setInterval and clearInterval in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Node.js code output?

Consider the following code snippet using setInterval and clearInterval. What will be printed to the console?

Node.js

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

A 1\n2\n3\n4 (then stops)

B 1\n2\n3 (then stops)

C 1\n2 (then stops)

D No output, code throws an error

💡 Hint

Think about when clearInterval stops the repeated action.

The interval runs every 1000ms, increasing count and printing it. When count reaches 3, clearInterval stops further calls, so output is 1, 2, 3 then stops.

📝 Syntax

intermediate

Which option correctly cancels a repeating timer?

You want to stop a repeating timer created by setInterval. Which of the following code snippets correctly cancels it?

Node.js

const timer = setInterval(() => console.log('tick'), 500);

A cancelInterval(timer);

B clearTimeout(timer);

C stopInterval(timer);

D clearInterval(timer);

💡 Hint

Remember the function name to stop intervals is similar to the one to start them.

clearInterval is the correct function to stop a timer created by setInterval. clearTimeout is for setTimeout. The others are invalid functions.

❓ state_output

advanced

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

Analyze the code below. What will be the value of counter after 3500 milliseconds?

Node.js

let counter = 0;
const id = setInterval(() => {
  counter += 2;
  if (counter >= 6) {
    clearInterval(id);
  }
}, 1000);

A 6

B 8

C 4

D 0

💡 Hint

Count how many times the interval callback runs before stopping.

The interval runs every 1000ms, adding 2 each time. After 3 runs (at 3000ms), counter is 6 and the interval stops. At 3500ms, counter remains 6.

🔧 Debug

advanced

Why does this code print odd numbers (1, 3, 5...) repeatedly?

Look at this code snippet. It prints '1', '3', '5', etc. every second instead of sequential numbers. What is the cause?

Node.js

let i = 0;
setInterval(() => {
  i++;
  console.log(i++);
}, 1000);

A Because <code>i++</code> returns the value before increment, causing unexpected output.

B Because <code>i</code> is not initialized properly.

C Because <code>setInterval</code> callback is missing a return statement.

D Because <code>console.log</code> is called outside the interval.

💡 Hint

Check how the increment operator works when used inside console.log.

i++ is the post-increment operator, which returns the value before incrementing. The standalone i++ increments i but discards the old value. Then console.log(i++) prints the value before the second increment, resulting in 1, 3, 5, etc. The other options are incorrect.

🧠 Conceptual

expert

What happens if you call clearInterval with an invalid ID?

In Node.js, if you call clearInterval with a value that is not a valid interval ID, what will happen?

A The program crashes and exits.

B A runtime error is thrown immediately.

C Nothing happens; the call is ignored without error.

D The last interval created is cleared instead.

💡 Hint

Think about how Node.js handles invalid timer IDs safely.

Calling clearInterval with an invalid or non-existent ID does nothing and does not throw an error. This design prevents crashes from accidental calls.