JavaScript MCQs - Event Loop, Promises and Async/Await (Set 2)

Q1. When do Promise .then() callbacks run relative to setTimeout callbacks?

a) After all setTimeout callbacks

b) Before setTimeout callbacks, because they are microtasks that run as soon as the call stack is empty

c) In random order

d) Only when the browser is idle

Q2. What is the output order?

console.log('A');
setTimeout(() => console.log('B'), 0);
queueMicrotask(() => console.log('C'));
console.log('D');

a) A D C B

b) A C D B

c) A B C D

d) A D B C

Q3. What is printed?

async function f() {
  return 1;
}
console.log(f() instanceof Promise);

a) 1

b) TypeError

c) false

d) true

Q4. What is the output order?

async function f() {
  console.log(1);
  await null;
  console.log(2);
}
f();
console.log(3);

a) 3 1 2

b) 1 2 3

c) 2 1 3

d) 1 3 2

Q5. How does Promise.all behave if one of the input promises rejects?

a) It rejects immediately with the first rejection reason

b) It waits for all promises and returns only the fulfilled ones

c) It retries the rejected promise

d) It resolves with undefined for the rejected one

Q6. What does Promise.allSettled resolve with?

a) The first settled value

b) An array of objects with a status of 'fulfilled' or 'rejected' plus a value or reason

c) Only the values of fulfilled promises

d) It never resolves if one promise rejects

Q7. Which statement about Promise.any is correct?

a) It is the same as Promise.all

b) It resolves with the first promise to settle, fulfilled or rejected

c) It rejects as soon as any promise rejects

d) It resolves with the first fulfilled value and rejects with an AggregateError only if all inputs reject

Q8. What is the output order?

new Promise((resolve) => {
  console.log('X');
  resolve();
});
console.log('Y');

a) Only Y

b) Only X

c) Y X

d) X Y

Q9. What is printed?

Promise.resolve(1)
  .then((x) => x + 1)
  .then((x) => { throw x * 2; })
  .catch((e) => e + 1)
  .then(console.log);

a) 4

b) Nothing, it is an unhandled rejection

c) 2

d) 5

Q10. What happens here?

try {
  Promise.reject(new Error('x'));
} catch (e) {
  console.log('caught');
}

a) Logs caught

b) Nothing is logged by catch and an unhandled rejection is reported

c) Throws a SyntaxError

d) Logs x

Q11. wait(ms) returns a promise that resolves after ms. Roughly how long does each version take?

// Version 1
await wait(1000);
await wait(1000);

// Version 2
await Promise.all([wait(1000), wait(1000)]);

a) Both about 2 seconds

b) Both about 1 second

c) Version 1 about 1 second, version 2 about 2 seconds

d) Version 1 about 2 seconds, version 2 about 1 second

Q12. What is the problem with this code?

items.forEach(async (item) => {
  await save(item);
});
console.log('done');

a) It throws a SyntaxError

b) 'done' is logged before the saves finish because forEach does not wait for async callbacks

c) There is no problem

d) save is called only once

Q13. What does setTimeout(fn, 0) guarantee?

a) fn runs before any Promise callback

b) fn runs immediately

c) fn runs exactly 0 ms later

d) fn runs after the current synchronous code and pending microtasks, after a minimum delay

Q14. What is printed?

Promise.resolve(2)
  .finally(() => 10)
  .then(console.log);

a) 12

b) 2

c) undefined

d) 10

Q15. What is the output order?

setTimeout(() => console.log('t1'), 0);
Promise.resolve()
  .then(() => {
    console.log('p1');
    setTimeout(() => console.log('t2'), 0);
  })
  .then(() => console.log('p2'));

a) p1 p2 t2 t1

b) p1 t1 p2 t2

c) t1 p1 p2 t2

d) p1 p2 t1 t2

Q16. Which statement about JavaScript concurrency in the browser is correct?

a) JavaScript runs every callback on a separate thread

b) Promises create new threads

c) Your code runs on a single main thread; timers and network I/O are handled by the browser and their callbacks are queued

d) async/await makes code run in parallel on multiple cores