Event loop mental model in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output order of this Node.js code?

Consider this Node.js code snippet. What will be the order of the printed lines in the console?

Node.js

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

A "Start", "Promise", "End", "Timeout"

B "Promise", "Start", "End", "Timeout"

C "Start", "Timeout", "Promise", "End"

D "Start", "End", "Promise", "Timeout"

💡 Hint

Remember that promises run before timers even if the timer delay is zero.

The synchronous logs run first: 'Start' then 'End'. Then microtasks like promises run before timers. So 'Promise' logs before 'Timeout'.

❓ state_output

intermediate

What is the final value of counter after this code runs?

Given this Node.js code, what is the final value of the variable counter after all asynchronous tasks complete?

Node.js

let counter = 0;
setTimeout(() => { counter += 1; }, 0);
Promise.resolve().then(() => { counter += 2; });
counter += 3;

A 6

B 3

C 5

D 2

💡 Hint

Synchronous code runs first, then microtasks, then timers.

Initially, counter is 0. Then synchronous counter += 3 makes it 3. Promise microtask adds 2 making 5. Timer adds 1 last making 6, but timer runs after microtasks and the question asks after all async tasks complete, so final is 6.

📝 Syntax

advanced

Which option causes a SyntaxError in this async code?

Identify which code snippet will cause a SyntaxError when run in Node.js 20+.

A async function test() { await 5; }

B async function test() { return await 5; }

C async function test() { await; }

D const test = async () => { await Promise.resolve(); };

💡 Hint

The await keyword must be followed by an expression.

Option C uses await without an expression, which is invalid syntax and causes a SyntaxError.

🔧 Debug

advanced

Why does this code never print 'Done'?

Examine the code below. Why does it never print 'Done' to the console?

Node.js

setImmediate(() => {
  while(true) {}
  console.log('Done');
});

A 'setImmediate' callbacks never run in Node.js.

B The infinite loop blocks the event loop, so 'Done' is never reached.

C The console.log is outside the callback function.

D The code throws a ReferenceError before running.

💡 Hint

Think about what happens when the event loop is blocked.

The infinite loop inside the setImmediate callback blocks the event loop, so the console.log('Done') line is never executed.

🧠 Conceptual

expert

Which statement best describes the Node.js event loop phases?

Select the option that correctly orders the main phases of the Node.js event loop.

A Timers → Pending Callbacks → Poll → Check → Close Callbacks

B Poll → Timers → Check → Pending Callbacks → Close Callbacks

C Timers → Poll → Pending Callbacks → Check → Close Callbacks

D Check → Timers → Poll → Pending Callbacks → Close Callbacks

💡 Hint

Remember the event loop starts with timers phase.

The Node.js event loop phases run in this order: Timers, Pending Callbacks, Poll, Check, then Close Callbacks.