Optimistic state updates in NextJS - Performance & Optimization

Performance: Optimistic state updates

MEDIUM IMPACT

Optimistic state updates improve user interaction speed by showing UI changes immediately before server confirmation, impacting input responsiveness and perceived performance.

Updating UI state immediately after user action while waiting for server response

NextJS

function handleClick() {
  const oldState = state;
  setState(newState); // update UI immediately
  fetch('/api/update', { method: 'POST' })
    .then(response => {
      if (!response.ok) {
        setState(oldState); // rollback on server error
        throw new Error();
      }
    })
    .catch(() => {
      setState(oldState); // rollback on network error
    });
}

UI updates instantly, improving input responsiveness and user experience.

📈 Performance GainReduces INP by 200-500ms, providing immediate feedback without waiting for server.

Updating UI state immediately after user action while waiting for server response

NextJS

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

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

📉 Performance CostBlocks interaction feedback until server responds, increasing INP by 200-500ms depending on network.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Wait for server response before UI update	Minimal until response	1 reflow after response	1 paint after response	[X] Bad
Optimistic UI update with rollback on failure	Immediate DOM update	1 reflow on update, possible 1 reflow on rollback	1 paint per update/rollback	[OK] Good

Rendering Pipeline

Optimistic updates trigger immediate state changes that cause React to re-render components and update the DOM before server confirmation.

→JavaScript Execution

→Reconciliation

→Layout

→Paint

→Composite

⚠️ BottleneckReconciliation and Layout stages can be costly if many components update or rollback frequently.

Core Web Vital Affected

INP

Optimistic state updates improve user interaction speed by showing UI changes immediately before server confirmation, impacting input responsiveness and perceived performance.

Optimization Tips

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

2Handle rollback carefully to avoid excessive layout shifts and reflows.

3Batch state updates to minimize rendering overhead.

Performance Quiz - 3 Questions

What is the main performance benefit of optimistic state updates?

A They eliminate the need for server communication.

B They reduce the total number of DOM nodes on the page.

C They show UI changes immediately, improving input responsiveness.

D They prevent any layout shifts from happening.

DevTools: Performance

How to check: Record a session while interacting with the UI; look for time between input event and next paint.

What to look for: Short input delay and immediate paint after user action indicate good optimistic update performance.

Practice

1. What is the main purpose of optimistic state updates in Next.js applications?

easy

A. To delay UI updates until the server responds

B. To prevent any UI changes until the user refreshes the page

C. To update the UI immediately before the server confirms the change

D. To disable user interactions during server requests

Solution

1. Step 1: Understand optimistic updates concept

   Optimistic updates update the UI immediately to make the app feel faster.

2. Step 2: Compare options with concept

   Only To update the UI immediately before the server confirms the change describes updating UI before server confirmation, matching optimistic updates.

3. Final Answer:

   To update the UI immediately before the server confirms the change -> Option C

4. Quick Check:

   Optimistic update = immediate UI change [OK]

Hint: Optimistic means acting before confirmation [OK]

Common Mistakes:

• Thinking UI waits for server response
• Confusing optimistic with pessimistic updates
• Assuming UI disables interactions

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

easy

A. Use setTimeout to delay state update by 2 seconds

B. Update state after server response inside then()

C. Do not update state at all until server confirms

D. Update state immediately before sending server request

Solution

1. Step 1: Recall optimistic update timing

   Optimistic updates change state immediately before server response.

2. Step 2: Match options to timing

   Update state immediately before sending server request updates state immediately before request, fitting optimistic update pattern.

3. Final Answer:

   Update state immediately before sending server request -> Option D

4. Quick Check:

   Immediate state update = optimistic update [OK]

Hint: Update state before server reply for optimism [OK]

Common Mistakes:

• Waiting for server response before updating state
• Delaying state update unnecessarily
• Not updating state at all

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

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

async function increment() {
  setCount(count + 1); // optimistic update
  try {
    await fetch('/api/increment', { method: 'POST' });
  } catch {
    setCount(count); // revert on failure
  }
}

What will be the displayed count if the server request fails?

medium

A. The count reverts to the original value before increment

B. The count becomes undefined

C. The count resets to 0 regardless

D. The count remains incremented by 1

Solution

1. Step 1: Analyze optimistic update and revert logic

   Count increments immediately, but on fetch failure, count resets to original value.

2. Step 2: Understand revert on failure

   The catch block sets count back to original, undoing optimistic increment.

3. Final Answer:

   The count reverts to the original value before increment -> Option A

4. Quick Check:

   Failed fetch triggers revert = original count [OK]

Hint: Revert state on error to original value [OK]

Common Mistakes:

• Assuming optimistic update stays after failure
• Thinking count resets to zero always
• Ignoring catch block revert

4. Given this code snippet for optimistic updates in Next.js:

const [items, setItems] = useState([]);

async function addItem(newItem) {
  setItems([...items, newItem]);
  try {
    await fetch('/api/add', { method: 'POST', body: JSON.stringify(newItem) });
  } catch {
    // What is missing here?
  }
}

What is the best fix to handle errors properly?

medium

A. Remove the optimistic update and update only after fetch

B. Add code to revert items state to previous value in catch

C. Ignore errors since optimistic update is enough

D. Add a console.log in catch without state change

Solution

1. Step 1: Identify missing error handling

   Optimistic update changes state immediately, but no revert on failure.

2. Step 2: Add revert logic in catch block

   To keep UI consistent, revert items to previous state if fetch fails.

3. Final Answer:

   Add code to revert items state to previous value in catch -> Option B

4. Quick Check:

   Revert state on error = correct error handling [OK]

Hint: Revert state in catch to fix errors [OK]

Common Mistakes:

• Not reverting state on failure
• Removing optimistic update unnecessarily
• Only logging errors without UI fix

5. You want to implement optimistic updates for a list of tasks in Next.js. The user can add a task, and the UI should update immediately. However, if the server rejects the addition, the UI must revert. Which approach correctly handles this scenario?

hard

A. Update the tasks state immediately, then on server failure, reset tasks to previous state saved before update

B. Wait for server confirmation before updating tasks state to avoid inconsistencies

C. Update tasks state immediately and do nothing on failure, assuming server will eventually succeed

D. Clear the tasks list on server failure to force user refresh

Solution

1. Step 1: Understand optimistic update with revert

   Optimistic update means immediate UI change, but must revert on failure to keep UI correct.

2. Step 2: Evaluate options for correct revert handling

   Update the tasks state immediately, then on server failure, reset tasks to previous state saved before update saves previous state and reverts on failure, matching best practice.

3. Final Answer:

   Update the tasks state immediately, then on server failure, reset tasks to previous state saved before update -> Option A

4. Quick Check:

   Optimistic update + revert = Update the tasks state immediately, then on server failure, reset tasks to previous state saved before update [OK]

Hint: Save old state to revert if server fails [OK]

Common Mistakes:

• Not reverting UI on failure
• Waiting for server before UI update
• Clearing list unnecessarily on error