Reusing logic with hooks in React - Performance & Optimization

Performance: Reusing logic with hooks

MEDIUM IMPACT

This affects interaction responsiveness and rendering efficiency by reducing unnecessary component updates and re-renders.

Sharing stateful logic across multiple components

React

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

function ComponentA() {
  const { count, increment } = useCounter();
  return <button onClick={increment}>Count: {count}</button>;
}

function ComponentB() {
  const { count, increment } = useCounter();
  return <button onClick={increment}>Count: {count}</button>;
}

Extracting logic into a custom hook avoids code duplication and ensures consistent state management with memoized callbacks.

📈 Performance GainSaves bundle size and improves INP by reducing unnecessary re-renders and re-creating functions.

Sharing stateful logic across multiple components

React

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

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

Duplicating state logic in multiple components causes repeated code and separate state management, increasing bundle size and potential bugs.

📉 Performance CostAdds extra bundle size and causes independent re-renders for each component.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Duplicated state logic in components	Multiple independent states	Multiple reflows per component update	Higher paint cost due to repeated renders	[X] Bad
Custom hook with memoized callbacks	Single state logic reused	Single reflow per state update	Lower paint cost due to fewer renders	[OK] Good

Rendering Pipeline

Custom hooks encapsulate state and logic, so React only re-renders components when state changes. Memoizing callbacks reduces layout recalculations and paint.

→JavaScript Execution

→Reconciliation

→Layout

→Paint

⚠️ BottleneckReconciliation stage due to unnecessary re-renders if hooks are not optimized

Core Web Vital Affected

INP

This affects interaction responsiveness and rendering efficiency by reducing unnecessary component updates and re-renders.

Optimization Tips

1Extract reusable logic into custom hooks to avoid code duplication.

2Use React.useCallback inside hooks to memoize functions and prevent unnecessary re-renders.

3Monitor component re-renders with React DevTools Profiler to ensure hooks improve performance.

Performance Quiz - 3 Questions

What is a main performance benefit of using custom hooks to reuse logic in React?

A Triggers more layout recalculations

B Increases bundle size by adding more code

C Reduces code duplication and prevents unnecessary re-renders

D Blocks rendering until all hooks finish

DevTools: React DevTools and Performance panel

How to check: Open React DevTools Profiler, record interactions, and observe component re-render counts and durations. Use Performance panel to check layout and paint times.

What to look for: Look for fewer component re-renders and shorter interaction times indicating better INP and less layout thrashing.

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