Mocking data fetching in NextJS - Performance & Optimization

Performance: Mocking data fetching

MEDIUM IMPACT

Mocking data fetching affects page load speed and interaction responsiveness by simulating network calls without real server delays.

Simulating data fetching during development

NextJS

export async function getServerSideProps() {
  const data = { id: 1, name: 'Mocked Item' };
  return { props: { data } };
}

Returns data immediately without network delay, speeding up page load and interaction.

📈 Performance Gainnon-blocking, zero network delay, faster INP

Simulating data fetching during development

NextJS

export async function getServerSideProps() {
  const res = await fetch('https://api.example.com/data');
  const data = await res.json();
  return { props: { data } };
}

This makes a real network request on every page load, slowing down development and testing.

📉 Performance Costblocks rendering for 200-500ms per request depending on network

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Real network fetch on each load	Minimal	1 reflow after data arrives	Moderate paint cost due to delayed content	[X] Bad
Mocked data returned immediately	Minimal	1 reflow immediately	Low paint cost with instant content	[OK] Good

Rendering Pipeline

Mocking data fetching bypasses the network request stage, allowing the browser to receive data immediately and proceed with rendering faster.

→Network Request

→JavaScript Execution

→Rendering

⚠️ BottleneckNetwork Request

Core Web Vital Affected

INP

Mocking data fetching affects page load speed and interaction responsiveness by simulating network calls without real server delays.

Optimization Tips

1Use mocked data to avoid network delays during development.

2Switch to real data fetching before production to ensure accuracy.

3Mocked data improves interaction responsiveness by eliminating fetch wait times.

Performance Quiz - 3 Questions

What is the main performance benefit of mocking data fetching in Next.js during development?

A It reduces the bundle size by removing data fetching code.

B It eliminates network latency, speeding up page load and interaction.

C It improves SEO by preloading real data.

D It decreases the number of DOM nodes rendered.

DevTools: Network

How to check: Open DevTools, go to Network tab, reload page, and observe if data fetch requests are made or if data is served instantly.

What to look for: Presence of network requests and their timing; mocked data shows no network delay.

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