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

Optimistic State Updates 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 when the user adds it, even before the server confirms it is saved. This technique is called optimistic state update.

🎯 Goal: Build a Next.js component that shows a list of tasks and lets the user add a new task. The new task should appear instantly in the list before the server responds.

📋 What You'll Learn

Create a React state variable to hold the list of tasks

Create a state variable to hold the new task text

Add a function to handle adding a new task with optimistic update

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 without waiting for the server. This improves user experience in chat apps, social media, and task managers.

💼 Career

Many modern web apps use optimistic UI updates. Knowing how to implement this pattern is valuable for frontend developers working with React and Next.js.

1

Set up initial tasks state

Create a React functional component called TaskList. Inside it, create a state variable called tasks initialized with an array containing these strings: 'Buy milk' and 'Walk dog'.

NextJS

import React, { useState } from 'react';

export default function TaskList() {
  // Your code here
}

Hint

Use useState to create tasks with the given initial array.

2

Add new task input state

Inside the TaskList component, create another state variable called newTask initialized as an empty string ''.

NextJS

import React, { useState } from 'react';

export default function TaskList() {
  const [tasks, setTasks] = useState(['Buy milk', 'Walk dog']);
  // Your code here
}

Hint

Create newTask state to hold the text typed by the user.

3

Add optimistic update function

Inside TaskList, write a function called addTask that does these steps: 1) Immediately add newTask to the tasks array using setTasks. 2) Clear newTask to an empty string. 3) Simulate a server call with setTimeout that waits 1 second and then logs 'Task saved'.

NextJS

import React, { useState } from 'react';

export default function TaskList() {
  const [tasks, setTasks] = useState(['Buy milk', 'Walk dog']);
  const [newTask, setNewTask] = useState('');

  // Your code here
}

Hint

Use setTasks to add the new task right away, then clear the input, then simulate saving with setTimeout.

4

Render tasks and input with button

Complete the TaskList component by returning JSX that shows: 1) An unordered list <ul> with each task in tasks as a list item <li>. 2) An input box with value bound to newTask and an onChange handler to update newTask. 3) A button labeled Add Task that calls addTask when clicked.

NextJS

import React, { useState } from 'react';

export default function TaskList() {
  const [tasks, setTasks] = useState(['Buy milk', 'Walk dog']);
  const [newTask, setNewTask] = useState('');

  function addTask() {
    setTasks([...tasks, newTask]);
    setNewTask('');
    setTimeout(() => {
      console.log('Task saved');
    }, 1000);
  }

  // Your code here
}

Hint

Use map to show tasks, bind input value and onChange, and add a button with onClick.

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