Why API routes serve backend logic in NextJS - See It in Action

Why API routes serve backend logic

📖 Scenario: You are building a simple Next.js app that needs to fetch data securely from a backend. Instead of putting all logic in the frontend, you will create an API route to handle backend tasks like data fetching and processing.

🎯 Goal: Build a Next.js API route that returns a JSON response with a greeting message. Then, configure a frontend page to call this API route and display the message.

📋 What You'll Learn

Create an API route file in the app/api/greet/route.js path

Write a GET handler function that returns a JSON object with a message key

Create a frontend React component that fetches data from the API route using fetch

Display the fetched message inside the component

💡 Why This Matters

🌍 Real World

API routes are used in real apps to handle backend tasks like authentication, database queries, and secure data fetching.

💼 Career

Understanding API routes is essential for full-stack Next.js developers to build secure and scalable web applications.

1

Create the API route file with a GET handler

Create a file at app/api/greet/route.js and export an async function called GET that returns a JSON response with { message: 'Hello from API' }.

NextJS

// Create the GET handler function that returns JSON
// Your code here

Hint

Use the Next.js API route pattern with an exported async GET function that returns a Response object containing JSON.

2

Create a frontend React component to fetch from the API

In app/page.js, add 'use client' directive and create a React functional component called HomePage. Inside it, create a state variable message initialized to an empty string. Use useEffect to fetch from /api/greet and update message with the JSON message value.

NextJS

// Create HomePage component with useState and useEffect to fetch /api/greet
// Your code here

Hint

Add 'use client' at the top since you're using client-side hooks. Use useState to hold the message and useEffect to fetch the API once when the component loads.

3

Add error handling in the fetch logic

Inside the fetchMessage async function in HomePage, add a try-catch block to catch fetch errors. If an error occurs, set message to 'Failed to load message'.

NextJS

'use client';

import React, { useState, useEffect } from 'react';

export default function HomePage() {
  const [message, setMessage] = useState('');

  useEffect(() => {
    async function fetchMessage() {
      // Add try-catch here
      // Your code here
    }
    fetchMessage();
  }, []);

  return {message};
}

Hint

Wrap the fetch call in try and catch. In catch, update the message state with an error string.

4

Add semantic HTML and accessibility features

In the HomePage component, wrap the content inside a <main> element with aria-live="polite" attribute for screen readers. Also, add a <header> with a heading <h1> that says 'Welcome to the Greeting App' above the message. Change the message to a <p> tag.

NextJS

'use client';

import React, { useState, useEffect } from 'react';

export default function HomePage() {
  const [message, setMessage] = useState('');

  useEffect(() => {
    async function fetchMessage() {
      try {
        const res = await fetch('/api/greet');
        const data = await res.json();
        setMessage(data.message);
      } catch {
        setMessage('Failed to load message');
      }
    }
    fetchMessage();
  }, []);

  // Add  with  and aria-live attribute on 
  // Your code here
}

Hint

Use semantic tags like <header> and <main>. Add aria-live="polite" to <main> so screen readers announce message changes.

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