useCallback hook in React - Mini Project: Build & Apply

Using the useCallback Hook in React

📖 Scenario: You are building a simple React app that counts clicks on two buttons. You want to make sure the click handler functions do not get recreated unnecessarily to keep your app efficient.

🎯 Goal: Build a React component with two buttons that share a click handler function memoized using the useCallback hook.

📋 What You'll Learn

Create a state variable count initialized to 0

Create a function handleClick that increases count by 1

Use the useCallback hook to memoize handleClick with count as dependency

Render two buttons that both use handleClick as their onClick handler

Display the current count value in a <p> tag

💡 Why This Matters

🌍 Real World

Using useCallback helps keep React apps fast by preventing unnecessary function recreations, especially when passing functions to child components.

💼 Career

Understanding useCallback is important for React developers to write efficient, maintainable code and improve app performance.

1

Set up state with useState

Import useState from React and create a state variable called count initialized to 0 inside a functional component named ClickCounter.

React

import React, { useState } from 'react';

export default function ClickCounter() {
  // Create state variable count with initial value 0
  // Your code here
}

Hint

Use const [count, setCount] = useState(0); inside the component.

2

Create the click handler function

Inside the ClickCounter component, create a function called handleClick that calls setCount to increase count by 1.

React

import React, { useState } from 'react';

export default function ClickCounter() {
  const [count, setCount] = useState(0);

  // Create handleClick function to increase count by 1
  // Your code here
}

Hint

Define handleClick as a function that calls setCount(count + 1).

3

Memoize the click handler with useCallback

Import useCallback from React and wrap the handleClick function with useCallback, passing count as a dependency.

React

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

export default function ClickCounter() {
  const [count, setCount] = useState(0);

  // Wrap handleClick with useCallback and count dependency
  // Your code here
}

Hint

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

4

Render buttons and display count

Render two <button> elements inside the component. Both buttons should use handleClick as their onClick handler. Also render a <p> tag that shows the current count value.

React

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

export default function ClickCounter() {
  const [count, setCount] = useState(0);

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

  // Render two buttons with onClick=handleClick and a  showing count
  // Your code here
}

Hint

Use JSX to render two buttons with onClick={handleClick} and a paragraph showing Count: {count}.

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