Unmounting phase in React - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to run cleanup when the component unmounts.

React

useEffect(() => {
  return () => {
    [1];
  };
}, []);

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

AfetchData()

BsetState(true)

Cconsole.log('Cleanup')

DrenderComponent()

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to update state inside cleanup causes errors.

Not returning a function inside useEffect for cleanup.

The cleanup function inside useEffect runs when the component unmounts. Logging 'Cleanup' shows this behavior.

2fill in blank

medium

Complete the code to clean up an event listener on unmount.

React

useEffect(() => {
  window.addEventListener('resize', handleResize);
  return () => {
    window.[1]('resize', handleResize);
  };
}, []);

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

AaddEventListener

BattachEvent

CdispatchEvent

DremoveEventListener

Attempts:

3 left

💡 Hint

Common Mistakes

Using addEventListener again in cleanup causes duplicate listeners.

Forgetting to remove event listeners leads to bugs.

To avoid memory leaks, remove event listeners in the cleanup function using removeEventListener.

3fill in blank

hard

Fix the error in the cleanup function to avoid memory leaks.

React

useEffect(() => {
  const timer = setTimeout(() => {
    console.log('Timer done');
  }, 1000);
  return () => {
    clearTimeout([1]);
  };
}, []);

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

AclearTimeout

Btimer

CsetTimeout

Dtimeout

Attempts:

3 left

💡 Hint

Common Mistakes

Using clearInterval instead of clearTimeout for timeouts.

Passing the wrong variable to clearTimeout.

The timer ID returned by setTimeout must be cleared with clearTimeout using the same variable timer.

4fill in blank

hard

Fill both blanks to properly clean up a subscription on unmount.

React

useEffect(() => {
  const subscription = api.subscribe(data => setData(data));
  return () => {
    subscription.[1]();
    console.[2]('Unsubscribed');
  };
}, []);

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

Aunsubscribe

Blog

Cwarn

Dsubscribe

Attempts:

3 left

💡 Hint

Common Mistakes

Calling subscribe instead of unsubscribe in cleanup.

Using console.warn instead of console.log without reason.

Call unsubscribe to stop the subscription and log a message with console.log during cleanup.

5fill in blank

hard

Fill all three blanks to clean up a WebSocket connection on unmount.

React

useEffect(() => {
  const socket = new WebSocket('wss://example.com');
  socket.onmessage = event => setMessage(event.data);
  return () => {
    socket.[1]();
    socket.[2] = null;
    console.[3]('WebSocket closed');
  };
}, []);

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

Aclose

Bonmessage

Clog

Dsend

Attempts:

3 left

💡 Hint

Common Mistakes

Calling send instead of close to end the connection.

Not removing the onmessage handler causes memory leaks.

Call close() to close the WebSocket, set onmessage to null to remove the handler, and log a message with console.log.

Practice

1. What is the main purpose of the unmounting phase in React components?

easy

A. To initialize state when the component first appears

B. To update the component when props change

C. To render the component's UI on the screen

D. To clean up resources like timers or event listeners before the component is removed

Solution

1. Step 1: Understand the unmounting phase

   The unmounting phase happens when a component is about to be removed from the screen.

2. Step 2: Identify the purpose of cleanup

   During unmounting, React lets you clean up things like timers or event listeners to avoid bugs and memory leaks.

3. Final Answer:

   To clean up resources like timers or event listeners before the component is removed -> Option D

4. Quick Check:

   Unmounting phase = cleanup before removal [OK]

Hint: Unmounting phase means cleaning up before component disappears [OK]

Common Mistakes:

• Confusing unmounting with mounting or updating phases
• Thinking unmounting initializes state
• Believing unmounting is for rendering UI

2. Which of the following is the correct way to add a cleanup function inside a React useEffect hook?

easy

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

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

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

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

Solution

1. Step 1: Recall useEffect cleanup syntax

   The cleanup function must be returned from the function passed to useEffect.

