Custom hook best practices in React - Performance & Optimization

Performance: Custom hook best practices

MEDIUM IMPACT

Custom hooks affect interaction responsiveness and rendering efficiency by managing state and side effects cleanly.

Sharing reusable logic without causing extra renders

React

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

Using useEffect with an empty dependency array runs fetch only once, avoiding repeated network calls and re-renders.

📈 Performance Gainsingle network request and minimal re-renders

Sharing reusable logic without causing extra renders

React

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

Fetching data directly in the hook body causes the fetch to run on every render, triggering many state updates and re-renders.

📉 Performance Costtriggers multiple re-renders and network requests per component render

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Fetching data in hook body	Minimal DOM changes	Multiple re-renders	Multiple paints	[X] Bad
Fetching data inside useEffect	Minimal DOM changes	Single re-render	Single paint	[OK] Good
State update in hook body	No DOM changes	Infinite re-renders	Continuous paints	[X] Bad
State update inside useEffect	No DOM changes	Single re-render	Single paint	[OK] Good
Inline function in hook	No DOM changes	Extra child re-renders	Extra paints	[X] Bad
Memoized function with useCallback	No DOM changes	Minimal re-renders	Minimal paints	[OK] Good

Rendering Pipeline

Custom hooks manage state and side effects that influence React's render cycle. Poor hook design can cause repeated state updates triggering multiple render passes and layout recalculations.

→JavaScript Execution

→React Reconciliation

→Layout

→Paint

⚠️ BottleneckReact Reconciliation due to repeated state changes causing many renders

Core Web Vital Affected

INP

Custom hooks affect interaction responsiveness and rendering efficiency by managing state and side effects cleanly.

Optimization Tips

1Run side effects like data fetching inside useEffect with proper dependencies.

2Avoid state updates directly in the hook body to prevent infinite render loops.

3Memoize functions returned from hooks with useCallback to reduce child re-renders.

Performance Quiz - 3 Questions

What is the best way to run a data fetch inside a custom hook to avoid multiple network requests?

A Run fetch inside a callback without dependencies

B Run fetch directly in the hook body

C Run fetch inside useEffect with empty dependency array

D Run fetch inside useMemo

DevTools: React DevTools and Chrome Performance

How to check: Use React DevTools Profiler to record interactions and check how many times components re-render. Use Chrome Performance tab to see scripting and rendering times.

What to look for: Look for excessive renders or repeated state updates causing long scripting times and layout thrashing.

Practice

1. Which of the following is a best practice when naming a custom React hook?

easy

A. Start the hook name with use to follow React conventions

B. Name the hook with a capital letter but no prefix

C. Use any name as long as it describes the functionality

D. Prefix the hook name with hook instead of use

Solution

1. Step 1: Understand React hook naming conventions

   React requires custom hooks to start with use so React can identify them as hooks.

2. Step 2: Evaluate the options

   Only Start the hook name with use to follow React conventions correctly follows this convention by starting the name with use.

3. Final Answer:

   Start the hook name with use to follow React conventions -> Option A

4. Quick Check:

   Hook names start with 'use' = B [OK]

Hint: Always start custom hook names with 'use' [OK]

Common Mistakes:

• Naming hooks without 'use' prefix
• Using 'hook' prefix instead of 'use'
• Starting hook names with lowercase letters without 'use'

2. Which of the following is the correct syntax to define a custom hook that returns a count state and an increment function?

easy

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

B. function countHook() { const [count, setCount] = useState(0); return [count, () => setCount(count + 1)]; }

C. function useCount() { const [count, setCount] = useState(0); return [count, () => setCount(count + 1)]; }

D. function useCount() { const count = 0; function increment() { count += 1; } return { count, increment }; }

Solution

1. Step 1: Check the hook naming and return type

   The hook must start with use and return state and updater properly. function useCount() { const [count, setCount] = useState(0); return [count, () => setCount(count + 1)]; } uses useCount and returns an array with count and increment function.

