Parallel data fetching pattern in NextJS - Mini Project: Build & Apply

Parallel Data Fetching Pattern in Next.js

📖 Scenario: You are building a Next.js app that shows user details and their posts side by side. To make the page load faster, you want to fetch user info and posts at the same time, not one after the other.

🎯 Goal: Build a Next.js server component that fetches user data and posts data in parallel using Promise.all and displays the user name and the titles of their posts.

📋 What You'll Learn

Create two async functions to fetch user data and posts data

Use a config variable for the user ID

Fetch both user and posts data in parallel using Promise.all

Render the user name and a list of post titles in the component

💡 Why This Matters

🌍 Real World

Fetching multiple data sources in parallel is common in web apps to improve performance and user experience.

💼 Career

Understanding parallel data fetching and server components is essential for building fast, scalable Next.js applications.

1

Create async functions to fetch user and posts data

Create two async functions called fetchUser and fetchPosts. fetchUser should fetch from https://jsonplaceholder.typicode.com/users/1. fetchPosts should fetch from https://jsonplaceholder.typicode.com/posts?userId=1. Both should return the JSON response.

NextJS

# Your code here

Hint

Use async function syntax and await fetch(...) to get data, then call res.json() to parse it.

2

Add a config variable for user ID

Create a constant called userId and set it to 1. Update the URLs in fetchUser and fetchPosts to use this userId variable instead of the hardcoded number.

NextJS

async function fetchUser() {
  const res = await fetch(`https://jsonplaceholder.typicode.com/users/1`);
  return res.json();
}

async function fetchPosts() {
  const res = await fetch(`https://jsonplaceholder.typicode.com/posts?userId=1`);
  return res.json();
}

# Your code here

Hint

Use a template string with backticks and ${userId} to insert the variable into the URL.

3

Fetch user and posts data in parallel

Create an async function called getData. Inside it, use Promise.all to run fetchUser() and fetchPosts() in parallel. Return an object with user and posts properties holding the results.

NextJS

const userId = 1;

async function fetchUser() {
  const res = await fetch(`https://jsonplaceholder.typicode.com/users/${userId}`);
  return res.json();
}

async function fetchPosts() {
  const res = await fetch(`https://jsonplaceholder.typicode.com/posts?userId=${userId}`);
  return res.json();
}

# Your code here

Hint

Use array destructuring to get user and posts from the array returned by Promise.all.

4

Create the Next.js server component to render user and posts

Create an async React component called Page. Inside it, call getData() to get user and posts. Return JSX that renders the user's name inside an <h1> and a list of post title inside <li> elements within a <ul>.

NextJS

const userId = 1;

async function fetchUser() {
  const res = await fetch(`https://jsonplaceholder.typicode.com/users/${userId}`);
  return res.json();
}

async function fetchPosts() {
  const res = await fetch(`https://jsonplaceholder.typicode.com/posts?userId=${userId}`);
  return res.json();
}

async function getData() {
  const [user, posts] = await Promise.all([fetchUser(), fetchPosts()]);
  return { user, posts };
}

# Your code here

Hint

Remember to use await when calling getData() inside the async component. Use map to render the list of posts.

Practice

1. What is the main benefit of using the Parallel data fetching pattern in Next.js?

easy

A. It delays fetching data until the user clicks a button.

B. It fetches multiple data sources at the same time to reduce loading time.

C. It fetches data one after another to ensure order.

D. It caches data permanently on the server.

Solution

1. Step 1: Understand parallel fetching concept

   Parallel fetching means requesting multiple data sources simultaneously rather than waiting for one to finish before starting the next.

2. Step 2: Identify the benefit in Next.js context

   Fetching data in parallel reduces total wait time, improving page load speed and user experience.

3. Final Answer:

   It fetches multiple data sources at the same time to reduce loading time. -> Option B

4. Quick Check:

   Parallel fetching = faster loading [OK]

Hint: Parallel means multiple fetches run together, not one by one [OK]

Common Mistakes:

• Thinking parallel means sequential fetching
• Confusing caching with parallel fetching
• Assuming parallel fetching waits for user action

2. Which of the following is the correct syntax to fetch two APIs in parallel using Next.js with Promise.all?

easy

A. const [data1, data2] = await Promise.all([fetch(url1), fetch(url2)]);

B. const data1 = await fetch(url1); const data2 = await fetch(url2);

C. const data1 = fetch(url1).then(); const data2 = fetch(url2).then();

D. const data1 = Promise.all(fetch(url1)); const data2 = Promise.all(fetch(url2));

Solution

1. Step 1: Recognize correct Promise.all usage

   Promise.all takes an array of promises and returns a single promise that resolves when all have resolved.

