Testing async code in Node.js - Mini Project: Build & Apply

Testing Async Code in Node.js

📖 Scenario: You are building a small Node.js module that fetches user data asynchronously. You want to write tests to make sure your async function works correctly.

🎯 Goal: Create a simple async function that returns user data after a delay, then write a test using async/await to verify it returns the expected result.

📋 What You'll Learn

Create an async function called fetchUser that returns a user object after a 100ms delay

Create a variable called expectedUser with the exact user data to compare

Write an async test function called testFetchUser that awaits fetchUser and checks the result

Add a final call to testFetchUser to run the test

💡 Why This Matters

🌍 Real World

Testing async functions is common when working with APIs, databases, or any delayed operations in Node.js applications.

💼 Career

Understanding how to test async code is essential for backend developers to ensure their code works reliably and to catch bugs early.

1

Create the async function fetchUser

Create an async function called fetchUser that returns the object { id: 1, name: 'Alice' } after a 100 millisecond delay using setTimeout inside a Promise.

Node.js

# Your code here

Need a hint?

Use return new Promise(resolve => { setTimeout(() => resolve(...), 100) }) inside your async function.

2

Create the expected user data variable

Create a variable called expectedUser and assign it the object { id: 1, name: 'Alice' }.

Node.js

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

// Create expectedUser variable here
# Your code here

Need a hint?

Use const expectedUser = { id: 1, name: 'Alice' };

3

Write the async test function testFetchUser

Write an async function called testFetchUser that calls fetchUser() with await, stores the result in a variable called user, and then uses if to check if JSON.stringify(user) === JSON.stringify(expectedUser). If true, return 'Test passed', else throw an error with message 'Test failed'.

Node.js

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

const expectedUser = { id: 1, name: 'Alice' };

// Write async testFetchUser function here
# Your code here

Need a hint?

Use const user = await fetchUser(); and compare with expectedUser using JSON.stringify.

4

Call the test function testFetchUser

Add a call to testFetchUser() and handle the returned promise by adding .then to log 'Test passed' and .catch to log the error message.

Node.js

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

const expectedUser = { id: 1, name: 'Alice' };

async function testFetchUser() {
    const user = await fetchUser();
    if (JSON.stringify(user) === JSON.stringify(expectedUser)) {
        return 'Test passed';
    } else {
        throw new Error('Test failed');
    }
}

// Call testFetchUser and handle promise
# Your code here

Need a hint?

Call testFetchUser() and add .then and .catch to handle success and error.