useCallback hook in React - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the useCallback hook from React.

React

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

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

AuseState

BuseEffect

CuseRef

DuseCallback

Attempts:

3 left

💡 Hint

Common Mistakes

Importing useState instead of useCallback

Forgetting to import useCallback

Importing useEffect by mistake

The useCallback hook is imported from React to memoize functions.

2fill in blank

medium

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

React

const handleClick = [1](() => {
  console.log('Clicked');
}, []);

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

AuseEffect

BuseMemo

CuseCallback

DuseState

Attempts:

3 left

💡 Hint

Common Mistakes

Using useEffect instead of useCallback

Using useMemo which memoizes values, not functions

Not passing a dependency array

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

3fill in blank

hard

Fix the error in the useCallback dependency array to include the correct dependency.

React

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

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

AsetCount

Bcount

Cincrement

DuseCallback

Attempts:

3 left

💡 Hint

Common Mistakes

Putting setCount in dependencies instead of count

Leaving dependency array empty causing stale closure

Putting the function name itself in dependencies

The count variable is used inside the callback, so it must be in the dependency array.

4fill in blank

hard

Fill both blanks to create a memoized function that depends on 'value' and logs it.

React

const logValue = useCallback(() => {
  console.log([1]);
}, [[2]]);

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

Avalue

Bconsole.log

DlogValue

Attempts:

3 left

💡 Hint

Common Mistakes

Logging console.log instead of value

Not including value in dependency array

Including the function name in dependencies

The function logs value and depends on value to update correctly.

5fill in blank

hard

Fill both blanks to create a memoized function that adds 'a' and 'b' and depends on both.

React

const add = useCallback(() => {
  return [1] + [2];
}, [[1], [2]]);

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

Aa

Bb

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names in the return statement and dependencies

Leaving dependency array empty

Using only one variable in dependencies

The function adds a and b, returns their sum, and depends on both.

Practice

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

easy

A. To memoize a function so it doesn't get recreated on every render unless dependencies change

B. To store a value that persists between renders

C. To fetch data from an API asynchronously

D. To create a new state variable

Solution

1. Step 1: Understand what useCallback does

   useCallback returns a memoized version of a function that only changes if its dependencies change.

2. Step 2: Compare options with this behavior

   Only To memoize a function so it doesn't get recreated on every render unless dependencies change describes memoizing a function to avoid recreating it unnecessarily.

3. Final Answer:

   To memoize a function so it doesn't get recreated on every render unless dependencies change -> Option A

4. Quick Check:

   useCallback = memoize function [OK]

Hint: Remember: useCallback memoizes functions to keep them stable [OK]

Common Mistakes:

• Confusing useCallback with useState
• Thinking useCallback stores values instead of functions
• Assuming useCallback fetches data

2. Which of the following is the correct syntax to use useCallback in a React functional component?

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: Recall the correct useCallback signature

   The hook takes a function and an array of dependencies: useCallback(fn, [deps]).

2. Step 2: Analyze each option

   const memoizedFn = useCallback(() => { doSomething(); }, [dependency]); correctly passes an inline function and dependencies array. const memoizedFn = useCallback(() => { doSomething(); }); misses dependencies array. const memoizedFn = useCallback(doSomething(), [dependency]); calls doSomething() immediately, passing its return value instead of the function. const memoizedFn = useCallback(doSomething, dependency); passes dependency without array brackets, invalid.

3. Final Answer:

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

4. Quick Check:

   useCallback(fn, [deps]) [OK]

Hint: useCallback needs a function and an array of dependencies [OK]

Common Mistakes:

• Omitting the dependencies array
• Passing dependencies without array brackets
• Passing function call instead of function reference

3. Consider this React component code:

const MyComponent = () => {
  const [count, setCount] = React.useState(0);
  const increment = React.useCallback(() => {
    setCount(count + 1);
  }, [count]);
  return <button onClick={increment}>Increment</button>;
};

What happens when the button is clicked multiple times quickly?

medium

A. The count increments correctly each time

B. The code throws an error because of missing dependencies

C. The count increments only once and then stops

D. The count increments but may lag behind due to stale closure

Solution

1. Step 1: Understand the closure inside useCallback

   The increment function uses count from its dependencies. Because count is in the dependency array, the function updates when count changes, but rapid clicks may use a stale value.

2. Step 2: Analyze behavior on quick clicks

   Rapid clicks may cause the function to use an outdated count value due to closure, causing increments to lag behind actual clicks.

3. Final Answer:

   The count increments but may lag behind due to stale closure -> Option D

4. Quick Check:

   useCallback with state dependency can cause stale closures [OK]

Hint: State in useCallback can be stale if updated rapidly [OK]

Common Mistakes:

• Assuming count always updates instantly
• Ignoring closure behavior in hooks
• Thinking useCallback prevents all stale data issues

4. Identify the bug in this code using useCallback:

const MyComponent = () => {
  const [value, setValue] = React.useState(0);
  const handleClick = React.useCallback(() => {
    setValue(value + 1);
  }, []);
  return <button onClick={handleClick}>Click</button>;
};

medium

A. Missing dependency value in the dependency array causes stale state updates

B. Using useCallback without dependencies causes infinite re-renders

C. setValue should not be called inside useCallback

D. The function should not be memoized with useCallback

Solution

1. Step 1: Check the dependency array of useCallback

   The callback uses value but the dependency array is empty, so the function never updates with new value.

2. Step 2: Understand the effect of missing dependencies

   Because value is missing, the callback uses the initial value forever, causing stale increments.

3. Final Answer:

   Missing dependency value in the dependency array causes stale state updates -> Option A

4. Quick Check:

   Dependencies must include all used variables [OK]

Hint: Always include all variables used inside useCallback in dependencies [OK]

Common Mistakes:

• Leaving dependency array empty when variables are used
• Thinking setState functions need dependencies
• Confusing useCallback with useMemo

5. You want to optimize a child component that receives a callback prop to avoid unnecessary re-renders. Which approach correctly uses useCallback to achieve this?

const Parent = () => {
  const [count, setCount] = React.useState(0);

  // Which is correct?
  const handleClick = ???

  return <Child onClick={handleClick} />;
};

hard

A. const handleClick = React.useCallback(() => setCount(count + 1), [count]);

B. const handleClick = React.useCallback(() => setCount(c => c + 1), []);

C. const handleClick = () => setCount(count + 1);

D. const handleClick = React.useCallback(() => setCount(count + 1));

Solution

1. Step 1: Understand the goal to avoid unnecessary re-renders

   We want handleClick to be stable and not change unless needed, so the child doesn't re-render unnecessarily.

2. Step 2: Analyze each option's dependency array and function

   const handleClick = React.useCallback(() => setCount(count + 1), [count]); depends on count, so it changes every time count changes, causing re-renders. const handleClick = React.useCallback(() => setCount(c => c + 1), []); uses functional update c => c + 1 and empty dependencies, so handleClick is stable. const handleClick = () => setCount(count + 1); recreates function every render. const handleClick = React.useCallback(() => setCount(count + 1)); misses dependencies array, which is invalid.

3. Final Answer:

   const handleClick = React.useCallback(() => setCount(c => c + 1), []); -> Option B

4. Quick Check:

   Stable callback with functional update and empty deps = const handleClick = React.useCallback(() => setCount(c => c + 1), []); [OK]

Hint: Use functional updates with empty deps to keep callback stable [OK]

Common Mistakes:

• Including state in dependencies causing new functions each render
• Not using functional updates when deps are empty
• Omitting dependency array causing bugs