2. Step 2: Check each option's syntax

   useEffect(() => { return () => { clearTimeout(timer); }; }, []); correctly returns a function that calls clearTimeout. Others either call clearTimeout immediately or do not return a function.

3. Final Answer:

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

4. Quick Check:

   Cleanup function is returned inside useEffect [OK]

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

Common Mistakes:

• Calling cleanup directly instead of returning a function
• Not returning anything from useEffect
• Returning the cleanup call instead of a function

3. What will be logged to the console when the following component unmounts?

import React, { useEffect } from 'react';

function Timer() {
  useEffect(() => {
    const id = setInterval(() => console.log('Tick'), 1000);
    return () => {
      clearInterval(id);
      console.log('Cleanup');
    };
  }, []);
  return <div>Timer running</div>;
}

medium

A. Nothing is logged on unmount

B. 'Tick' then 'Cleanup' immediately on unmount

C. 'Cleanup' logged once on unmount

D. Multiple "Tick" logs after unmount

Solution

1. Step 1: Understand the effect and cleanup

   The effect sets an interval that logs 'Tick' every second. The cleanup clears the interval and logs 'Cleanup'.

2. Step 2: What happens on unmount?

   When the component unmounts, the cleanup runs once, clearing the interval and logging 'Cleanup'. No more 'Tick' logs happen after unmount.

3. Final Answer:

   'Cleanup' logged once on unmount -> Option C

4. Quick Check:

   Cleanup logs once on unmount [OK]

Hint: Cleanup runs once on unmount, stopping intervals [OK]

Common Mistakes:

• Thinking 'Tick' logs after unmount
• Expecting no logs on unmount
• Confusing cleanup timing with effect start

4. Identify the error in this React component related to unmounting:

import React, { useEffect } from 'react';

function Listener() {
  useEffect(() => {
    window.addEventListener('resize', () => console.log('Resized'));
  }, []);
  return <div>Listening</div>;
}

medium

A. The event listener callback should be named function, not arrow function

B. Missing cleanup function to remove the event listener on unmount

C. useEffect should not have an empty dependency array

D. Event listener should be added inside the return statement

Solution

1. Step 1: Check event listener setup

   The component adds a resize event listener on mount but does not remove it on unmount.

2. Step 2: Identify missing cleanup

   Without cleanup, the listener stays active after unmount, causing memory leaks or bugs.

3. Final Answer:

   Missing cleanup function to remove the event listener on unmount -> Option B

4. Quick Check:

   Always remove listeners in cleanup [OK]

Hint: Always remove event listeners in cleanup function [OK]

Common Mistakes:

• Forgetting to remove event listeners on unmount
• Thinking arrow functions can't be event callbacks
• Misusing dependency arrays in useEffect

5. You have a React component that sets up a WebSocket connection in useEffect. Which is the best way to ensure the connection closes properly when the component unmounts?

useEffect(() => {
  const socket = new WebSocket('wss://example.com');
  socket.onmessage = (event) => console.log(event.data);
  // What should go here?
}, []);

hard

A. Return a cleanup function that calls socket.close() to close the connection

B. Call socket.close() immediately after setting onmessage

C. Do nothing; WebSocket closes automatically on unmount

D. Set socket = null in the cleanup function

Solution

1. Step 1: Understand WebSocket lifecycle in React

   WebSocket connections stay open until explicitly closed. They do not close automatically on component unmount.

2. Step 2: Implement cleanup to close connection

   Returning a cleanup function that calls socket.close() ensures the connection closes when the component unmounts, preventing resource leaks.

3. Final Answer:

   Return a cleanup function that calls socket.close() to close the connection -> Option A

4. Quick Check:

   Close WebSocket in cleanup function [OK]

Hint: Close WebSocket inside cleanup function returned by useEffect [OK]

Common Mistakes:

• Closing WebSocket immediately instead of on unmount
• Assuming WebSocket closes automatically
• Setting socket to null without closing