Optimistic updates pattern in NextJS - Performance & Optimization

Performance: Optimistic updates pattern

MEDIUM IMPACT

This pattern affects interaction responsiveness and perceived speed by updating UI before server confirmation.

Updating UI immediately after user action without waiting for server response

NextJS

function handleClick() {
  setData(prev => ({ ...prev, updated: true })); // update UI immediately
  fetch('/api/update', { method: 'POST' }).catch(() => {
    setData(prev => ({ ...prev, updated: false })); // rollback on error
  });
}

<button onClick={handleClick}>Update</button>

UI updates instantly on user action, improving perceived speed and responsiveness.

📈 Performance GainReduces input delay to near zero, improving INP metric significantly.

Updating UI immediately after user action without waiting for server response

NextJS

async function handleClick() {
  const response = await fetch('/api/update', { method: 'POST' });
  if (response.ok) {
    setData(await response.json());
  }
}

<button onClick={handleClick}>Update</button>

UI waits for server response before updating, causing input delay and poor responsiveness.

📉 Performance CostBlocks interaction feedback for 200-500ms depending on network, increasing INP.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Waiting for server response before UI update	Multiple DOM updates after async call	Triggers 1 reflow per update delay	Higher paint cost due to delayed changes	[X] Bad
Optimistic UI update before server response	Single immediate DOM update	Single reflow triggered instantly	Lower paint cost with smooth interaction	[OK] Good

Rendering Pipeline

Optimistic updates modify the UI state immediately, triggering style recalculation and layout before server response arrives.

→Style Calculation

→Layout

→Paint

→Composite

⚠️ BottleneckLayout and Paint stages due to immediate DOM updates

Core Web Vital Affected

INP

This pattern affects interaction responsiveness and perceived speed by updating UI before server confirmation.

Optimization Tips

1Update UI state immediately on user input to improve responsiveness.

2Rollback UI changes if server response indicates failure.

3Batch DOM updates to minimize layout thrashing and paint cost.

Performance Quiz - 3 Questions

What is the main performance benefit of using optimistic updates in a Next.js app?

A Improves input responsiveness by updating UI immediately

B Reduces bundle size by removing server calls

C Eliminates all layout recalculations

D Prevents any paint operations on user input

DevTools: Performance

How to check: Record a session while interacting with the UI; look for long tasks or delays between input and next paint.

What to look for: Short input-to-next-paint times indicate good optimistic updates; long delays show poor responsiveness.

Practice

1. What is the main idea behind the optimistic updates pattern in Next.js?

easy

A. Update the UI immediately before server confirmation to improve user experience

B. Wait for server response before updating the UI to ensure accuracy

C. Reload the entire page after every data change to keep UI fresh

D. Disable user input until the server confirms the update

Solution

1. Step 1: Understand the purpose of optimistic updates

   Optimistic updates aim to make the app feel faster by showing changes immediately.

2. Step 2: Compare UI update timing

   Instead of waiting for the server, the UI updates first, then confirms or reverts based on server response.

3. Final Answer:

   Update the UI immediately before server confirmation to improve user experience -> Option A

4. Quick Check:

   Optimistic updates = Immediate UI update [OK]

Hint: UI updates first, server confirms later [OK]

Common Mistakes:

• Waiting for server before UI update
• Reloading entire page unnecessarily
• Disabling user input during update

2. Which of the following is the correct way to implement an optimistic update in Next.js using React hooks?

easy

A. Use useEffect to update state only after server confirms

B. Send API request first, then call setState after response

C. Call setState to update UI, then send API request, revert on error

D. Reload the page after API call to update UI

Solution

1. Step 1: Identify optimistic update flow

   Optimistic update means updating UI state immediately with setState.

2. Step 2: Confirm API call and handle errors

   Send API request after UI update, and revert state if the request fails.

3. Final Answer:

   Call setState to update UI, then send API request, revert on error -> Option C

4. Quick Check:

   Update state first, then API call [OK]

Hint: Update state first, then call API [OK]

Common Mistakes:

• Waiting for API before updating state
• Using useEffect incorrectly for optimistic update
• Reloading page instead of updating state

3. Consider this Next.js React component snippet using optimistic updates:

const [likes, setLikes] = useState(0);

async function handleLike() {
  setLikes(likes + 1);
  try {
    await fetch('/api/like', { method: 'POST' });
  } catch {
    setLikes(likes); // revert on error
  }
}

What will the UI show if the API call fails?

medium

A. The likes count stays incremented by 1

B. The likes count increments by 2

C. The likes count resets to zero

D. The likes count reverts to the original value

Solution

1. Step 1: Analyze optimistic update behavior

   The UI increments likes immediately by 1 using setLikes(likes + 1).

2. Step 2: Check error handling

   If the API call fails, setLikes(likes) resets likes to the original value before increment.

3. Final Answer:

   The likes count reverts to the original value -> Option D

4. Quick Check:

   API error triggers revert to old likes [OK]

Hint: Revert state on API failure [OK]

Common Mistakes:

• Assuming UI stays incremented after failure
• Resetting likes to zero incorrectly
• Incrementing likes twice by mistake

4. You wrote this optimistic update code in Next.js but the UI never reverts on API failure:

const [count, setCount] = useState(0);

async function increment() {
  setCount(count + 1);
  try {
    await fetch('/api/increment', { method: 'POST' });
  } catch {
    setCount(count - 1);
  }
}

What is the bug causing the revert to fail?

medium

A. Using stale count value inside catch block

B. Not awaiting the setCount call

C. Missing return statement after setCount

D. API call method should be GET, not POST

Solution

1. Step 1: Understand state closure issue

   The count variable inside catch is the old value before increment.

2. Step 2: Explain why revert fails

   Subtracting 1 from stale count does not revert to original because count was already incremented in UI.

3. Final Answer:

   Using stale count value inside catch block -> Option A

4. Quick Check:

   State closure causes revert failure [OK]

Hint: Avoid stale state in async error handlers [OK]

Common Mistakes:

• Expecting setState to be async
• Ignoring stale closure of state variable
• Wrong HTTP method for API call

5. You want to implement optimistic updates for a comment submission in Next.js. Which approach best handles UI update, server confirmation, and error rollback?

hard

A. Reload the page after comment submission to show new comment

B. Add comment to UI state immediately, send API request, remove comment if API fails

C. Add comment to UI state only after API confirms success

D. Send API request first, then add comment to UI state after success

Solution

1. Step 1: Apply optimistic update pattern

   Update UI immediately by adding comment to state to improve responsiveness.

2. Step 2: Handle server confirmation and errors

   Send API request; if it fails, remove the comment from UI to keep data consistent.

3. Final Answer:

   Add comment to UI state immediately, send API request, remove comment if API fails -> Option B

4. Quick Check:

   UI update first, revert on error [OK]

Hint: Add UI item first, remove on failure [OK]

Common Mistakes:

• Waiting for API before UI update
• Adding comment only after success
• Reloading page instead of updating state