Promise chaining in Node.js - Mini Project: Build & Apply

Promise chaining

📖 Scenario: You are building a simple Node.js program that simulates fetching user data and then fetching that user's posts from a server. Each fetch returns a Promise that resolves after a short delay.

🎯 Goal: Build a chain of Promises where you first fetch a user, then fetch the posts of that user, and finally log the posts.

📋 What You'll Learn

Create a function fetchUser that returns a Promise resolving to the user object { id: 1, name: 'Alice' } after 100ms.

Create a function fetchPosts that takes a user ID and returns a Promise resolving to an array of posts [{ id: 1, title: 'Hello World' }, { id: 2, title: 'Learning Promises' }] after 100ms.

Create a variable userId to store the fetched user's ID.

Chain the Promises so that fetchUser is called first, then fetchPosts is called with the userId, and finally the posts are logged.

💡 Why This Matters

🌍 Real World

Promise chaining is used in Node.js to handle sequences of asynchronous tasks, such as fetching data from APIs or databases in order.

💼 Career

Understanding Promise chaining is essential for backend developers working with asynchronous JavaScript, enabling them to write clean, readable, and maintainable code.

1

Create the fetchUser function

Create a function called fetchUser that returns a Promise. This Promise should resolve after 100 milliseconds with the object { id: 1, name: 'Alice' }.

Node.js

// Your code here

Need a hint?

Use new Promise and setTimeout to simulate a delayed response.

2

Create the fetchPosts function

Create a function called fetchPosts that takes a parameter userId and returns a Promise. This Promise should resolve after 100 milliseconds with the array [{ id: 1, title: 'Hello World' }, { id: 2, title: 'Learning Promises' }].

Node.js

function fetchUser() {
    return new Promise((resolve) => {
        setTimeout(() => {
            resolve({ id: 1, name: 'Alice' });
        }, 100);
    });
}

// Your code here

Need a hint?

Use userId as a parameter but you don't need to use it inside the function body for this example.

3

Create the userId variable

Create a variable called userId and set it to null. This will hold the ID of the fetched user.

Node.js

function fetchUser() {
    return new Promise((resolve) => {
        setTimeout(() => {
            resolve({ id: 1, name: 'Alice' });
        }, 100);
    });
}

function fetchPosts(userId) {
    return new Promise((resolve) => {
        setTimeout(() => {
            resolve([
                { id: 1, title: 'Hello World' },
                { id: 2, title: 'Learning Promises' }
            ]);
        }, 100);
    });
}

// Your code here

Need a hint?

Use let to declare userId and set it to null.

4

Chain the Promises to fetch user and posts

Use fetchUser() and chain a .then() to get the user object. Inside this .then(), set userId to the user's id and return fetchPosts(userId). Chain another .then() to receive the posts and log them using console.log(posts).

Node.js

function fetchUser() {
    return new Promise((resolve) => {
        setTimeout(() => {
            resolve({ id: 1, name: 'Alice' });
        }, 100);
    });
}

function fetchPosts(userId) {
    return new Promise((resolve) => {
        setTimeout(() => {
            resolve([
                { id: 1, title: 'Hello World' },
                { id: 2, title: 'Learning Promises' }
            ]);
        }, 100);
    });
}

let userId = null;

// Your code here

Need a hint?

Remember to return fetchPosts(userId) inside the first .then() to chain the Promises.