async/await Interview Questions in JavaScript

async/await Interview Questions in JavaScript

async/await is a top interview topic. Here are the most asked questions with answers.

Q1: What is async/await?

Syntactic sugar over promises. async functions return promises. await pauses until a promise settles. Makes async code look synchronous.

Q2: What is the output?

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

Answer: 1, 3, 2. await pauses foo; "3" logs first (sync), then "2" (microtask).

Q3: Does an async function always return a promise?

Yes. Return a value -> Promise.resolve(value). Throw -> Promise.reject(error). Return a promise -> returned as-is.

Q4: What is the output?

async function foo() {
  return 42;
}
foo().then((v) => console.log(v));

Answer: 42. async wraps the return value in a promise.

Q5: How do you handle errors with async/await?

Use try/catch around await. If any await rejects, the catch runs.

Q6: What is the output?

async function foo() {
  try {
    await Promise.reject("err");
  } catch (err) {
    console.log("caught", err);
  }
}
foo();

Answer: caught err. try/catch catches the rejection.

Q7: How do you run async operations in parallel?

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

Q8: Does await block the main thread?

No. await pauses the async function and yields to the caller. The main thread continues.

Q9: What happens if you use await in forEach?

urls.forEach(async (url) => {
  await fetch(url);
});

forEach does not await. The loop finishes immediately; fetches run in parallel in the background. Use for...of or Promise.all instead.

Q10: What is the difference between async/await and promises?

async/await is syntactic sugar over promises. Same mechanism, cleaner syntax. try/catch instead of .catch. Better stack traces. Works well with loops.

The Takeaway

async/await interview questions test: the definition, the 1, 3, 2 output (microtask), always returning a promise, error handling (try/catch), parallel (Promise.all), not blocking the main thread, forEach not awaiting, and the relationship to promises.