Mocking data fetching in NextJS

Introduction

Mocking data fetching helps you test and build your app without needing a real server. It lets you pretend to get data so you can see how your app works.

You want to build a page before the real API is ready.

You want to test how your app behaves with different data.

You want to avoid slow or unreliable network calls during development.

You want to show a loading state or error state without real data.

You want to practice fetching data without setting up a backend.

Syntax

NextJS

async function fetchData() {
  // Simulate a delay
  await new Promise(resolve => setTimeout(resolve, 1000));
  // Return mock data
  return { id: 1, name: 'Mock User' };
}

Use setTimeout inside a Promise to simulate network delay.

Return simple objects or arrays that look like real data from your API.

Examples

Returns mock user data immediately without delay.

NextJS

async function fetchUser() {
  return { id: 1, name: 'Alice' };
}

Simulates a 500ms delay before returning a list of posts.

NextJS

async function fetchPosts() {
  await new Promise(r => setTimeout(r, 500));
  return [{ id: 1, title: 'Hello' }, { id: 2, title: 'World' }];
}

Simulates a failed fetch to test error handling.

NextJS

async function fetchWithError() {
  await new Promise(r => setTimeout(r, 300));
  throw new Error('Failed to fetch');
}

Sample Program

This React component uses a mocked fetch function to simulate loading user data. It shows a loading message, then displays the user info once loaded. If an error happens, it shows an error message.

NextJS

import { useState, useEffect } from 'react';

async function mockFetchUser() {
  await new Promise(resolve => setTimeout(resolve, 1000));
  return { id: 1, name: 'Mock User' };
}

export default function UserProfile() {
  const [user, setUser] = useState(null);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState(null);

  useEffect(() => {
    mockFetchUser()
      .then(data => {
        setUser(data);
        setLoading(false);
      })
      .catch(() => {
        setError('Could not load user');
        setLoading(false);
      });
  }, []);

  if (loading) return <p>Loading user...</p>;
  if (error) return <p role="alert">{error}</p>;

  return (
    <section aria-label="User Profile">
      <h1>User Profile</h1>
      <p><strong>ID:</strong> {user.id}</p>
      <p><strong>Name:</strong> {user.name}</p>
    </section>
  );
}

Important Notes

Mocking helps you build UI before backend is ready.

Always simulate delays to see loading states realistically.

Use error mocks to test how your app handles failures.

Summary

Mocking data fetching lets you pretend to get data for testing and development.

Use simple async functions with delays and fake data.

Helps you build and test UI without a real server.

Practice

1. What is the main purpose of mocking data fetching in a Next.js app?

easy

A. To encrypt data before sending it

B. To speed up the production server

C. To permanently store data on the client

D. To simulate data retrieval without needing a real server

Solution

1. Step 1: Understand mocking concept

   Mocking means pretending to get data, so you don't rely on a real server.

2. Step 2: Identify purpose in Next.js

   This helps build and test UI without waiting for real backend responses.

3. Final Answer:

   To simulate data retrieval without needing a real server -> Option D

4. Quick Check:

   Mocking = Simulate data fetching [OK]

Hint: Mocking means faking data to test without a real server [OK]

Common Mistakes:

• Thinking mocking speeds up production
• Confusing mocking with data storage
• Assuming mocking encrypts data

2. Which of the following is the correct way to create a mock async data fetch function in Next.js?

easy

A. async function fetchData() { return new Promise(resolve => setTimeout(() => resolve({ data: [] }), 1000)); }

B. const fetchData = () => { return { data: [] }; }

C. function fetchData() { setTimeout(() => { return { data: [] }; }, 1000); }

D. const fetchData = async () => { return setTimeout(() => ({ data: [] }), 1000); }

Solution

1. Step 1: Check async function syntax

   async function fetchData() { return new Promise(resolve => setTimeout(() => resolve({ data: [] }), 1000)); } correctly returns a Promise that resolves after 1 second with data.

2. Step 2: Identify incorrect patterns

   const fetchData = () => { return { data: [] }; } is synchronous, C does not return a Promise, D returns timeout ID, not data.

