Event loop phases and timer execution in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding setTimeout and setImmediate order

Consider this Node.js code snippet:

setTimeout(() => console.log('timeout'), 0);
setImmediate(() => console.log('immediate'));

What is the most likely output order when this code runs?

Node.js

setTimeout(() => console.log('timeout'), 0);
setImmediate(() => console.log('immediate'));

A timeout only

B immediate\ntimeout

C timeout\nimmediate

D immediate only

💡 Hint

Think about which phase runs first after the current poll phase.

In Node.js, setTimeout callbacks with 0 delay run in the timers phase, which happens before the check phase where setImmediate callbacks run. So timeout logs before immediate.

❓ component_behavior

intermediate

Behavior of process.nextTick in event loop

What happens when you schedule a callback with process.nextTick inside a setTimeout callback?

setTimeout(() => {
  console.log('timeout');
  process.nextTick(() => console.log('nextTick inside timeout'));
}, 0);

What will be the output order?

Node.js

setTimeout(() => {
  console.log('timeout');
  process.nextTick(() => console.log('nextTick inside timeout'));
}, 0);

A timeout\nnextTick inside timeout

B nextTick inside timeout\ntimeout

C Only timeout

D Only nextTick inside timeout

💡 Hint

Remember when nextTick callbacks run relative to the current phase.

process.nextTick callbacks run immediately after the current operation completes, before the event loop continues. So inside the setTimeout callback, timeout logs first, then the nextTick callback runs.

📝 Syntax

advanced

Identify error in timer callback usage

Which option will cause a runtime error when executed in Node.js?

setTimeout(() => console.log('Hello'), '1000');

Node.js

setTimeout(() => console.log('Hello'), '1000');

A ReferenceError because setTimeout is undefined

B TypeError because delay must be a number, not string

C SyntaxError due to arrow function

D No error, prints 'Hello' after 1 second

💡 Hint

Check how Node.js treats string delays in setTimeout.

Node.js converts string delays to numbers automatically, so no error occurs. The callback runs after approximately 1 second.

🔧 Debug

advanced

Why does this setInterval never stop?

Consider this code:

let count = 0;
setInterval(() => {
  if (count === 3) clearInterval();
  console.log(count);
  count++;
}, 1000);

Why does the interval never stop?

Node.js

let count = 0;
const intervalId = setInterval(() => {
  if (count === 3) clearInterval(intervalId);
  console.log(count);
  count++;
}, 1000);

A clearInterval is called without interval ID, so it does nothing

B count never reaches 3 because of async behavior

C Syntax error due to missing semicolon

D setInterval is not cleared because count is reset

💡 Hint

What does clearInterval need to stop an interval?

clearInterval requires the interval ID returned by setInterval. Calling it without arguments does nothing, so the interval keeps running.

❓ lifecycle

expert

Order of phases with nested timers and nextTick

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

console.log('start');
setTimeout(() => {
  console.log('timeout 1');
  process.nextTick(() => console.log('nextTick inside timeout 1'));
}, 0);
setImmediate(() => {
  console.log('immediate 1');
  setTimeout(() => console.log('timeout 2'), 0);
});
process.nextTick(() => console.log('nextTick 1'));
console.log('end');

Node.js

console.log('start');
setTimeout(() => {
  console.log('timeout 1');
  process.nextTick(() => console.log('nextTick inside timeout 1'));
}, 0);
setImmediate(() => {
  console.log('immediate 1');
  setTimeout(() => console.log('timeout 2'), 0);
});
process.nextTick(() => console.log('nextTick 1'));
console.log('end');

A start\nend\nnextTick 1\ntimeout 1\nnextTick inside timeout 1\nimmediate 1\ntimeout 2

B start\nend\nnextTick 1\nimmediate 1\ntimeout 1\nnextTick inside timeout 1\ntimeout 2

C start\nend\nimmediate 1\nnextTick 1\ntimeout 1\nnextTick inside timeout 1\ntimeout 2

D start\nend\nnextTick 1\ntimeout 1\nimmediate 1\nnextTick inside timeout 1\ntimeout 2

💡 Hint

Remember process.nextTick runs immediately after current operation, and setImmediate runs after poll phase.

Execution order:
1. 'start' and 'end' log immediately.
2. After main script completes (end of poll phase), process.nextTick callbacks run: 'nextTick 1' logs.
3. Check phase runs: 'immediate 1' logs, schedules 'timeout 2'.
4. Timers phase (next iteration): 'timeout 1' logs.
5. After 'timeout 1' callback, process.nextTick runs: 'nextTick inside timeout 1' logs.
6. Next timers phase: 'timeout 2' logs.