JavaScript Async/Await Interview Questions

One of the most commonly asked JavaScript interview questions is:

What are async/await and how do they work?

Most developers can write:

async function fetchUsers() {
  const users = await getUsers();
}

But interviewers often go deeper:

• Is async/await different from Promises?
• Does await block the main thread?
• How does await interact with the Event Loop?
• Can multiple awaits run in parallel?
• What happens when an error occurs?

Let's understand async/await from first principles.

1. Why Do We Need Async/Await?

Consider fetching user data from an API.

Using callbacks:

getUser(function (user) {
  getPosts(user.id, function (posts) {
    getComments(posts[0].id, function (comments) {
      console.log(comments);
    });
  });
});

This quickly becomes:

Callback Hell

Problems:

• Hard to read
• Hard to debug
• Difficult error handling
• Nested code

Promises improved this:

getUser()
  .then(getPosts)
  .then(getComments)
  .then(console.log)
  .catch(console.error);

Better, but still chain-heavy.

Async/await was introduced to make asynchronous code look synchronous.

2. What is an Async Function?

The async keyword transforms a function into an asynchronous function.

async function greet() {
  return "Hello";
}

Although it appears to return a string:

greet();

actually returns:

Promise { "Hello" }

Equivalent to:

function greet() {
  return Promise.resolve("Hello");
}

Example

async function getMessage() {
  return "FrontendPrep";
}
 
console.log(getMessage());

Output:

Promise { "FrontendPrep" }

Important interview point:

Every async function always returns a Promise.

3. What is Await?

await pauses execution inside an async function until a Promise settles.

Example:

async function fetchData() {
  const data = await getUsers();
 
  console.log(data);
}

Execution flow:

Call Function
      ↓
Wait For Promise
      ↓
Promise Resolves
      ↓
Continue Execution

Simple Example

function delay() {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve("Done");
    }, 2000);
  });
}
 
async function run() {
  const result = await delay();
 
  console.log(result);
}
 
run();

Output after 2 seconds:

Done

Does Await Block JavaScript?

A common interview question.

Many developers answer:

Yes

This is incorrect.

Wrong

await blocks JavaScript

Correct

await pauses only the current async function

The Event Loop continues running.

Example:

async function demo() {
  console.log("Start");
 
  await Promise.resolve();
 
  console.log("End");
}
 
demo();
 
console.log("Outside");

Output:

Start
Outside
End

If await blocked JavaScript:

Start
End
Outside

would appear.

It doesn't.

4. Async/Await Under the Hood

Consider:

async function getUser() {
  const user = await fetch("/api/user");
 
  return user.json();
}

JavaScript roughly converts this into:

function getUser() {
  return fetch("/api/user").then((user) => {
    return user.json();
  });
}

Important interview point:

Async/await is syntax sugar over Promises.

Execution Order Interview Question

What is the output?

console.log(1);
 
async function demo() {
  console.log(2);
 
  await Promise.resolve();
 
  console.log(3);
}
 
demo();
 
console.log(4);

Answer

Output:

1
2
4
3

Why?

1 → Sync
2 → Sync
await → Pause function
4 → Sync
3 → Microtask Queue

5. Error Handling with Async/Await

Promises:

fetchData().then(handleSuccess).catch(handleError);

Async/Await:

async function loadData() {
  try {
    const data = await fetchData();
 
    console.log(data);
  } catch (error) {
    console.error(error);
  }
}

Cleaner and easier to read.

What Happens When Await Rejects?

async function demo() {
  await Promise.reject("Failed");
}

Equivalent to:

throw "Failed";

The function immediately exits unless handled.

Example

async function demo() {
  try {
    await Promise.reject("Error");
 
    console.log("Never runs");
  } catch (error) {
    console.log(error);
  }
}

Output:

Error

6. Optimization: Sequential vs. Parallel Awaits

Consider:

const user = await getUser();
const posts = await getPosts();

Execution:

Get User
   ↓
Wait
   ↓
Get Posts
   ↓
Wait

Total time:

Time1 + Time2

Interview Optimization Question

What is wrong here?

const users = await fetchUsers();
const posts = await fetchPosts();

If both requests are independent:

Unnecessary waiting

Better Solution

const [users, posts] = await Promise.all([fetchUsers(), fetchPosts()]);

Execution:

Start Both Requests
         ↓
Wait Once
         ↓
Receive Results

Total time:

max(Time1, Time2)

instead of:

Time1 + Time2

Async/Await vs Promise.all

Sequential

await task1();
await task2();
await task3();

Timeline:

Task1
 ↓
Task2
 ↓
Task3

Concurrent

await Promise.all([task1(), task2(), task3()]);

Timeline:

Task1
Task2
Task3
 ↓
Wait Once

Much faster.

7. Top-Level Await

No.

Invalid:

function demo() {
  const data = await fetchUsers();
}

Error:

await is only valid in async functions

Top-Level Await

Modern JavaScript allows:

const users = await fetchUsers();

inside ES modules.

Example:

// users.js
 
const users = await fetch("/users");
 
export default users;

8. Async Function Execution Without Await

async function demo() {
  return 10;
}

Perfectly valid.

Output:

Promise { 10 }

9. Common Interview Q&A

Does Async/Await Replace Promises?

No.

Async/await is built on top of Promises.

Promises still power everything underneath.

Does Await Create a New Thread?

No.

JavaScript remains single-threaded.

The Event Loop handles scheduling.

Can Multiple Awaits Run Simultaneously?

Not automatically.

This:

await task1();
await task2();

runs sequentially.

Use:

Promise.all();

for concurrency.

Can Async Functions Return Values?

Yes.

async function getName() {
  return "John";
}

Actually returns:

Promise.resolve("John");

What Happens If an Async Function Throws?

async function demo() {
  throw new Error("Failed");
}

Equivalent to:

return Promise.reject(new Error("Failed"));

Senior-Level Interview Answer

Async/await is syntactic sugar built on top of Promises that allows asynchronous code to be written in a synchronous-looking manner. An async function always returns a Promise, and the await keyword pauses execution of the current async function until a Promise settles. Await does not block the JavaScript thread; it simply yields control back to the Event Loop. Under the hood, async/await is transformed into Promise chains. For independent asynchronous operations, Promise.all should be used to execute tasks concurrently and improve performance.

Common Interview Mistakes

❌ "await blocks JavaScript."

Correct:

await pauses only the current async function.

❌ "async functions return values."

Correct:

async functions always return Promises.

❌ "async/await is different from Promises."

Correct:

async/await is built on top of Promises.

❌ "Multiple awaits run in parallel."

Correct:

Awaits are sequential unless explicitly parallelized.

Key Takeaways

• Syntactic Sugar: Built on top of Promises to write cleaner, synchronous-looking asynchronous code.
• Execution Flow: An async function returns a Promise; await pauses execution until the Promise resolves.
• Error Handling: Uses native try/catch blocks instead of .catch() chains for asynchronous operations.
• Task Orchestration: Awaiting tasks sequentially can block execution; use Promise.all to run requests in parallel.
• Top-Level Await: Supported in modern modules to pause importing modules until assets resolve.