Async/await syntax in Node.js - Mini Project: Build & Apply

Using Async/Await Syntax in Node.js

📖 Scenario: You are building a simple Node.js program that fetches user data from a mock API. You want to use modern async/await syntax to handle asynchronous calls clearly and simply.

🎯 Goal: Create a function that fetches user data asynchronously using async/await syntax and logs the user's name.

📋 What You'll Learn

Create a function called fetchUser that returns a Promise resolving to an object with name and age.

Create a variable called userId with the value 42.

Create an async function called getUserName that uses await to get the user data from fetchUser.

Inside getUserName, return the name property from the fetched user object.

💡 Why This Matters

🌍 Real World

Async/await is used in Node.js to handle operations like reading files, making network requests, or querying databases without blocking the program.

💼 Career

Understanding async/await is essential for backend developers working with Node.js to write clean, readable asynchronous code.

1

Create the mock fetchUser function

Create a function called fetchUser that takes a parameter id and returns a Promise. The Promise should resolve to an object with name set to 'Alice' and age set to 30.

Node.js

// Your code here

Hint

Use new Promise and call resolve with the user object inside.

2

Create the userId variable

Create a variable called userId and set it to the number 42.

Node.js

function fetchUser(id) {
    return new Promise((resolve) => {
        resolve({ name: 'Alice', age: 30 });
    });
}

// Your code here

Hint

Use const userId = 42; to create the variable.

3

Create the async getUserName function

Create an async function called getUserName that takes no parameters. Inside it, use await to call fetchUser with userId and store the result in a variable called user. Then return user.name.

Node.js

function fetchUser(id) {
    return new Promise((resolve) => {
        resolve({ name: 'Alice', age: 30 });
    });
}

const userId = 42;

// Your code here

Hint

Remember to mark the function with async and use await before fetchUser(userId).

4

Call getUserName and handle the result

Call getUserName() and use .then() to receive the returned name. Inside the then callback, assign the received name to a variable called userName.

Node.js

function fetchUser(id) {
    return new Promise((resolve) => {
        resolve({ name: 'Alice', age: 30 });
    });
}

const userId = 42;

async function getUserName() {
    const user = await fetchUser(userId);
    return user.name;
}

// Your code here

Hint

Use getUserName().then((name) => { const userName = name; }) to handle the async result.

Practice

1. What does the await keyword do inside an async function in Node.js?

easy

A. It converts a promise into a callback function.

B. It makes the function run faster by skipping the promise.

C. It stops the entire program until the promise finishes.

D. It pauses the function execution until the promise resolves or rejects.

Solution

1. Step 1: Understand the role of await

   The await keyword pauses the execution of the async function until the promise it waits for settles (resolves or rejects).

2. Step 2: Differentiate from blocking behavior

   This pause only affects the async function, not the entire program, allowing other code to run concurrently.

3. Final Answer:

   It pauses the function execution until the promise resolves or rejects. -> Option D

4. Quick Check:

   await pauses async function = C [OK]

Hint: Remember: await pauses only async function, not whole program [OK]

Common Mistakes:

• Thinking await blocks the entire program
• Confusing await with callbacks
• Believing await speeds up code by skipping promises

2. Which of the following is the correct syntax to declare an async function in Node.js?

easy

A. function async myFunc() {}

B. async function myFunc() {}

C. function myFunc async() {}

D. async: function myFunc() {}

Solution

1. Step 1: Recall async function declaration syntax

   In Node.js, the correct way to declare an async function is by placing the async keyword before the function keyword.

2. Step 2: Check each option

   async function myFunc() {} matches the correct syntax: async function myFunc() {}. Others are invalid syntax.

3. Final Answer:

   async function myFunc() {} -> Option B

4. Quick Check:

   async before function keyword = B [OK]

Hint: Put async right before function keyword to declare async function [OK]

Common Mistakes:

• Placing async after function name
• Using colons or other symbols incorrectly
• Writing async inside parentheses

3. What will be the output of the following code?

async function getNumber() {
  return 42;
}

async function main() {
  const result = await getNumber();
  console.log(result);
}

main();

medium

A. 42

B. Error: await used outside async function

C. undefined

D. Promise { 42 }

Solution

1. Step 1: Understand async function return values

   The function getNumber is async and returns 42, which means it returns a promise that resolves to 42.

2. Step 2: Await the promise in main

   The await keyword waits for the promise to resolve, so result gets the value 42, which is then logged.

3. Final Answer:

   42 -> Option A

4. Quick Check:

   await unwraps promise value = D [OK]

Hint: Await unwraps promise to get actual value inside async function [OK]

Common Mistakes:

• Expecting a Promise object printed
• Forgetting await causes Promise logged
• Using await outside async function

4. Identify the error in this code snippet:

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

fetchData().then(console.log);

medium

A. Missing await before data.json() call

B. fetch cannot be used in Node.js

C. async keyword is missing before fetchData

D. Cannot return data from async function

Solution

1. Step 1: Analyze the fetchData function

   The function awaits the fetch call, which returns a Response object. Calling data.json() returns a promise.

2. Step 2: Check promise handling for data.json()

   Since data.json() returns a promise, it should be awaited to get the parsed JSON before returning.

3. Final Answer:

   Missing await before data.json() call -> Option A

4. Quick Check:

   Await promises before returning parsed data = A [OK]

Hint: Await all promises inside async functions before returning [OK]

Common Mistakes:

• Not awaiting nested promises like data.json()
• Assuming fetch is unavailable in Node.js (modern Node supports it)
• Forgetting async keyword on async functions

5. You want to fetch user data and then fetch posts for that user sequentially using async/await. Which code snippet correctly handles errors and ensures posts are fetched only after user data is received?

async function getUserAndPosts() {
  try {
    const user = await fetchUser();
    const posts = await fetchPosts(user.id);
    return { user, posts };
  } catch (error) {
    console.error('Error fetching data:', error);
    return null;
  }
}

hard

A. fetchPosts runs before fetchUser completes

B. Does not handle errors because try/catch is missing

C. Correctly handles errors and fetches posts after user data

D. Returns posts without waiting for fetchPosts promise

Solution

1. Step 1: Check async/await sequence

   The code awaits fetchUser() first, then uses the user ID to await fetchPosts(), ensuring sequential execution.

2. Step 2: Verify error handling

   The try/catch block correctly catches any errors from either await call and logs them, returning null on failure.

3. Final Answer:

   Correctly handles errors and fetches posts after user data -> Option C

4. Quick Check:

   Try/catch with sequential await = A [OK]

Hint: Use try/catch around awaits to handle errors sequentially [OK]

Common Mistakes:

• Omitting try/catch for error handling
• Calling second await before first completes
• Returning promises without awaiting them