Multiple effects in a component in React - Mini Project: Build & Apply

Multiple effects in a component

📖 Scenario: You are building a simple React component that shows a user's name and the current window width. You want to update the document title with the user's name and also update the width value whenever the window is resized.

🎯 Goal: Create a React functional component that uses two separate useEffect hooks: one to update the document title with the user's name, and another to listen for window resize events and update the width state.

📋 What You'll Learn

Create a state variable name with initial value 'Alice'.

Create a state variable width with initial value equal to window.innerWidth.

Use one useEffect to update document.title to `User: ${name}` whenever name changes.

Use another useEffect to add a resize event listener on window that updates width state, and clean up the listener on unmount.

Render the name and width values inside <div> elements.

💡 Why This Matters

🌍 Real World

Many React apps need to handle multiple side effects like updating the page title and responding to window size changes separately.

💼 Career

Understanding multiple useEffect hooks is essential for React developers to manage side effects cleanly and avoid bugs.

1

Set up state variables

Create a React functional component called UserInfo. Inside it, create a state variable name with initial value 'Alice' using useState. Also create a state variable width with initial value window.innerWidth using useState.

React

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

function UserInfo() {
  // Create state variables name and width here
  // Your code here
}

Hint

Use const [name, setName] = useState('Alice') and const [width, setWidth] = useState(window.innerWidth) inside the component.

2

Add effect to update document title

Add a useEffect hook inside UserInfo that updates document.title to `User: ${name}` whenever the name state changes.

React

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

function UserInfo() {
  const [name, setName] = useState('Alice');
  const [width, setWidth] = useState(window.innerWidth);

  // Add useEffect to update document.title when name changes
  // Your code here
}

Hint

Use useEffect with a dependency array containing name. Inside, set document.title = `User: ${name}`.

3

Add effect to handle window resize

Add another useEffect hook inside UserInfo that adds a resize event listener on window. The listener should update the width state with window.innerWidth. Also clean up the event listener when the component unmounts.

React

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

function UserInfo() {
  const [name, setName] = useState('Alice');
  const [width, setWidth] = useState(window.innerWidth);

  useEffect(() => {
    document.title = `User: ${name}`;
  }, [name]);

  // Add useEffect to handle window resize and update width
  // Your code here
}

Hint

Inside useEffect, define handleResize function that calls setWidth(window.innerWidth). Add it as a listener to resize event on window. Return a cleanup function that removes the listener.

4

Render the component output

Inside the UserInfo component, return a <div> that contains two <p> elements. The first shows the text Name: {name} and the second shows Window width: {width}.

React

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

function UserInfo() {
  const [name, setName] = useState('Alice');
  const [width, setWidth] = useState(window.innerWidth);

  useEffect(() => {
    document.title = `User: ${name}`;
  }, [name]);

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

  // Return JSX with name and width displayed
  // Your code here
}

Hint

Return a <div> with two <p> elements showing Name: {name} and Window width: {width}.

Practice

1. What is the main benefit of using multiple useEffect hooks in a React component?

easy

A. It combines all side effects into one function for faster execution.

B. It separates different side effects for better readability and maintenance.

C. It prevents the component from rendering multiple times.

D. It automatically optimizes the component's performance.

Solution

1. Step 1: Understand the purpose of multiple useEffect hooks

   Multiple useEffect hooks allow you to separate different side effects logically in a component.

2. Step 2: Recognize the benefit of separation

   This separation makes the component easier to read and maintain because each effect handles a specific task.

3. Final Answer:

   It separates different side effects for better readability and maintenance. -> Option B

4. Quick Check:

   Multiple effects = separation and clarity [OK]

Hint: Multiple effects separate tasks for clarity and easier updates [OK]

Common Mistakes:

• Thinking multiple effects combine tasks into one
• Believing multiple effects prevent re-renders
• Assuming multiple effects auto-optimize performance

2. Which of the following is the correct syntax to use two separate useEffect hooks in a React functional component?

easy

A. useEffect(() => { /* effect 1 */ }, []); useEffect(() => { /* effect 2 */ }, []);

B. useEffect(() => { /* effect 1 */ }, []) useEffect(() => { /* effect 2 */ }, []);

C. useEffect(() => { /* effect 1 */ } useEffect(() => { /* effect 2 */ });

D. useEffect(() => { /* effect 1 */ }, [] && []) useEffect(() => { /* effect 2 */ }, []);

Solution

1. Step 1: Recall correct useEffect syntax

   The useEffect hook takes a function and a dependency array as arguments, separated by a comma.

