useCallback optimization in React Native - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the useCallback hook from React.

React Native

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

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

AuseState

BuseEffect

CuseCallback

DuseRef

Attempts:

3 left

💡 Hint

Common Mistakes

Importing useState instead of useCallback

Forgetting to import useCallback

The useCallback hook is imported from React to memoize functions and optimize performance.

2fill in blank

medium

Complete the code to memoize the handlePress function using useCallback with an empty dependency array.

React Native

const handlePress = [1](() => {
  console.log('Button pressed');
}, []);

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

AuseCallback

BuseEffect

CuseMemo

DuseState

Attempts:

3 left

💡 Hint

Common Mistakes

Using useEffect instead of useCallback

Not passing the dependency array

useCallback memoizes the function so it doesn't get recreated on every render.

3fill in blank

hard

Fix the error in the dependency array to correctly update the memoized function when count changes.

React Native

const increment = useCallback(() => {
  setCount(count + 1);
}, [[1]]);

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

Acount

Bincrement

CsetCount

DuseCallback

Attempts:

3 left

💡 Hint

Common Mistakes

Putting setCount in dependencies instead of count

Leaving dependency array empty

The dependency array must include count so the function updates when count changes.

4fill in blank

hard

Fill both blanks to create a memoized callback that uses userId inside and includes it in dependencies.

React Native

const fetchUserData = useCallback(() => {
  fetchData([1]);
}, [[2]]);

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

AuserId

BfetchData

Cuser

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names

Putting fetchData instead of userId in dependencies

The callback uses userId inside, so userId must be included in the dependency array to avoid stale closures. fetchData is stable.

5fill in blank

hard

Fill all three blanks to create a memoized function that toggles a boolean state and depends on isActive.

React Native

const toggleActive = useCallback(() => {
  [1](![2]);
}, [[3]]);

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

AsetIsActive

BisActive

CsetActive

DisActiveState

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong setter function name

Putting setIsActive in dependencies instead of isActive

The function calls setIsActive with the opposite of isActive. The dependency array must include isActive so the function uses the current value and avoids stale closures. setIsActive is stable.

Practice

1. What is the main purpose of useCallback in React Native?

easy

A. To create new state variables

B. To prevent functions from being recreated on every render

C. To style components dynamically

D. To fetch data from an API

Solution

1. Step 1: Understand function recreation in React

   Functions inside components are recreated on every render by default, which can cause unnecessary work.

2. Step 2: Role of useCallback

   useCallback memoizes the function, so it keeps the same reference unless dependencies change.

3. Final Answer:

   To prevent functions from being recreated on every render -> Option B

4. Quick Check:

   useCallback = prevent function recreation [OK]

Hint: useCallback keeps functions stable between renders [OK]

Common Mistakes:

• Thinking useCallback creates state
• Confusing useCallback with styling
• Assuming useCallback fetches data

2. Which of the following is the correct syntax to use useCallback?

easy

A. const memoizedFn = useCallback(doSomething, dependency);

B. const memoizedFn = useCallback(() => { doSomething(); });

C. const memoizedFn = useCallback(() => { doSomething(); }, [dependency]);

D. const memoizedFn = useCallback(doSomething, [dependency]);

Solution

1. Step 1: Check useCallback parameters

   useCallback takes a function and an array of dependencies as parameters.

2. Step 2: Validate correct syntax

   const memoizedFn = useCallback(() => { doSomething(); }, [dependency]); correctly passes an arrow function and an array of dependencies.

3. Final Answer:

   const memoizedFn = useCallback(() => { doSomething(); }, [dependency]); -> Option C

4. Quick Check:

   useCallback syntax = function + [dependencies] [OK]

Hint: useCallback needs a function and dependency array [OK]

Common Mistakes:

• Omitting dependency array
• Passing dependencies without array brackets
• Passing function reference without wrapping in arrow function

3. What will be the output of the following code snippet?

const [count, setCount] = useState(0);
const increment = useCallback(() => setCount(count + 1), []);

increment();
increment();
console.log(count);

medium

A. 2

B. Error

C. 1

D. 0

Solution

1. Step 1: Analyze useCallback dependencies

   The callback uses count but has an empty dependency array, so it captures the initial value 0.

2. Step 2: Effect of calling increment()

   Calling increment() calls setCount(0 + 1) twice, but React batches updates and count remains 0 in this synchronous context.

3. Final Answer:

   0 -> Option D

4. Quick Check:

   Empty dependencies cause stale count = 0 [OK]

Hint: Empty deps cause stale closure values [OK]

Common Mistakes:

• Assuming count updates immediately
• Ignoring stale closure from empty deps
• Expecting increment to add twice

4. Identify the issue in this code and how to fix it:

const [value, setValue] = useState(0);
const updateValue = useCallback(() => {
  setValue(value + 1);
}, []);

medium

A. Missing dependency 'value' in useCallback array; add [value]

B. setValue should be outside useCallback

C. useCallback should not be used here

D. No issue; code is correct

Solution

1. Step 1: Check dependencies in useCallback

   The callback uses value but the dependency array is empty, causing stale closure.

2. Step 2: Fix by adding dependency

   Add value to the dependency array to update the callback when value changes.

3. Final Answer:

   Missing dependency 'value' in useCallback array; add [value] -> Option A

4. Quick Check:

   Dependencies must include all used variables [OK]

Hint: Always include variables used inside useCallback deps [OK]

Common Mistakes:

• Leaving dependency array empty
• Removing useCallback instead of fixing deps
• Misplacing setValue call

5. You have a parent component passing a callback to a child. The callback depends on multiple state variables. How should you optimize it with useCallback to avoid unnecessary re-renders of the child?

hard

A. Wrap the callback with useCallback including all dependent state variables in the dependency array

B. Wrap the callback with useCallback but leave dependency array empty

C. Do not use useCallback; define the callback inline in the child

D. Use useMemo instead of useCallback for the callback

Solution

1. Step 1: Understand callback dependencies

   The callback depends on multiple state variables, so it must update when any of them change.

2. Step 2: Use useCallback with all dependencies

   Include all dependent state variables in the dependency array to keep the callback updated and stable.

3. Step 3: Avoid empty dependency array

   Leaving it empty causes stale values and bugs; defining inline causes new function each render.

4. Final Answer:

   Wrap the callback with useCallback including all dependent state variables in the dependency array -> Option A

5. Quick Check:

   All dependencies in useCallback deps = stable, updated callback [OK]

Hint: Include all used states in useCallback deps to optimize [OK]

Common Mistakes:

• Leaving deps empty causing stale closures
• Defining callback inline causing re-renders
• Confusing useMemo with useCallback