Unmounting phase in React - Mini Project: Build & Apply

React Component Unmounting Phase

📖 Scenario: You are building a simple React app that shows a message when a component is visible. When the component is removed from the screen, you want to run some cleanup code to log a message.

🎯 Goal: Create a React functional component that displays a message. Add a button to toggle showing or hiding this component. Use the unmounting phase to log a message when the component is removed.

📋 What You'll Learn

Create a React functional component called Message that returns a <div> with the text 'Hello, I am here!'.

Create a state variable called showMessage in the main component to control if Message is shown.

Add a button that toggles showMessage between true and false.

Use useEffect with a cleanup function inside Message to log 'Message component is unmounting' when it is removed.

💡 Why This Matters

🌍 Real World

React components often need to clean up resources like timers or subscriptions when they are removed from the screen to avoid bugs or memory leaks.

💼 Career

Understanding the unmounting phase is essential for React developers to manage component lifecycle and resource cleanup properly.

1

Create the Message component

Create a React functional component called Message that returns a <div> with the text 'Hello, I am here!'.

React

import React from 'react';

// Create Message component here

export default function App() {
  return null;
}

Hint

Define a function named Message that returns a <div> with the exact text.

2

Add state to toggle Message visibility

In the App component, create a state variable called showMessage using useState and set its initial value to true.

React

import React, { useState } from 'react';

function Message() {
  return Hello, I am here!;
}

export default function App() {
  const [showMessage, setShowMessage] = useState(true);
  // Your code here
}

Hint

Import useState and create showMessage with initial value true.

3

Add button to toggle Message display

Inside the App component's return, add a <button> that toggles showMessage between true and false when clicked. Also, conditionally render the Message component only when showMessage is true.

React

import React, { useState } from 'react';

function Message() {
  return Hello, I am here!;
}

export default function App() {
  const [showMessage, setShowMessage] = useState(true);
  return (
    
      {/* Add button and conditional Message here */}
    
  );
}

Hint

Add a button with an onClick that flips showMessage. Render <Message /> only if showMessage is true.

4

Add cleanup in Message unmounting phase

Inside the Message component, use useEffect to add a cleanup function that logs 'Message component is unmounting' when the component is removed from the screen.

React

import React, { useState, useEffect } from 'react';

function Message() {
  // Add useEffect with cleanup here
  return Hello, I am here!;
}

export default function App() {
  const [showMessage, setShowMessage] = useState(true);
  return (
    
       setShowMessage(!showMessage)}>
        Toggle Message
      
      {showMessage && }
    
  );
}

Hint

Use useEffect with an empty dependency array and return a cleanup function that logs the message.

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