Cleanup function in React - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a cleanup function inside useEffect.

React

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

  return [1];
}, []);

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

AclearTimeout(timer)

B() => clearTimeout(timer)

C() => setTimeout(timer)

DclearInterval(timer)

Attempts:

3 left

💡 Hint

Common Mistakes

Returning clearTimeout(timer) directly instead of a function.

Using clearInterval instead of clearTimeout.

The cleanup function must be a function that clears the timeout using clearTimeout(timer). So we return a function () => clearTimeout(timer).

2fill in blank

medium

Complete the code to clean up an event listener in useEffect.

React

useEffect(() => {
  function onResize() {
    console.log('Resized');
  }
  window.addEventListener('resize', onResize);

  return [1];
}, []);

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

Awindow.removeEventListener('resize', onResize)

Bwindow.addEventListener('resize', onResize)

C() => window.removeEventListener('resize', onResize)

D() => window.addEventListener('resize', onResize)

Attempts:

3 left

💡 Hint

Common Mistakes

Calling removeEventListener directly instead of returning a function.

Adding the event listener again in cleanup.

The cleanup function must remove the event listener by returning a function that calls window.removeEventListener('resize', onResize).

3fill in blank

hard

Fix the error in the cleanup function that clears an interval.

React

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

  return [1];
}, []);

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

A() => clearInterval(id)

BclearInterval(id)

C() => clearTimeout(id)

DclearTimeout(id)

Attempts:

3 left

💡 Hint

Common Mistakes

Returning clearInterval(id) directly instead of a function.

Using clearTimeout instead of clearInterval.

The cleanup function must be a function that calls clearInterval(id). Returning clearInterval(id) directly causes an error because cleanup must be a function.

4fill in blank

hard

Fill both blanks to add and clean up a window scroll event listener.

React

useEffect(() => {
  function handleScroll() {
    console.log('Scrolled');
  }
  window.[1]('scroll', handleScroll);

  return () => {
    window.[2]('scroll', handleScroll);
  };
}, []);

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

AaddEventListener

BremoveEventListener

CattachEvent

DdetachEvent

Attempts:

3 left

💡 Hint

Common Mistakes

Using attachEvent or detachEvent which are old and not standard.

Mixing addEventListener and removeEventListener incorrectly.

We add the event listener with addEventListener and clean it up with removeEventListener.

5fill in blank

hard

Fill all three blanks to create a cleanup function that cancels a subscription and clears a timeout.

React

useEffect(() => {
  const subscription = api.subscribe();
  const timeoutId = setTimeout(() => {
    console.log('Timeout done');
  }, 2000);

  return () => {
    subscription.[1]();
    clearTimeout([2]);
    console.log([3]);
  };
}, []);

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

Aunsubscribe

BtimeoutId

C'Cleanup complete'

Dsubscribe

Attempts:

3 left

💡 Hint

Common Mistakes

Calling subscribe instead of unsubscribe.

Using wrong variable name for timeout.

Not returning a function for cleanup.

The cleanup calls subscription.unsubscribe(), clears the timeout with clearTimeout(timeoutId), and logs a message.

Practice

1. What is the main purpose of a cleanup function in a React useEffect hook?

easy

A. To initialize state variables before rendering

B. To remove timers, listeners, or subscriptions when the component unmounts or before the effect reruns

C. To fetch data from an API after the component mounts

D. To render JSX elements conditionally

Solution

1. Step 1: Understand the role of cleanup functions

   Cleanup functions are used to tidy up side effects like timers or event listeners to prevent memory leaks.

2. Step 2: Identify when cleanup runs

   Cleanup runs when a component unmounts or before the effect runs again, ensuring resources are freed.

3. Final Answer:

   To remove timers, listeners, or subscriptions when the component unmounts or before the effect reruns -> Option B

4. Quick Check:

   Cleanup function purpose = Remove resources [OK]

Hint: Cleanup frees resources when component unmounts or effect reruns [OK]

Common Mistakes:

• Thinking cleanup initializes state
• Confusing cleanup with data fetching
• Assuming cleanup runs after every render

2. Which of the following is the correct syntax to define a cleanup function inside useEffect?

easy

A. useEffect(() => { () => { clearTimeout(timer); }; }, []);

B. useEffect(() => { clearTimeout(timer); return; }, []);

C. useEffect(() => { return () => { clearTimeout(timer); }; }, []);

D. useEffect(() => { return clearTimeout(timer); }, []);

Solution

1. Step 1: Recognize cleanup function syntax

   The cleanup function must be returned from the effect callback as a function.

