Cleanup function in React - Performance & Optimization

Performance: Cleanup function

MEDIUM IMPACT

This affects interaction responsiveness and memory usage by properly cleaning up side effects in React components.

Managing subscriptions or timers inside React useEffect

React

useEffect(() => {
  const id = setInterval(() => {
    console.log('tick');
  }, 1000);
  return () => clearInterval(id);
}, []);

Cleanup stops the interval when the component unmounts, freeing resources and improving responsiveness.

📈 Performance GainPrevents memory leaks and unnecessary CPU use, improving INP and overall app performance.

Managing subscriptions or timers inside React useEffect

React

useEffect(() => {
  const id = setInterval(() => {
    console.log('tick');
  }, 1000);
  // No cleanup function
}, []);

Without cleanup, intervals keep running even after component unmounts, causing memory leaks and wasted CPU.

📉 Performance CostCauses memory leaks and continuous CPU usage, increasing INP and slowing user interactions.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Effect without cleanup	No extra DOM nodes	No reflows	No paint cost	[X] Bad - causes memory leaks and CPU waste
Effect with cleanup	No extra DOM nodes	No reflows	No paint cost	[OK] Good - frees resources and improves responsiveness

Rendering Pipeline

React effects run after rendering. Cleanup functions run before the next effect or on unmount to remove side effects, preventing extra work and memory use.

→Commit phase

→Effect cleanup

→JavaScript execution

⚠️ BottleneckJavaScript execution during effect cleanup if missing or inefficient

Core Web Vital Affected

INP

This affects interaction responsiveness and memory usage by properly cleaning up side effects in React components.

Optimization Tips

1Always return a cleanup function from useEffect to clear timers, subscriptions, or event listeners.

2Cleanup functions run before the next effect or on unmount to prevent memory leaks.

3Proper cleanup improves interaction responsiveness and reduces CPU waste.

Performance Quiz - 3 Questions

What is the main performance benefit of using a cleanup function in React's useEffect?

A It speeds up the initial render of the component.

B It reduces the size of the JavaScript bundle.

C It prevents memory leaks and reduces CPU usage after component unmount.

D It improves CSS selector matching speed.

DevTools: Performance

How to check: Record a session while mounting and unmounting the component. Look for long tasks or continuous CPU usage after unmount.

What to look for: If CPU usage continues or memory grows after unmount, cleanup is missing or inefficient.

Practice

1. What is the main purpose of a cleanup function in a React useEffect hook?

easy

A. To initialize state variables before rendering

B. To remove timers, listeners, or subscriptions when the component unmounts or before the effect reruns

C. To fetch data from an API after the component mounts

D. To render JSX elements conditionally

Solution

1. Step 1: Understand the role of cleanup functions

   Cleanup functions are used to tidy up side effects like timers or event listeners to prevent memory leaks.

2. Step 2: Identify when cleanup runs

   Cleanup runs when a component unmounts or before the effect runs again, ensuring resources are freed.

3. Final Answer:

   To remove timers, listeners, or subscriptions when the component unmounts or before the effect reruns -> Option B

4. Quick Check:

   Cleanup function purpose = Remove resources [OK]

Hint: Cleanup frees resources when component unmounts or effect reruns [OK]

Common Mistakes:

• Thinking cleanup initializes state
• Confusing cleanup with data fetching
• Assuming cleanup runs after every render

2. Which of the following is the correct syntax to define a cleanup function inside useEffect?

easy

A. useEffect(() => { () => { clearTimeout(timer); }; }, []);

B. useEffect(() => { clearTimeout(timer); return; }, []);

C. useEffect(() => { return () => { clearTimeout(timer); }; }, []);

D. useEffect(() => { return clearTimeout(timer); }, []);

Solution

1. Step 1: Recognize cleanup function syntax

   The cleanup function must be returned from the effect callback as a function.

