Event loop overview in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code snippet?

Consider the following JavaScript code. What will be printed to the console?

Javascript

console.log('Start');
setTimeout(() => console.log('Timeout'), 0);
Promise.resolve().then(() => console.log('Promise'));
console.log('End');

A

Start
Promise
End
Timeout

B

Start
End
Promise
Timeout

C

Start
End
Timeout
Promise

D

Start
Timeout
Promise
End

💡 Hint

Remember that Promise callbacks go to the microtask queue, which runs before the macrotask queue where setTimeout callbacks go.

The synchronous console.log calls run first: 'Start' then 'End'. The Promise callback runs next because microtasks have priority, printing 'Promise'. Finally, the setTimeout callback runs, printing 'Timeout'.

❓ Predict Output

intermediate

What will this code print and why?

Analyze the following code and select the correct console output.

Javascript

console.log('1');
setTimeout(() => console.log('2'), 100);
setTimeout(() => console.log('3'), 0);
Promise.resolve().then(() => console.log('4'));
console.log('5');

A

1
5
3
4
2

B

1
5
2
3
4

C

1
4
5
3
2

D

1
5
4
3
2

💡 Hint

Microtasks run before any timers, even if the timer delay is zero.

First, synchronous logs '1' and '5'. Then microtask logs '4'. Then timers with 0ms delay logs '3', and finally timer with 100ms delay logs '2'.

❓ Predict Output

advanced

What is the output of this nested asynchronous code?

Look at this code with nested setTimeout and Promise. What will it print?

Javascript

console.log('A');
setTimeout(() => {
  console.log('B');
  Promise.resolve().then(() => console.log('C'));
}, 0);
Promise.resolve().then(() => {
  console.log('D');
  setTimeout(() => console.log('E'), 0);
});
console.log('F');

A

A
F
D
B
C
E

B

A
F
B
D
C
E

C

A
F
D
B
E
C

D

A
F
D
C
B
E

💡 Hint

Remember microtasks run immediately after the current task, before the next macrotask.

First synchronous logs 'A' and 'F'. Then microtask logs 'D' and schedules a timer for 'E'. Next macrotask logs 'B' and schedules a microtask for 'C'. That microtask runs immediately after 'B', printing 'C'. Finally, the timer for 'E' runs.

❓ Predict Output

advanced

What error does this code produce?

Consider this code snippet. What error will it raise when run?

Javascript

setTimeout(() => {
  throw new Error('Oops');
}, 0);

Promise.reject('Fail').catch(e => console.log('Caught:', e));

A Caught: Fail

B Uncaught Error: Oops

C No output, silent failure

D Caught: Oops

💡 Hint

Errors thrown inside setTimeout callbacks are not caught by Promise.catch.

The Promise.reject is caught and logs 'Caught: Fail'. The error thrown inside setTimeout is asynchronous and not caught here, so it causes an uncaught error in the environment but does not affect this output.

🧠 Conceptual

expert

Which option correctly describes the event loop phases order?

Which of the following lists the correct order of event loop phases in Node.js?

A Timers → Poll → Pending callbacks → Check → Close callbacks → Idle, prepare

B Poll → Timers → Check → Pending callbacks → Close callbacks → Idle, prepare

C Timers → Pending callbacks → Idle, prepare → Poll → Check → Close callbacks

D Pending callbacks → Timers → Poll → Check → Close callbacks → Idle, prepare

💡 Hint

Recall the Node.js event loop phases as documented in official docs.

The Node.js event loop phases run in this order: Timers, Pending callbacks, Idle/prepare, Poll, Check, and Close callbacks.