Async/await syntax in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of async function with await

What will be the output of this Node.js code snippet?

Node.js

async function fetchData() {
  return 'data';
}

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

main();

A Error: await is only valid in async function

B Promise { 'data' }

C undefined

D data

💡 Hint

Remember that await unwraps the resolved value of a promise inside an async function.

The async function fetchData returns a resolved promise with 'data'. Await unwraps it, so console.log prints 'data'.

❓ component_behavior

intermediate

Behavior of async function returning a rejected promise

What happens when this async function is called without a try/catch block?

Node.js

async function fail() {
  throw new Error('fail');
}

fail();

A The error is silently ignored

B The program crashes immediately

C A rejected promise is returned but no error is thrown synchronously

D The error is logged to console automatically

💡 Hint

Async functions always return promises, even when throwing errors.

Throwing inside async returns a rejected promise. Without await or catch, no synchronous error occurs.

📝 Syntax

advanced

Identify the incorrect usage in async/await

Which option contains incorrect usage of async/await?

Node.js

async function test() {
  const data = await fetchData();
  return data;
}

A const data = await fetchData;

B const data = await fetchData();

C await fetchData();

D async function test() { const data = await fetchData(); return data; }

💡 Hint

Remember that await expects a function call returning a promise, not the function itself.

Option A misses parentheses after fetchData, so await is applied to the function object instead of calling it.

🔧 Debug

advanced

Why does this async function never resolve?

Consider this code. Why does the console.log never run?

Node.js

async function waitForever() {
  await new Promise(() => {});
  console.log('Done');
}

waitForever();

A The promise never resolves or rejects, so await waits forever

B The console.log is unreachable due to syntax error

C The function throws an error before console.log

D The await keyword is used incorrectly

💡 Hint

What happens if a promise never settles?

The promise passed to await never calls resolve or reject, so await waits forever and console.log never runs.

🧠 Conceptual

expert

Effect of missing await in async function call

What is the value of variable result after running this code?

Node.js

async function getValue() {
  return 42;
}

const result = getValue();

A 42

B Promise { 42 }

C undefined

D Error: Missing await

💡 Hint

Async functions always return promises, even if you don't use await.

Calling an async function returns a promise immediately. Without await, result is that promise, not the resolved value.

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