setTimeout and clearTimeout in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to schedule a function to run after 2 seconds.

Node.js

setTimeout(() => { console.log('Hello!'); }, [1]);

Drag options to blanks, or click blank then click option'

A2000

B2

C0.002

D20

Attempts:

3 left

💡 Hint

Common Mistakes

Using 2 instead of 2000 for 2 seconds delay.

Using 0.002 which is 2 microseconds, too short.

The setTimeout delay is in milliseconds, so 2000 means 2 seconds.

2fill in blank

medium

Complete the code to cancel a scheduled timeout using its ID.

Node.js

const timerId = setTimeout(() => { console.log('Run later'); }, 3000);
clearTimeout([1]);

Drag options to blanks, or click blank then click option'

AsetTimeout

B3000

Cnull

DtimerId

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the delay time (3000) instead of the timer ID.

Passing null or the function name.

You must pass the timer ID returned by setTimeout to clearTimeout to cancel it.

3fill in blank

hard

Fix the error in the code to properly cancel the timeout.

Node.js

let id = setTimeout(() => console.log('Oops'), 5000);
clearTimeout([1]);

Drag options to blanks, or click blank then click option'

AtimeoutId

B5000

Cid

DsetTimeout

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong variable name like timeoutId.

Passing the delay time (5000) instead of the ID.

The variable id holds the timer ID and must be passed to clearTimeout.

4fill in blank

hard

Fill both blanks to schedule a message and then cancel it before it runs.

Node.js

const [1] = setTimeout(() => console.log('Bye!'), 4000);
[2]([1]);

Drag options to blanks, or click blank then click option'

Atimer

BclearTimeout

CsetTimeout

Dtimeout

Attempts:

3 left

💡 Hint

Common Mistakes

Using setTimeout instead of clearTimeout to cancel.

Not storing the timer ID in a variable.

Store the timer ID in timer and pass it to clearTimeout to cancel.

5fill in blank

hard

Fill all three blanks to create a timeout, cancel it, and then schedule a new one.

Node.js

const [1] = setTimeout(() => console.log('First'), 3000);
[2]([1]);
const [3] = setTimeout(() => console.log('Second'), 3000);

Drag options to blanks, or click blank then click option'

AfirstTimer

BclearTimeout

CsecondTimer

DsetTimeout

Attempts:

3 left

💡 Hint

Common Mistakes

Not cancelling the first timer before creating the second.

Using the same variable name for both timers.

Use firstTimer to store the first timeout, cancel it with clearTimeout, then create secondTimer for the new timeout.

Practice

1. What does the setTimeout function do in Node.js?

easy

A. Stops a running function immediately

B. Runs a function repeatedly at fixed intervals

C. Schedules a function to run only when the program ends

D. Runs a function once after a specified delay

Solution

1. Step 1: Understand setTimeout purpose

   setTimeout schedules a function to run once after a delay in milliseconds.

2. Step 2: Compare options with definition

   Only Runs a function once after a specified delay matches this behavior. Runs a function repeatedly at fixed intervals describes setInterval, the option about stopping a running function immediately is incorrect, and the option about scheduling when the program ends is incorrect.

3. Final Answer:

   Runs a function once after a specified delay -> Option D

4. Quick Check:

   setTimeout = run once after delay [OK]

Hint: Remember: setTimeout runs once after delay [OK]

Common Mistakes:

• Confusing setTimeout with setInterval
• Thinking setTimeout repeats automatically
• Believing setTimeout stops functions

2. Which of the following is the correct syntax to cancel a timeout set by setTimeout?

easy

A. stopTimeout(timeoutId);

B. cancelTimeout(timeoutId);

C. clearTimeout(timeoutId);

D. clearInterval(timeoutId);

Solution

1. Step 1: Recall the function to cancel setTimeout

   The correct function to cancel a timeout is clearTimeout with the timeout ID.

2. Step 2: Check each option's validity

   Only clearTimeout(timeoutId); uses the correct function name. cancelTimeout and stopTimeout do not exist. clearInterval(timeoutId); is for intervals, not timeouts.

