` }) } else { res.status(400).json({ error: 'Invalid request' }) } } [OK]

Hint: Check method and required fields before responding [OK]

Common Mistakes:

• Not checking request method
• Ignoring required fields in body
• Always returning success without validation