Why custom hooks are used in React - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a custom hook that uses state.

React

import { useState } from 'react';

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

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

AuseRef

BuseEffect

CuseContext

DuseState

Attempts:

3 left

💡 Hint

Common Mistakes

Using useEffect instead of useState to create state.

Trying to use useContext without a context provider.

The useState hook is used to add state inside a custom hook.

2fill in blank

medium

Complete the code to use the custom hook inside a component.

React

function Counter() {
  const [count, setCount] = [1]();
  return <button onClick={() => setCount(count + 1)}>{count}</button>;
}

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

AuseCounter

BuseState

CuseEffect

DuseRef

Attempts:

3 left

💡 Hint

Common Mistakes

Calling useState directly instead of the custom hook.

Not calling the hook as a function.

The custom hook useCounter is called like a regular function to get state and updater.

3fill in blank

hard

Fix the error in the custom hook to avoid violating React rules.

React

function useCounter() {
  if (true) {
    const [count, setCount] = [1](0);
  }
  return [count, setCount];
}

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

AuseEffect

BuseState

CuseContext

DuseRef

Attempts:

3 left

💡 Hint

Common Mistakes

Calling hooks inside if statements or loops.

Trying to conditionally call hooks.

Hooks must be called unconditionally at the top level of the function, not inside if statements.

4fill in blank

hard

Fill both blanks to create a custom hook that fetches data and stores it in state.

React

import { useState, useEffect } from 'react';

function useData(url) {
  const [data, setData] = useState(null);
  useEffect(() => {
    fetch(url)
      .then(response => response.[1]())
      .then(json => setData(json));
  }, [[2]]);
  return data;
}

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

Ajson

Burl

Ctext

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using response.text() instead of response.json().

Leaving the dependency array empty or incorrect.

The response.json() method parses the response body as JSON.
The effect depends on url so it runs again if the URL changes.

5fill in blank

hard

Fill all three blanks to create a custom hook that toggles a boolean state.

React

import { useState } from 'react';

function useToggle(initialValue = false) {
  const [value, setValue] = useState([1]);
  const toggle = () => setValue([2] => ![3]);
  return [value, toggle];
}

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

AinitialValue

BprevValue

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Not using the previous state in the toggle function.

Using a fixed value instead of toggling.

The state starts with initialValue.
The toggle function uses the previous value to set the opposite boolean.

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