JavaScript Async Await Interview Questions 2026: 26 Q&A

Last Updated: June 2026 | Level: Freshers to Mid-Level | Read Time: ~16 min

Asynchronous JavaScript separates juniors from mid-level developers in interviews. Candidates report that the event loop, promise combinators, and async error handling are guaranteed for frontend and Node roles. This guide covers 26 async questions with full answers, runnable code, and predict-the-output traps. Behaviour reflects the official MDN documentation; confirm any event-loop nuance on MDN, since ordering questions hinge on a single rule.

Pair this with JavaScript Interview Questions for Freshers 2026 and JavaScript Output Questions 2026.

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Table of Contents

1. Promises Basics (Q1 to Q9)
2. async await (Q10 to Q17)
3. Event Loop and Ordering (Q18 to Q22)
4. Error Handling and Combinators (Q23 to Q26)
5. Predict the Output
6. Quick Revision Table
7. Frequently Asked Questions

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Promises Basics

Q1. What is a promise? Easy

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Q2. What are the states of a promise? Easy

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Q3. What is the difference between a callback and a promise? Medium

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Q4. What does Promise.resolve do? Easy

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Q5. How does promise chaining work? Medium

fetchUser()
  .then(u => fetchOrders(u.id))
  .then(orders => render(orders))
  .catch(err => showError(err));

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Q6. What does returning a value vs a promise from then do? Medium

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Q7. What is promise flattening? Hard

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Q8. What does .finally do? Easy

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Q9. Can you cancel a promise? Hard

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async await

Q10. What is async await? Easy

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Q11. What does an async function return? Medium

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Q12. What does await actually do? Medium

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Q13. How do you run async operations in parallel? Medium

const [a, b] = await Promise.all([fetchA(), fetchB()]); // parallel

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Q14. What is the cost of sequential awaits? Medium

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Q15. Can you use await at the top level? Medium

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Q16. What happens if you forget to await a promise? Hard

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Q17. How do you await inside a loop correctly? Hard

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Event Loop and Ordering

Q18. What is the event loop? Medium

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Q19. What is the difference between microtasks and macrotasks? Hard

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Q20. Why does a promise callback run before setTimeout(0)? Hard

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Q21. What is queueMicrotask? Medium

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Q22. Does await block the thread? Hard

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Error Handling and Combinators

Q23. How do you handle errors with async await? Medium

async function load() {
  try { return await fetchData(); }
  catch (e) { return fallback; }
}

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Q24. What is the difference between Promise.all and Promise.allSettled? Hard

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Q25. What do Promise.race and Promise.any do? Hard

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Q26. Why can a synchronous try/catch miss a promise rejection? Hard

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Predict the Output

Snippet 1

console.log("1");
setTimeout(() => console.log("2"), 0);
Promise.resolve().then(() => console.log("3"));
console.log("4");

Output:

1
4
3
2

Why: Sync first (1, 4), then microtask (3), then macrotask (2).

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Snippet 2

async function f() {
  console.log("a");
  await Promise.resolve();
  console.log("b");
}
console.log("start");
f();
console.log("end");

Output:

start
a
end
b

Why: Code up to the await runs synchronously; the continuation is a microtask after the current stack clears.

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Snippet 3

Promise.all([
  Promise.resolve(1),
  Promise.reject("x"),
  Promise.resolve(3)
]).then(console.log).catch(e => console.log("caught", e));

Output:

caught x

Why: Promise.all rejects on the first rejection, so the catch fires with "x" and the resolved values are discarded.

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Quick Revision Table

Concept	Key takeaway
Promise states	pending, fulfilled, rejected
async returns	always a promise
await	unwraps and yields a microtask
parallel	Promise.all, not sequential awaits
microtask vs macrotask	promises before setTimeout
all vs allSettled	reject early vs wait for all
race vs any	first settle vs first success

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Frequently Asked Questions

What async question is most asked in 2026?

Candidates report the event-loop ordering of synchronous code, microtasks, and macrotasks, plus the difference between Promise.all and Promise.allSettled, are the two most repeated async questions across frontend and Node interviews.

Is async await just syntax sugar over promises?

Yes. An async function returns a promise and await pauses until a promise settles, but it does not change the underlying event loop. Understanding that await yields a microtask is what interviewers test.

How do I handle errors with async await?

Wrap awaits in try/catch, or attach .catch to the returned promise. A common bug is expecting a synchronous try/catch to catch an unawaited rejected promise, which it cannot.

Why are sequential awaits slow?

Each await waits for its operation before the next starts, so durations add up. If the operations are independent, fire them together and use Promise.all so they overlap and the total is roughly the slowest one.

What is the difference between Promise.race and Promise.any?

race settles with the first promise to settle, whether fulfilled or rejected, while any fulfils with the first success and only rejects if every promise rejects. Use any for first-success-wins logic.

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Related Articles

• JavaScript Interview Questions for Freshers 2026, the master set
• JavaScript Output Questions 2026, event-loop traps
• JavaScript Closures Interview Questions 2026, closures in depth
• JavaScript Coding Questions With Solutions 2026, solved problems

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Confirm any event-loop or promise detail on the official MDN documentation before your interview. This guide reflects candidate-reported patterns and public preparation resources as of June 2026.