Optimistic updates pattern in NextJS

Introduction

Optimistic updates make your app feel faster by showing changes right away before the server confirms them.

When you want to update a list item immediately after a user edits it, without waiting for the server.

When a user likes or votes on something and you want to show the new count instantly.

When submitting a form and you want to show the new data right away to keep the user engaged.

When deleting an item and you want to remove it from the UI immediately for a smooth experience.

Syntax

NextJS

const [data, setData] = useState(initialData);

async function updateData(newData) {
  const previousData = data;
  // 1. Update UI immediately
  setData(newData);

  try {
    // 2. Send update to server
    await fetch('/api/update', { method: 'POST', body: JSON.stringify(newData), headers: { 'Content-Type': 'application/json' } });
  } catch (error) {
    // 3. Revert UI if server fails
    setData(previousData);
  }
}

Update the UI state first, then send the server request.

If the server fails, revert the UI to the old state to keep data correct.

Examples

Instantly increase like count, then send to server. If server fails, go back.

NextJS

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

function like() {
  const previousCount = count;
  setCount(previousCount + 1); // update UI immediately
  fetch('/api/like', { method: 'POST' }).catch(() => setCount(previousCount)); // revert if error
}

Add a new item to the list immediately, then confirm with server.

NextJS

const [items, setItems] = useState(['apple', 'banana']);

function addItem(newItem) {
  const previousItems = items;
  setItems([...previousItems, newItem]); // show new item right away
  fetch('/api/add', { method: 'POST', body: JSON.stringify(newItem), headers: { 'Content-Type': 'application/json' } })
    .catch(() => setItems(previousItems)); // undo if error
}

Sample Program

This Next.js component lets you add todos instantly. It updates the list right away, sends the new todo to the server, and if the server fails, it removes the new todo and shows an alert.

It uses accessible labels and keyboard support for better user experience.

NextJS

import { useState } from 'react';

export default function OptimisticUpdate() {
  const [todos, setTodos] = useState(['Learn Next.js', 'Build app']);
  const [input, setInput] = useState('');

  async function addTodo() {
    if (!input.trim()) return;

    const previousTodos = todos;
    const newTodos = [...todos, input];
    setTodos(newTodos); // update UI immediately

    try {
      const res = await fetch('/api/todos', {
        method: 'POST',
        headers: { 'Content-Type': 'application/json' },
        body: JSON.stringify({ todo: input })
      });

      if (!res.ok) throw new Error('Failed to save');

      setInput(''); // clear input on success
    } catch {
      setTodos(previousTodos); // revert UI if error
      alert('Failed to add todo. Please try again.');
    }
  }

  return (
    <main>
      <h1>My Todos</h1>
      <ul>
        {todos.map((todo, i) => <li key={i}>{todo}</li>)}
      </ul>
      <input
        aria-label="New todo"
        value={input}
        onChange={e => setInput(e.target.value)}
        onKeyDown={e => e.key === 'Enter' && addTodo()}
      />
      <button onClick={addTodo} aria-label="Add todo">Add</button>
    </main>
  );
}

Important Notes

Always handle errors to avoid showing wrong data.

Optimistic updates improve user experience but need careful rollback logic.

Use accessible labels and keyboard events for better usability.

Summary

Optimistic updates show changes immediately to make apps feel faster.

Update UI first, then confirm with server, and revert if needed.

Good error handling and accessibility improve the pattern's success.

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