2. Step 2: Match syntax with parallel fetch

   Using await Promise.all([fetch(url1), fetch(url2)]) runs both fetches at the same time and waits for both results.

3. Final Answer:

   const [data1, data2] = await Promise.all([fetch(url1), fetch(url2)]); -> Option A

4. Quick Check:

   Promise.all with array = parallel fetch [OK]

Hint: Use await Promise.all with an array of fetch calls [OK]

Common Mistakes:

• Awaiting fetch calls one after another (sequential)
• Using Promise.all incorrectly on single fetch
• Not destructuring the array result

3. Given this Next.js code snippet, what will be logged to the console?

async function fetchData() {
  const [res1, res2] = await Promise.all([
    fetch('https://api.example.com/data1'),
    fetch('https://api.example.com/data2')
  ]);
  console.log(res1.ok, res2.ok);
}
fetchData();

medium

A. [false, false] always because Promise.all fails

B. SyntaxError because Promise.all cannot destructure

C. [true, false] if first fetch succeeds and second fails

D. [true, true] if both fetches succeed

Solution

1. Step 1: Understand Promise.all behavior

   Promise.all resolves only if all promises succeed; otherwise, it rejects.

2. Step 2: Analyze console.log output

   If both fetches succeed, res1.ok and res2.ok will be true, so console logs [true, true].

3. Final Answer:

   [true, true] if both fetches succeed -> Option D

4. Quick Check:

   Promise.all success = all true [OK]

Hint: Promise.all resolves only if all fetches succeed [OK]

Common Mistakes:

• Assuming Promise.all returns false on failure instead of rejecting
• Thinking destructuring causes syntax error
• Confusing fetch response with data content

4. Identify the error in this Next.js parallel fetching code:

async function getData() {
  const data1 = fetch('/api/data1');
  const data2 = fetch('/api/data2');
  const results = await Promise.all(data1, data2);
  return results;
}

medium

A. You cannot use Promise.all with fetch in Next.js.

B. fetch calls must be awaited before Promise.all.

C. Promise.all expects an array, but data1 and data2 are passed as separate arguments.

D. The function should not be async.

Solution

1. Step 1: Check Promise.all argument

   Promise.all requires a single array of promises, but here two promises are passed as separate arguments.

2. Step 2: Correct usage

   It should be Promise.all([data1, data2]) to run fetches in parallel correctly.

3. Final Answer:

   Promise.all expects an array, but data1 and data2 are passed as separate arguments. -> Option C

4. Quick Check:

   Promise.all needs array argument [OK]

Hint: Always pass an array to Promise.all, not separate arguments [OK]

Common Mistakes:

• Passing multiple arguments instead of an array to Promise.all
• Awaiting fetch before Promise.all (sequential fetch)
• Misunderstanding async function requirements

5. You want to fetch user info and user posts in parallel in Next.js, but only display posts if user info fetch succeeds. Which pattern correctly handles this?

hard

A. Fetch user info first, then fetch posts only if user info fetch succeeds.

B. Use Promise.all to fetch both, then check user info before rendering posts.

C. Fetch posts first, then fetch user info regardless of posts result.

D. Fetch both sequentially without checking results.

Solution

1. Step 1: Understand dependency between data

   Posts depend on user info, so user info must be fetched and verified first.

2. Step 2: Choose correct fetching order

   Fetch user info first; if successful, then fetch posts. This avoids unnecessary posts fetch if user info fails.

3. Final Answer:

   Fetch user info first, then fetch posts only if user info fetch succeeds. -> Option A

4. Quick Check:

   Dependent fetches require sequential order [OK]

Hint: Fetch dependent data sequentially, independent data in parallel [OK]

Common Mistakes:

• Fetching dependent data in parallel without checks
• Ignoring fetch failure conditions
• Fetching unnecessary data causing wasted resources