Reusing logic with hooks in React - Mini Project: Build & Apply

Reusing Logic with Hooks in React

📖 Scenario: You are building a simple React app that shows a counter. You want to reuse the counting logic in multiple places without repeating code.

🎯 Goal: Create a custom hook called useCounter that manages a counter value and a function to increase it. Then use this hook inside a functional component to display the count and a button to increase it.

📋 What You'll Learn

Create a custom hook named useCounter that returns a count state and an increment function.

Initialize the count state to 0 inside the hook.

Create a functional component named CounterDisplay that uses the useCounter hook.

Render the current count and a button labeled Increase inside CounterDisplay.

Clicking the button should increase the count by 1.

💡 Why This Matters

🌍 Real World

Custom hooks help you reuse logic like counters, form handling, or data fetching across many components without repeating code.

💼 Career

Understanding custom hooks is essential for React developers to write clean, maintainable, and reusable code in real projects.

1

Create the custom hook useCounter with initial count

Create a custom hook named useCounter that uses useState to create a state variable called count initialized to 0.

React

import React, { useState } from 'react';

// Your code here

Hint

Use useState(0) inside useCounter to create count and setCount.

2

Add an increment function inside useCounter

Inside the useCounter hook, create a function called increment that increases count by 1 using setCount. Return increment along with count from the hook.

React

import React, { useState } from 'react';

function useCounter() {
  const [count, setCount] = useState(0);

  // Your code here

  return { count, increment };
}

Hint

Define increment as a function that calls setCount(count + 1). Return it from the hook.

3

Create CounterDisplay component using useCounter

Create a functional component named CounterDisplay. Inside it, call the useCounter hook to get count and increment. Render a <div> with a <p> showing the text Count: {count} and a <button> with the label Increase that calls increment when clicked.

React

import React, { useState } from 'react';

function useCounter() {
  const [count, setCount] = useState(0);

  function increment() {
    setCount(count + 1);
  }

  return { count, increment };
}

// Your code here

Hint

Use object destructuring to get count and increment from useCounter(). Render them inside JSX.

4

Export CounterDisplay as default export

Add a default export statement for the CounterDisplay component at the end of the file.

React

import React, { useState } from 'react';

function useCounter() {
  const [count, setCount] = useState(0);

  function increment() {
    setCount(count + 1);
  }

  return { count, increment };
}

function CounterDisplay() {
  const { count, increment } = useCounter();

  return (
    
      Count: {count}
      Increase
    
  );
}

// Your code here

Hint

Use export default CounterDisplay; to export the component.

Practice

1. What is the main purpose of creating a custom hook in React?

easy

A. To create new HTML elements

B. To reuse stateful logic across multiple components

C. To style components with CSS

D. To replace React components entirely

Solution

1. Step 1: Understand what hooks do

   Hooks let you use React features like state and effects inside functions.

2. Step 2: Identify the role of custom hooks

   Custom hooks let you reuse this logic in many components without repeating code.

3. Final Answer:

   To reuse stateful logic across multiple components -> Option B

4. Quick Check:

   Custom hooks = reuse logic [OK]

Hint: Custom hooks start with 'use' and share logic [OK]

Common Mistakes:

• Thinking hooks create UI elements
• Confusing hooks with styling tools
• Believing hooks replace components

2. Which of the following is the correct way to define a custom hook in React?

easy

A. function useCounter() { const [count, setCount] = useState(0); return [count, setCount]; }

B. function counterHook() { const [count, setCount] = useState(0); return [count, setCount]; }

C. function UseCounter() { const [count, setCount] = useState(0); return [count, setCount]; }

D. function counter() { const [count, setCount] = useState(0); return [count, setCount]; }

Solution

1. Step 1: Recall naming rules for custom hooks

   Custom hooks must start with 'use' to follow React conventions and enable linting.

2. Step 2: Check function names

   Only function useCounter() { const [count, setCount] = useState(0); return [count, setCount]; } starts with 'use' and uses camelCase correctly.