2. Step 2: Verify state management correctness

   function useCount() { const [count, setCount] = useState(0); return [count, () => setCount(count + 1)]; } uses useState correctly and returns a function that updates state with setCount(count + 1). This is the correct pattern.

3. Final Answer:

   function useCount() { const [count, setCount] = useState(0); return [count, () => setCount(count + 1)]; } -> Option C

4. Quick Check:

   Custom hook syntax with useState and return array = C [OK]

Hint: Custom hooks return state and updater, named starting with 'use' [OK]

Common Mistakes:

• Not starting function name with 'use'
• Returning object instead of array without reason
• Not using useState for state management
• Mutating state directly instead of using setter

3. Given this custom hook code:

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

function Component() {
  const [isOn, toggle] = useToggle(false);
  toggle();
  return 
{isOn ? 'ON' : 'OFF'}
;
}

What will the component render after calling toggle() once?

medium

A. ON

B. false

C. OFF

D. Error because toggle is called outside an event

Solution

1. Step 1: Analyze where toggle is called

   toggle() is called directly in the component body, which executes during every render phase.

2. Step 2: Understand React render rules

   State updates during render trigger re-renders. Since toggle() is unconditional, it flips state every render, causing an infinite loop. React prohibits this and produces errors/warnings (e.g., "Cannot update a component while rendering a different component" or too many re-renders).

3. Final Answer:

   Error because toggle is called outside an event -> Option D

4. Quick Check:

   State updaters only in events/effects, not render body [OK]

Hint: Never call state updaters during render; use events or useEffect [OK]

Common Mistakes:

• Assuming async update shows initial state without considering re-render loop
• Expecting 'ON' due to toggle flip
• Confusing boolean with string output
• Thinking render body calls are safe without infinite loop

4. What is wrong with this custom hook code?

function useCounter() {
  let count = 0;
  const increment = () => { count += 1; };
  return { count, increment };
}

medium

A. It does not use React state, so count won't trigger re-renders

B. The hook name should not start with 'use'

C. The increment function should not modify count

D. It should return an array, not an object

Solution

1. Step 1: Check state management in the hook

   The hook uses a local variable count that resets to 0 on every call. It does not use useState, so React won't track changes or re-render.

2. Step 2: Understand React state importance

   Without useState, updating count does not cause component updates, making the hook ineffective.

3. Final Answer:

   It does not use React state, so count won't trigger re-renders -> Option A

4. Quick Check:

   Local variable won't update UI without useState [OK]

Hint: Always use useState for reactive state in hooks [OK]

Common Mistakes:

• Using local variables instead of useState
• Expecting UI to update without state
• Returning object instead of array (not always wrong)
• Misnaming hook without 'use' prefix

5. You want to create a custom hook useFetch that fetches data from a URL and returns loading state, data, and error. Which approach follows best practices?

hard

A. Fetch data directly in the hook body without useEffect, return data and error only

B. Use useEffect inside the hook to fetch data on mount, use useState for loading, data, and error, and return them as an object

C. Use useMemo to fetch data and return loading and data states

D. Fetch data outside the hook and pass it as a parameter to the hook

Solution

1. Step 1: Understand data fetching in hooks

   Fetching data should happen inside useEffect to avoid running on every render. State variables track loading, data, and error.

2. Step 2: Evaluate options for best practice

   Use useEffect inside the hook to fetch data on mount, use useState for loading, data, and error, and return them as an object correctly uses useEffect and useState to manage asynchronous fetch and returns all relevant states as an object for clarity.

3. Final Answer:

   Use useEffect inside the hook to fetch data on mount, use useState for loading, data, and error, and return them as an object -> Option B

4. Quick Check:

   Fetch in useEffect with state management = A [OK]

Hint: Fetch inside useEffect, manage state with useState [OK]

Common Mistakes:

• Fetching data directly in hook body causing infinite loops
• Not managing loading or error states
• Using useMemo for side effects
• Passing fetched data as parameter instead of fetching inside hook