Why custom hooks are used in React

Introduction

Custom hooks help you reuse code easily in React. They keep your components clean and simple by sharing logic.

When you want to share the same logic between multiple components.

When your component code is getting too long or complicated.

When you want to separate concerns for better readability.

When you need to reuse stateful logic without repeating code.

When you want to keep your code organized and easier to test.

Syntax

React

function useCustomHook() {
  // use React hooks here
  return someValue;
}

Custom hooks always start with the word 'use' to follow React rules.

You can use built-in hooks like useState or useEffect inside your custom hook.

Examples

This custom hook manages a counter state and provides a function to increase it.

React

import { useState } from 'react';

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

This custom hook tracks the window width and updates it when the window is resized.

React

import { useState, useEffect } from 'react';

function useWindowWidth() {
  const [width, setWidth] = useState(window.innerWidth);
  useEffect(() => {
    const handleResize = () => setWidth(window.innerWidth);
    window.addEventListener('resize', handleResize);
    return () => window.removeEventListener('resize', handleResize);
  }, []);
  return width;
}

Sample Program

This example shows a custom hook useToggle that manages a boolean state. The component uses it to toggle text between ON and OFF with a button.

React

import React, { useState } from 'react';

function useToggle(initialValue = false) {
  const [value, setValue] = useState(initialValue);
  const toggle = () => setValue(!value);
  return [value, toggle];
}

export default function ToggleComponent() {
  const [isOn, toggleIsOn] = useToggle();
  return (
    <>
      <p>The switch is {isOn ? 'ON' : 'OFF'}</p>
      <button onClick={toggleIsOn} aria-pressed={isOn} aria-label="Toggle switch">
        Toggle
      </button>
    </>
  );
}

Important Notes

Always name custom hooks starting with 'use' so React knows they follow hook rules.

Custom hooks help keep your components smaller and easier to read.

You can combine multiple hooks inside a custom hook to share complex logic.

Summary

Custom hooks let you reuse stateful logic across components.

They keep your code clean and organized.

Always start their names with 'use' to follow React conventions.

Practice

1. Why do React developers create custom hooks?

easy

A. To reuse stateful logic across multiple components

B. To style components with CSS

C. To replace React components entirely

D. To write HTML inside JavaScript

Solution

1. Step 1: Understand the purpose of hooks

   Hooks in React let you use state and other React features without writing a class.

2. Step 2: Identify why custom hooks are made

   Custom hooks let you reuse stateful logic easily across different components, avoiding repetition.

3. Final Answer:

   To reuse stateful logic across multiple components -> Option A

4. Quick Check:

   Custom hooks = reuse logic [OK]

Hint: Custom hooks reuse logic, not styles or HTML [OK]

Common Mistakes:

• Thinking custom hooks are for styling
• Believing custom hooks replace components
• Confusing hooks with HTML syntax

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

easy

A. useFetchData

B. fetchDataHook

C. FetchData

D. hookUseFetch

Solution

1. Step 1: Recall React naming conventions for hooks

   React requires custom hooks to start with the word 'use' to identify them as hooks.

2. Step 2: Check each option

   Only 'useFetchData' starts with 'use', following React's convention.

3. Final Answer:

   useFetchData -> Option A

4. Quick Check:

   Custom hook names start with 'use' [OK]

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

Common Mistakes:

• Not starting the name with 'use'
• Using camel case without 'use' prefix
• Confusing component names with hook names

3. What will be the output of this React component using a custom hook?

function useCounter() {
  const [count, setCount] = React.useState(0);
  React.useEffect(() => {
    const timer = setInterval(() => setCount(c => c + 1), 1000);
    return () => clearInterval(timer);
  }, []);
  return count;
}

function Counter() {
  const count = useCounter();
  return 
{count}
;
}

medium

A. Throws an error because hooks can't be used here

B. Always shows 0 and never changes

C. A number that increases by 1 every second starting from 0

D. Shows a static string 'count'

Solution

1. Step 1: Analyze the custom hook useCounter

   It initializes count to 0 and sets up a timer to increase count by 1 every second.

2. Step 2: Understand the component output

   The Counter component uses useCounter and displays the count, which updates every second.

3. Final Answer:

   A number that increases by 1 every second starting from 0 -> Option C

4. Quick Check:

   useCounter returns incrementing count [OK]

Hint: Custom hooks can return changing state values [OK]

Common Mistakes:

• Thinking count stays 0 forever
• Believing hooks can't be used in functions
• Assuming output is a string, not a number

4. Identify the error in this custom hook usage:

function useData() {
  const [data, setData] = React.useState(null);
  fetch('https://api.example.com/data')
    .then(res => res.json())
    .then(json => setData(json));
  return data;
}

function App() {
  const data = useData();
  return <div>{JSON.stringify(data)}</div>;
}

medium

A. Custom hooks cannot return data

B. The fetch call should be inside useEffect to avoid infinite loops

C. useState is used incorrectly and should be replaced with useRef

D. The component should not call the custom hook

Solution

1. Step 1: Check where fetch is called in useData

   fetch is called directly inside the hook body, causing it to run on every render, leading to infinite loops.

2. Step 2: Correct usage with useEffect

   fetch should be inside a useEffect with an empty dependency array to run only once on mount.

3. Final Answer:

   The fetch call should be inside useEffect to avoid infinite loops -> Option B

4. Quick Check:

   Side effects go inside useEffect [OK]

Hint: Put side effects like fetch inside useEffect [OK]

Common Mistakes:

• Calling fetch directly in hook body
• Misusing useState instead of useEffect
• Thinking hooks can't return values

5. You want to share logic for fetching user data and managing loading state across several components. Which approach best uses custom hooks to solve this?

hard

A. Store user data in a global variable outside React

B. Write the fetch logic separately in each component to avoid dependencies

C. Use a class component instead of hooks for shared logic

D. Create a custom hook that fetches user data and returns both data and loading state

Solution

1. Step 1: Understand the need for reusable logic

   Fetching data and managing loading state is stateful logic that can be reused with a custom hook.

2. Step 2: Evaluate options for sharing logic

   Creating a custom hook that returns data and loading state allows multiple components to share this logic cleanly.

3. Final Answer:

   Create a custom hook that fetches user data and returns both data and loading state -> Option D

4. Quick Check:

   Custom hooks share stateful logic best [OK]

Hint: Custom hooks bundle related state and logic for reuse [OK]

Common Mistakes:

• Duplicating fetch code in every component
• Using class components instead of hooks
• Using global variables that break React's state flow