2. Step 2: Check each option

   useEffect(() => { return () => { clearTimeout(timer); }; }, []); correctly returns a function that calls clearTimeout. Others either don't return a function or return the wrong value.

3. Final Answer:

   useEffect(() => { return () => { clearTimeout(timer); }; }, []); -> Option C

4. Quick Check:

   Cleanup function = returned function inside useEffect [OK]

Hint: Cleanup must be a returned function inside useEffect [OK]

Common Mistakes:

• Not returning a function from useEffect
• Returning the result of cleanup call instead of a function
• Placing cleanup code outside the returned function

3. What will be logged to the console when the component unmounts in this code?

function Timer() {
  React.useEffect(() => {
    const id = setInterval(() => console.log('tick'), 1000);
    return () => {
      clearInterval(id);
      console.log('cleanup');
    };
  }, []);
  return null;
}

medium

A. Nothing, because the component never unmounts

B. 'tick' once and no 'cleanup'

C. 'cleanup' every second and no 'tick'

D. 'tick' every second and then 'cleanup' once when unmounting

Solution

1. Step 1: Understand the effect behavior

   The effect sets an interval that logs 'tick' every second and returns a cleanup function that clears the interval and logs 'cleanup'.

2. Step 2: Trace what happens on unmount

   When the component unmounts, the cleanup runs once, clearing the interval and logging 'cleanup'.

3. Final Answer:

   'tick' every second and then 'cleanup' once when unmounting -> Option D

4. Quick Check:

   Interval logs 'tick'; cleanup logs 'cleanup' once [OK]

Hint: Cleanup logs once on unmount, interval logs repeatedly [OK]

Common Mistakes:

• Thinking cleanup runs repeatedly
• Confusing interval clearing with immediate stop
• Expecting no logs at all

4. Identify the error in this React effect with cleanup:

useEffect(() => {
  window.addEventListener('resize', handleResize);
  return window.removeEventListener('resize', handleResize);
}, []);

medium

A. The cleanup function should be a function returned, not the result of removeEventListener call

B. The event listener should be added inside the cleanup function

C. The effect should not have a dependency array

D. handleResize should be defined inside useEffect

Solution

1. Step 1: Check cleanup function syntax

   The cleanup must be a function returned from useEffect, not the result of calling removeEventListener immediately.

2. Step 2: Identify correct cleanup pattern

   The cleanup should be: return () => window.removeEventListener('resize', handleResize); to run on unmount.

3. Final Answer:

   The cleanup function should be a function returned, not the result of removeEventListener call -> Option A

4. Quick Check:

   Cleanup must be a returned function [OK]

Hint: Return a function for cleanup, not the cleanup call itself [OK]

Common Mistakes:

• Calling cleanup immediately instead of returning a function
• Adding event listener inside cleanup
• Ignoring dependency array effects

5. You want to subscribe to a WebSocket in a React component and clean up properly. Which code correctly uses a cleanup function to close the socket when the component unmounts or the URL changes?

function SocketComponent({ url }) {
  React.useEffect(() => {
    const socket = new WebSocket(url);
    socket.onmessage = (event) => console.log(event.data);
    // What cleanup code is correct?
  }, [url]);
  return null;
}

hard

A. return () => { socket.close(); };

B. socket.close();

C. return socket.close();

D. return () => socket.onmessage = null;

Solution

1. Step 1: Understand cleanup for WebSocket

   To avoid leaks, the socket must be closed when the component unmounts or URL changes.

2. Step 2: Identify correct cleanup syntax

   The cleanup must be a function returned from useEffect that calls socket.close(). return () => { socket.close(); }; does this correctly.

3. Final Answer:

   return () => { socket.close(); }; -> Option A

4. Quick Check:

   Cleanup returns function closing socket [OK]

Hint: Return a function that closes socket for cleanup [OK]

Common Mistakes:

• Calling close immediately instead of returning a function
• Returning the result of close() instead of a function
• Only nullifying event handlers without closing socket