3. Final Answer:

   async function fetchData() { return new Promise(resolve => setTimeout(() => resolve({ data: [] }), 1000)); } -> Option A

4. Quick Check:

   Async + Promise + delay = async function fetchData() { return new Promise(resolve => setTimeout(() => resolve({ data: [] }), 1000)); } [OK]

Hint: Async functions must return a Promise to mock delays [OK]

Common Mistakes:

• Returning data synchronously without Promise
• Using setTimeout without Promise
• Returning timeout ID instead of data

3. Given this mock fetch function in Next.js:

async function fetchData() {
  return new Promise(resolve => setTimeout(() => resolve({ users: ["Alice", "Bob"] }), 500));
}

fetchData().then(data => console.log(data.users));

What will be logged to the console?

medium

A. Promise { <pending> }

B. undefined

C. ["Alice", "Bob"]

D. Error: fetchData is not a function

Solution

1. Step 1: Understand the Promise resolution

   The function returns a Promise that resolves after 500ms with an object containing users array.

2. Step 2: Check the then callback

   The then callback logs data.users, which is ["Alice", "Bob"].

3. Final Answer:

   ["Alice", "Bob"] -> Option C

4. Quick Check:

   Promise resolves to users array logged [OK]

Hint: Promise.then logs resolved data, not the Promise itself [OK]

Common Mistakes:

• Expecting immediate data without waiting
• Logging the Promise object instead of resolved value
• Confusing function call with undefined

4. Identify the error in this mock data fetch function:

async function fetchData() {
  setTimeout(() => {
    return { data: [1, 2, 3] };
  }, 1000);
}

medium

A. setTimeout cannot be used inside async functions

B. The function does not return a Promise, so data is undefined

C. The return statement inside setTimeout is invalid syntax

D. The function should not be async if using setTimeout

Solution

1. Step 1: Analyze return behavior

   The function does not return anything; setTimeout callback's return is ignored.

2. Step 2: Understand async function expectations

   Async functions should return a Promise; here, no Promise is returned, so caller gets undefined.

3. Final Answer:

   The function does not return a Promise, so data is undefined -> Option B

4. Quick Check:

   Async function must return Promise, not just use setTimeout [OK]

Hint: Return a Promise to deliver data after delay [OK]

Common Mistakes:

• Assuming setTimeout return affects function return
• Thinking async forbids setTimeout
• Ignoring that async functions return Promises

5. You want to mock fetching user profiles with a delay and filter out users with empty names. Which code correctly mocks this in Next.js?

hard

A. async function fetchUsers() {
const users = [{ name: "", id: 1 }, { name: "Eve", id: 2 }];
return new Promise(resolve => setTimeout(() => resolve(users.filter(u => u.name)), 1000));
}

B. function fetchUsers() {
return [{ name: "", id: 1 }, { name: "Eve", id: 2 }].filter(u => u.name);
}

C. async function fetchUsers() {
const users = [{ name: "", id: 1 }, { name: "Eve", id: 2 }];
return users.filter(u => u.name);
}

D. async function fetchUsers() {
return new Promise(resolve => setTimeout(() => resolve([{ name: "", id: 1 }, { name: "Eve", id: 2 }]), 1000));
}

Solution

1. Step 1: Identify filtering and delay

   async function fetchUsers() { const users = [{ name: "", id: 1 }, { name: "Eve", id: 2 }]; return new Promise(resolve => setTimeout(() => resolve(users.filter(u => u.name)), 1000)); } filters out users with empty names and returns a Promise resolving after 1 second.

2. Step 2: Check other options

   A returns unfiltered data with delay, B performs filtering but is synchronous and non-async without delay, C returns filtered data synchronously without delay.

3. Final Answer:

   async function fetchUsers() { const users = [{ name: "", id: 1 }, { name: "Eve", id: 2 }]; return new Promise(resolve => setTimeout(() => resolve(users.filter(u => u.name)), 1000)); } -> Option A

4. Quick Check:

   Filter then delay with Promise = async function fetchUsers() { const users = [{ name: "", id: 1 }, { name: "Eve", id: 2 }]; return new Promise(resolve => setTimeout(() => resolve(users.filter(u => u.name)), 1000)); } [OK]

Hint: Filter data before resolving Promise with delay [OK]

Common Mistakes:

• Not filtering data before resolving
• Returning data synchronously without delay
• Using non-async functions for async behavior