3. Final Answer:

   function useCounter() { const [count, setCount] = useState(0); return [count, setCount]; } -> Option A

4. Quick Check:

   Custom hook names start with 'use' [OK]

Hint: Custom hooks always start with 'use' prefix [OK]

Common Mistakes:

• Not starting function name with 'use'
• Using uppercase first letter in hook name
• Naming hooks like regular functions

3. Given this custom hook code, what will be logged when ExampleComponent renders?

function useToggle(initial) {
  const [on, setOn] = React.useState(initial);
  const toggle = () => setOn(!on);
  return [on, toggle];
}

function ExampleComponent() {
  const [isOn, toggle] = useToggle(false);
  React.useEffect(() => {
    console.log(isOn);
  }, [isOn]);
  return Toggle;
}

medium

A. undefined on first render, then false after button click

B. true on first render, then false after button click

C. No output because console.log is inside useEffect

D. false on first render, then true after button click

Solution

1. Step 1: Analyze initial state in useToggle

   The hook starts with initial = false, so isOn is false initially.

2. Step 2: Understand useEffect logging

   useEffect logs isOn after render; initially logs false, after toggle logs true.

3. Final Answer:

   false on first render, then true after button click -> Option D

4. Quick Check:

   Initial state false, toggles true [OK]

Hint: Initial state logs first, toggle changes it [OK]

Common Mistakes:

• Assuming initial state is true
• Thinking console.log won't run inside useEffect
• Confusing toggle function behavior

4. What is wrong with this custom hook code?

function useCounter() {
  const [count, setCount] = React.useState(0);
  function increment() {
    count = count + 1;
  }
  return { count, increment };
}

medium

A. Directly modifying count variable instead of using setCount

B. Missing React import for useState

C. increment function should return count

D. Custom hook name should not start with 'use'

Solution

1. Step 1: Check how state is updated

   State variables like count must be updated via setCount, not by direct assignment.

2. Step 2: Identify the error in increment

   increment tries to assign count = count + 1, which does not update React state.

3. Final Answer:

   Directly modifying count variable instead of using setCount -> Option A

4. Quick Check:

   State updates need setCount [OK]

Hint: Always use setState to update state, never assign directly [OK]

Common Mistakes:

• Trying to assign state variable directly
• Forgetting to call setCount
• Confusing variable assignment with state update

5. You want to create a custom hook useFetch that fetches data from a URL and returns loading state, data, and error. Which approach correctly reuses logic and handles updates safely?

function useFetch(url) {
  const [data, setData] = React.useState(null);
  const [loading, setLoading] = React.useState(true);
  const [error, setError] = React.useState(null);

  React.useEffect(() => {
    setLoading(true);
    fetch(url)
      .then(res => res.json())
      .then(json => {
        setData(json);
        setLoading(false);
      })
      .catch(err => {
        setError(err.message);
        setLoading(false);
      });
  }, [url]);

  return { data, loading, error };
}

hard

A. This hook will cause infinite loops because setState is inside useEffect

B. This hook should not use useEffect for fetching data

C. This hook correctly reuses fetch logic and updates state on URL change

D. This hook must return an array, not an object

Solution

1. Step 1: Check useEffect dependency

   useEffect runs when URL changes, so fetch updates correctly.

2. Step 2: Verify state updates inside fetch

   State updates happen after fetch resolves or errors, no infinite loops occur.

3. Step 3: Confirm return value format

   Returning an object with data, loading, error is a common pattern and valid.

4. Final Answer:

   This hook correctly reuses fetch logic and updates state on URL change -> Option C

5. Quick Check:

   useEffect fetches on URL change, updates state [OK]

Hint: useEffect with URL dependency safely fetches data [OK]

Common Mistakes:

• Thinking setState inside useEffect causes loops
• Not using useEffect for side effects
• Confusing return types of hooks