2. Step 2: Identify correct separation of two hooks

   Two separate useEffect hooks should be called one after another, each with its own function and dependency array, ending with semicolons.

3. Final Answer:

   useEffect(() => { /* effect 1 */ }, []); useEffect(() => { /* effect 2 */ }, []); -> Option A

4. Quick Check:

   Separate hooks with own deps and semicolons [OK]

Hint: Each useEffect needs its own function and dependency array [OK]

Common Mistakes:

• Using commas instead of semicolons between hooks
• Combining dependency arrays incorrectly
• Omitting dependency arrays causing unwanted runs

3. Consider this React component code:

function Example({ count }) {
  useEffect(() => {
    console.log('Effect A:', count);
  }, [count]);

  useEffect(() => {
    console.log('Effect B: component mounted');
  }, []);

  return <div>Count: {count}</div>;
}

What will be logged when the component first mounts with count = 5?

medium

A. Effect B: component mounted only

B. Effect B: component mountedEffect A: 5

C. Effect A: 5 only

D. Effect A: 5Effect B: component mounted

Solution

1. Step 1: Understand useEffect execution order on mount

   All useEffect hooks run after the first render. They run in the order they appear in code.

2. Step 2: Identify the order of console logs

   The first useEffect logs 'Effect A: 5', the second logs 'Effect B: component mounted'. So logs appear in code order.

3. Final Answer:

   Effect A: 5
   Effect B: component mounted -> Option D

4. Quick Check:

   Effects run in code order on mount [OK]

Hint: Effects run in code order after first render [OK]

Common Mistakes:

• Assuming effects run in reverse order
• Thinking effects run before render
• Confusing dependency arrays impact on first run

4. What is wrong with this React component using multiple effects?

function Demo({ value }) {
  useEffect(() => {
    console.log('Effect 1:', value);
  }, [value]);

  useEffect(() => {
    console.log('Effect 2');
  });

  return <div>Value: {value}</div>;
}

medium

A. The first useEffect should not have a dependency array.

B. You cannot use two useEffect hooks in one component.

C. The second useEffect has no dependency array, so it runs after every render.

D. The console.log statements are invalid inside useEffect.

Solution

1. Step 1: Check dependency arrays in useEffect hooks

   The first useEffect has a dependency array [value], so it runs only when value changes.

2. Step 2: Analyze second useEffect without dependency array

   The second useEffect has no dependency array, so it runs after every render, which might be unintended.

3. Final Answer:

   The second useEffect has no dependency array, so it runs after every render. -> Option C

4. Quick Check:

   Missing deps means effect runs every render [OK]

Hint: Always add dependency array to control effect runs [OK]

Common Mistakes:

• Thinking multiple useEffects are not allowed
• Ignoring missing dependency arrays
• Believing console.log is invalid inside useEffect

5. You want to fetch user data when the component mounts and also update the document title whenever the user's name changes. How should you organize your useEffect hooks?

hard

A. Use two separate useEffect hooks: one with empty deps for fetch, one with [user.name] for title.

B. Use one useEffect to fetch data and update title together with both dependencies.

C. Use one useEffect with no dependencies to do both tasks on every render.

D. Use two useEffect hooks both with empty dependency arrays.

Solution

1. Step 1: Separate concerns with multiple useEffect hooks

   Fetching data should happen once on mount, so useEffect with empty dependency array is best.

2. Step 2: Update document title when user.name changes

   Use a separate useEffect with [user.name] dependency to update title only when name changes.

3. Final Answer:

   Use two separate useEffect hooks: one with empty deps for fetch, one with [user.name] for title. -> Option A

4. Quick Check:

   Separate effects by task and dependencies [OK]

Hint: Separate effects by when and why they run [OK]

Common Mistakes:

• Combining unrelated tasks in one effect
• Missing dependencies causing stale updates
• Running fetch on every render unintentionally