Multiple effects in a component in React - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the React hook used for side effects.

React

import React, { [1] } from 'react';

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

AuseState

BuseEffect

CuseRef

DuseContext

Attempts:

3 left

💡 Hint

Common Mistakes

Using useState instead of useEffect for side effects.

Forgetting to import useEffect.

The useEffect hook lets you perform side effects in React components.

2fill in blank

medium

Complete the code to add a side effect that logs 'Hello' once when the component mounts.

React

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

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

A[]

B[true]

C[null]

D[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving out the dependency array causes the effect to run after every render.

Using non-empty arrays causes repeated runs.

An empty dependency array [] makes the effect run only once after the first render.

3fill in blank

hard

Fix the error in the effect that should update the document title when count changes.

React

useEffect(() => { document.title = `Count: ${count}`; }, [1]);

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

A[count]

Bcount

C[]

D[setCount]

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-array dependency causes errors.

Leaving the dependency array empty causes stale updates.

The effect depends on count, so it must be in the dependency array to update correctly.

4fill in blank

hard

Fill both blanks to create two effects: one logs on mount, the other logs count changes.

React

useEffect(() => { console.log('Mounted'); }, [1]);
useEffect(() => { console.log(count); }, [2]);

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

A[]

B[count]

C[true]

D[null]

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same dependency array for both effects.

Forgetting to include count in the second effect's dependencies.

The first effect runs once on mount with an empty array. The second runs when count changes.

5fill in blank

hard

Fill all three blanks to create effects: one on mount, one on count change, and one on user change.

React

useEffect(() => { console.log('Start'); }, [1]);
useEffect(() => { console.log(count); }, [2]);
useEffect(() => { console.log(user); }, [3]);

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

A[]

B[count]

C[user]

D[true]

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same dependency array for all effects.

Leaving out dependencies causing stale or missing updates.

The first effect runs once on mount. The second runs when count changes. The third runs when user changes.

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