2. Step 2: Check each option

   useEffect(() => { return () => { clearTimeout(timer); }; }, []); correctly returns a function that calls clearTimeout. Others either don't return a function or return the wrong value.

3. Final Answer:

   useEffect(() => { return () => { clearTimeout(timer); }; }, []); -> Option C

4. Quick Check:

   Cleanup function = returned function inside useEffect [OK]

Hint: Cleanup must be a returned function inside useEffect [OK]

Common Mistakes:

• Not returning a function from useEffect
• Returning the result of cleanup call instead of a function
• Placing cleanup code outside the returned function

3. What will be logged to the console when the component unmounts in this code?

function Timer() {
  React.useEffect(() => {
    const id = setInterval(() => console.log('tick'), 1000);
    return () => {
      clearInterval(id);
      console.log('cleanup');
    };
  }, []);
  return null;
}

medium

A. Nothing, because the component never unmounts

B. 'tick' once and no 'cleanup'

C. 'cleanup' every second and no 'tick'

D. 'tick' every second and then 'cleanup' once when unmounting

Solution

1. Step 1: Understand the effect behavior

   The effect sets an interval that logs 'tick' every second and returns a cleanup function that clears the interval and logs 'cleanup'.

2. Step 2: Trace what happens on unmount

   When the component unmounts, the cleanup runs once, clearing the interval and logging 'cleanup'.

3. Final Answer:

   'tick' every second and then 'cleanup' once when unmounting -> Option D

4. Quick Check:

   Interval logs 'tick'; cleanup logs 'cleanup' once [OK]

Hint: Cleanup logs once on unmount, interval logs repeatedly [OK]

Common Mistakes:

• Thinking cleanup runs repeatedly
• Confusing interval clearing with immediate stop
• Expecting no logs at all

4. Identify the error in this React effect with cleanup:

useEffect(() => {
  window.addEventListener('resize', handleResize);
  return window.removeEventListener('resize', handleResize);
}, []);

medium

A. The cleanup function should be a function returned, not the result of removeEventListener call

B. The event listener should be added inside the cleanup function

C. The effect should not have a dependency array

D. handleResize should be defined inside useEffect

Solution

1. Step 1: Check cleanup function syntax

   The cleanup must be a function returned from useEffect, not the result of calling removeEventListener immediately.

2. Step 2: Identify correct cleanup pattern

   The cleanup should be: return () => window.removeEventListener('resize', handleResize); to run on unmount.

3. Final Answer:

   The cleanup function should be a function returned, not the result of removeEventListener call -> Option A

4. Quick Check:

   Cleanup must be a returned function [OK]

Hint: Return a function for cleanup, not the cleanup call itself [OK]

Common Mistakes:

• Calling cleanup immediately instead of returning a function
• Adding event listener inside cleanup
• Ignoring dependency array effects

5. You want to subscribe to a WebSocket in a React component and clean up properly. Which code correctly uses a cleanup function to close the socket when the component unmounts or the URL changes?

function SocketComponent({ url }) {
  React.useEffect(() => {
    const socket = new WebSocket(url);
    socket.onmessage = (event) => console.log(event.data);
    // What cleanup code is correct?
  }, [url]);
  return null;
}

hard

A. return () => { socket.close(); };

B. socket.close();

C. return socket.close();

D. return () => socket.onmessage = null;

Solution

1. Step 1: Understand cleanup for WebSocket

   To avoid leaks, the socket must be closed when the component unmounts or URL changes.

2. Step 2: Identify correct cleanup syntax

   The cleanup must be a function returned from useEffect that calls socket.close(). return () => { socket.close(); }; does this correctly.

3. Final Answer:

   return () => { socket.close(); }; -> Option A

4. Quick Check:

   Cleanup returns function closing socket [OK]

Hint: Return a function that closes socket for cleanup [OK]

Common Mistakes:

• Calling close immediately instead of returning a function
• Returning the result of close() instead of a function
• Only nullifying event handlers without closing socket