setTimeout and clearTimeout in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What will this Node.js code output?

Consider the following code snippet using setTimeout and clearTimeout. What will be printed to the console?

Node.js

const timer = setTimeout(() => {
  console.log('Hello after 1 second');
}, 1000);

clearTimeout(timer);

console.log('Timer cleared');

A Timer cleared

B No output

C Hello after 1 second

D Hello after 1 second\nTimer cleared

💡 Hint

Think about what clearTimeout does to a scheduled timer.

The setTimeout schedules a function to run after 1 second. However, clearTimeout cancels this scheduled function before it runs. So only the immediate console.log('Timer cleared') runs.

❓ state_output

intermediate

What is the value of count after this code runs?

Look at this Node.js code using setTimeout. What will be the final value of count after 1500 milliseconds?

Node.js

let count = 0;

const timer = setTimeout(() => {
  count += 1;
}, 1000);

setTimeout(() => {
  clearTimeout(timer);
}, 500);

setTimeout(() => {
  console.log(count);
}, 1500);

A 1

B undefined

C 0

D Throws an error

💡 Hint

When is the timer cleared compared to when it would run?

The first timer is set to increment count after 1000ms. But it is cleared after 500ms, so it never runs. Therefore, count remains 0.

📝 Syntax

advanced

Which option correctly cancels a repeating timer?

You want to stop a repeating timer created with setInterval. Which code snippet correctly cancels it?

Node.js

const intervalId = setInterval(() => {
  console.log('Tick');
}, 1000);

// Which line correctly stops the interval?

A stopInterval(intervalId);

B clearInterval(intervalId);

C cancelTimeout(intervalId);

D clearTimeout(intervalId);

💡 Hint

Remember the difference between setTimeout and setInterval cancellation functions.

setInterval timers must be cancelled with clearInterval. Using clearTimeout will not stop the interval.

🔧 Debug

advanced

Why does this timer never stop?

This code is intended to print 'Hello' every second and stop after 3 seconds. Why does it keep printing forever?

Node.js

const timer = setTimeout(function repeat() {
  console.log('Hello');
  setTimeout(repeat, 1000);
}, 1000);

setTimeout(() => {
  clearTimeout(timer);
  console.log('Stopped');
}, 3000);

A Because clearTimeout only cancels the first timer, but new timers keep being created inside repeat.

B Because clearTimeout is called too late after 3 seconds.

C Because setTimeout cannot be used recursively.

D Because the timer variable is not declared with let or const.

💡 Hint

Look at how many timers are created and which one is cleared.

The code creates a new timer inside the repeat function each time it runs. The clearTimeout(timer) only cancels the original timer, but the new timers keep running indefinitely.

🧠 Conceptual

expert

What is the behavior of this code snippet?

Analyze this Node.js code. What will be the output and why?

Node.js

let count = 0;

const timer = setTimeout(() => {
  console.log('Timeout 1:', count);
}, 1000);

const timer2 = setTimeout(() => {
  console.log('Timeout 2:', count);
}, 1500);

count = 5;

clearTimeout(timer);

setTimeout(() => {
  console.log('Final count:', count);
}, 2000);

A Timeout 1: 5\nFinal count: 5

B Timeout 1: 0\nTimeout 2: 5\nFinal count: 5

C Timeout 1: 5\nTimeout 2: 5\nFinal count: 5

D Timeout 2: 5\nFinal count: 5

💡 Hint

Consider which timers are cleared and when the variable count changes.

The first timer is cleared before it runs, so it never prints. The second timer runs after 1.5 seconds and prints the current value of count, which is 5. The final timer prints the count after 2 seconds, also 5.