3. Final Answer:

   clearTimeout(timeoutId); -> Option C

4. Quick Check:

   Cancel timeout = clearTimeout [OK]

Hint: Use clearTimeout with the ID from setTimeout [OK]

Common Mistakes:

• Using clearInterval to cancel setTimeout
• Using non-existent functions like cancelTimeout
• Not passing the timeout ID to clearTimeout

3. What will be the output of the following code?

const id = setTimeout(() => console.log('Hello'), 1000);
clearTimeout(id);
console.log('Done');

medium

A. Done

B. Hello\nDone

C. Hello

D. No output

Solution

1. Step 1: Analyze setTimeout and clearTimeout usage

   The timeout is set to print 'Hello' after 1 second, but immediately cleared with clearTimeout(id).

2. Step 2: Determine what prints immediately

   The console.log('Done') runs immediately, so only 'Done' is printed.

3. Final Answer:

   Done -> Option A

4. Quick Check:

   clearTimeout cancels delayed output [OK]

Hint: clearTimeout stops delayed code; immediate logs still run [OK]

Common Mistakes:

• Expecting 'Hello' to print after clearTimeout
• Thinking both 'Hello' and 'Done' print
• Confusing order of console logs

4. Identify the error in this code snippet:

const timer = setTimeout(() => console.log('Run'), 2000);
clearTimeout(timer);
clearTimeout(timer);

medium

A. No error; calling clearTimeout multiple times is safe

B. setTimeout callback will still run despite clearTimeout

C. Missing parentheses in clearTimeout calls

D. Calling clearTimeout twice causes an error

Solution

1. Step 1: Understand clearTimeout behavior

   Calling clearTimeout multiple times on the same ID does not cause errors; it safely ignores subsequent calls.

2. Step 2: Check syntax and callback execution

   Syntax is correct, and the callback will not run because the timeout was cleared.

3. Final Answer:

   No error; calling clearTimeout multiple times is safe -> Option A

4. Quick Check:

   Multiple clearTimeout calls are safe [OK]

Hint: clearTimeout can be called repeatedly without error [OK]

Common Mistakes:

• Thinking multiple clearTimeout calls cause errors
• Believing callback runs after clearTimeout
• Confusing syntax with missing parentheses

5. You want to print "Start", then after 2 seconds print "Middle", but if a user clicks a button before 2 seconds, cancel "Middle" and print "Cancelled" immediately. Which code snippet correctly implements this behavior? Options: A)

console.log('Start');
const id = setTimeout(() => console.log('Middle'), 2000);
button.onclick = () => {
  clearTimeout(id);
  console.log('Cancelled');
};

B)

console.log('Start');
setTimeout(() => console.log('Middle'), 2000);
button.onclick = () => {
  clearTimeout();
  console.log('Cancelled');
};

C)

console.log('Start');
const id = setTimeout(() => console.log('Middle'), 2000);
button.onclick = () => {
  console.log('Cancelled');
  clearTimeout();
};

D)

console.log('Start');
const id = setTimeout(() => console.log('Middle'), 2000);
button.onclick = () => {
  clearInterval(id);
  console.log('Cancelled');
};

hard

A. clearTimeout called without ID, so timeout not canceled

B. Correctly cancels timeout and prints Cancelled on click

C. Prints Cancelled then calls clearTimeout without ID, so timeout not canceled

D. Uses clearInterval instead of clearTimeout, so fails

Solution

1. Step 1: Check timeout setup and cancellation

   Correctly cancels timeout and prints Cancelled on click, which stores the timeout ID and uses it in clearTimeout inside the click handler, correctly canceling the delayed print.

2. Step 2: Verify order of console logs and function calls

   Correctly cancels timeout and prints Cancelled on click prints 'Start' immediately, then 'Cancelled' if clicked before 2 seconds, preventing 'Middle' from printing.

3. Step 3: Analyze other options for errors

   The second option does not store the timeout ID and calls clearTimeout() without an argument (no effect). The third option stores the ID but calls clearTimeout() without passing the ID (no effect). The fourth option uses clearInterval(id), which cannot cancel a timeout.

4. Final Answer:

   Correctly cancels timeout and prints Cancelled on click -> Option B

5. Quick Check:

   Use clearTimeout with stored ID to cancel delayed action [OK]

Hint: Store timeout ID and clearTimeout with it on event [OK]

Common Mistakes:

• Not passing timeout ID to clearTimeout
• Using clearInterval instead of clearTimeout
• Calling clearTimeout without arguments