Optimistic updates pattern in NextJS - Mini Project: Build & Apply

Optimistic Updates Pattern in Next.js

📖 Scenario: You are building a simple task list app where users can add tasks quickly. To make the app feel fast, you want to show the new task immediately on the screen before the server confirms it is saved. This is called an optimistic update.

🎯 Goal: Create a Next.js component that shows a list of tasks and allows adding a new task with optimistic updates. The new task should appear instantly in the list, and if the server call fails, the task should be removed.

📋 What You'll Learn

Create a React functional component called TaskList with initial tasks data

Add a state variable tasks to hold the list of tasks

Add a state variable newTask to hold the input value

Create a function addTask that adds a new task optimistically

Simulate a server call with setTimeout and handle success or failure

Render the list of tasks and an input with a button to add tasks

💡 Why This Matters

🌍 Real World

Optimistic updates make apps feel faster by showing changes immediately, improving user experience in social media, chat apps, and task managers.

💼 Career

Understanding optimistic updates is important for frontend developers working with React or Next.js to build responsive and user-friendly interfaces that handle asynchronous data.

1

Set up initial tasks data

Create a React functional component called TaskList. Inside it, create a state variable called tasks using useState with the initial value as an array of two task objects: { id: 1, text: 'Buy groceries' } and { id: 2, text: 'Walk the dog' }.

NextJS

import React, { useState } from 'react';

export default function TaskList() {
  // Create tasks state with initial two tasks
  // Your code here
}

Hint

Use useState to create tasks with the exact two task objects inside an array.

2

Add input state for new task

Inside the TaskList component, create a state variable called newTask using useState with an initial empty string ''.

NextJS

import React, { useState } from 'react';

export default function TaskList() {
  const [tasks, setTasks] = useState([
    { id: 1, text: 'Buy groceries' },
    { id: 2, text: 'Walk the dog' }
  ]);
  // Create newTask state with empty string
  // Your code here
}

Hint

Use useState to create newTask with an empty string.

3

Implement optimistic addTask function

Inside the TaskList component, create a function called addTask. This function should: - Create a new task object with a unique id using Date.now() and text from newTask. - Update tasks state immediately by adding the new task at the end (optimistic update). - Clear the newTask state to an empty string. - Simulate a server call with setTimeout of 1 second. - Inside setTimeout, randomly decide success or failure (use Math.random() < 0.7 for success). - If failure, remove the new task from tasks state by filtering it out using its id.

NextJS

import React, { useState } from 'react';

export default function TaskList() {
  const [tasks, setTasks] = useState([
    { id: 1, text: 'Buy groceries' },
    { id: 2, text: 'Walk the dog' }
  ]);
  const [newTask, setNewTask] = useState('');

  // Create addTask function with optimistic update and simulated server call
  // Your code here
}

Hint

Remember to add the new task immediately to tasks state, then simulate a server call with setTimeout. On failure, remove the task by filtering it out.

4

Render tasks list and input with add button

Inside the TaskList component, return JSX that: - Renders an unordered list <ul> with each task's text inside a <li>. Use task.id as the key. - Renders an input field with value bound to newTask and onChange updating newTask. - Renders a button with text Add Task that calls addTask on click. - Use semantic HTML and add aria-label to the input for accessibility. - Wrap the list and input/button inside a <section>.

NextJS

import React, { useState } from 'react';

export default function TaskList() {
  const [tasks, setTasks] = useState([
    { id: 1, text: 'Buy groceries' },
    { id: 2, text: 'Walk the dog' }
  ]);
  const [newTask, setNewTask] = useState('');

  function addTask() {
    const task = { id: Date.now(), text: newTask };
    setTasks(prev => [...prev, task]);
    setNewTask('');

    setTimeout(() => {
      const success = Math.random()  prev.filter(t => t.id !== task.id));
      }
    }, 1000);
  }

  // Return JSX rendering tasks list, input with aria-label, and add button
  // Your code here
}

Hint

Render the tasks inside a <ul> with keys, an input with aria-label, and a button that calls addTask.

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