Why custom hooks are used in React - Performance Evidence

Performance: Why custom hooks are used

MEDIUM IMPACT

Custom hooks affect component rendering efficiency and code reuse, impacting interaction responsiveness and bundle size.

Reusing stateful logic across multiple components

React

function useCounter() {
  const [count, setCount] = React.useState(0);
  React.useEffect(() => {
    console.log('Count changed:', count);
  }, [count]);
  return [count, setCount];
}

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

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

Extracts shared logic into a single reusable hook, reducing code duplication and ensuring consistent behavior.

📈 Performance GainReduces bundle size slightly and improves maintainability, leading to fewer bugs and smoother updates.

Reusing stateful logic across multiple components

React

function ComponentA() {
  const [count, setCount] = React.useState(0);
  React.useEffect(() => {
    console.log('Count changed:', count);
  }, [count]);
  return <button onClick={() => setCount(count + 1)}>{count}</button>;
}

function ComponentB() {
  const [count, setCount] = React.useState(0);
  React.useEffect(() => {
    console.log('Count changed:', count);
  }, [count]);
  return <button onClick={() => setCount(count + 1)}>{count}</button>;
}

Duplicated state and effect logic in multiple components causes more code and potential inconsistent updates.

📉 Performance CostIncreases bundle size and may cause redundant renders if logic is not consistent.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Duplicated state logic in components	Multiple state hooks per component	Multiple re-renders on state change	Higher paint cost due to redundant updates	[X] Bad
Custom hook for shared logic	Single state hook reused	Fewer re-renders due to consistent logic	Lower paint cost with optimized updates	[OK] Good

Rendering Pipeline

Custom hooks encapsulate state and side effects, affecting the React rendering lifecycle by controlling when components re-render and update.

→Component Render

→State Update

→Effect Execution

⚠️ BottleneckUnnecessary re-renders caused by duplicated or poorly optimized state logic

Core Web Vital Affected

INP

Custom hooks affect component rendering efficiency and code reuse, impacting interaction responsiveness and bundle size.

Optimization Tips

1Use custom hooks to avoid repeating stateful logic in multiple components.

2Centralize side effects in custom hooks to reduce inconsistent updates.

3Monitor component renders with React Profiler to verify performance improvements.

Performance Quiz - 3 Questions

How do custom hooks improve React app performance?

A By reusing stateful logic and reducing code duplication

B By increasing the number of components rendered

C By blocking rendering until all hooks finish

D By adding more event listeners to the DOM

DevTools: React DevTools Profiler

How to check: Record interactions and look for repeated renders of components using duplicated logic versus those using custom hooks.

What to look for: Lower render counts and faster update times indicate better performance with custom hooks.

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