Loading states for data in NextJS - Mini Project: Build & Apply

Loading states for data

📖 Scenario: You are building a simple Next.js app that fetches user data from an API. To improve user experience, you want to show a loading message while the data is being fetched.

🎯 Goal: Create a Next.js functional component that fetches user data and shows a loading message until the data is ready.

📋 What You'll Learn

Create a React functional component named UserList.

Use useState to hold the user data and loading state.

Use useEffect to fetch data from https://jsonplaceholder.typicode.com/users when the component mounts.

Show a Loading... message while fetching data.

Display the list of user names after data is loaded.

💡 Why This Matters

🌍 Real World

Loading states are common in web apps to inform users that data is being fetched, improving user experience.

💼 Career

Understanding loading states and data fetching is essential for frontend developers working with React and Next.js.

1

Set up the component and state variables

Create a React functional component called UserList. Inside it, create two state variables using useState: users initialized to an empty array, and loading initialized to true.

NextJS

import React, { useState, useEffect } from 'react';

export default function UserList() {
  // Create state variables users and loading
  // Your code here
}

Hint

Use const [users, setUsers] = useState([]); and const [loading, setLoading] = useState(true); inside the component.

2

Add data fetching with useEffect

Inside the UserList component, add a useEffect hook that fetches data from https://jsonplaceholder.typicode.com/users. After fetching, update users with the data and set loading to false.

NextJS

import React, { useState, useEffect } from 'react';

export default function UserList() {
  const [users, setUsers] = useState([]);
  const [loading, setLoading] = useState(true);

  // Add useEffect to fetch user data
  // Your code here
}

Hint

Use useEffect with an empty dependency array to fetch data once on mount. Use fetch and update states inside then.

3

Render loading message conditionally

Inside the UserList component's return statement, add JSX that shows a <p>Loading...</p> message if loading is true. Otherwise, show the list of user names.

NextJS

import React, { useState, useEffect } from 'react';

export default function UserList() {
  const [users, setUsers] = useState([]);
  const [loading, setLoading] = useState(true);

  useEffect(() => {
    fetch('https://jsonplaceholder.typicode.com/users')
      .then(response => response.json())
      .then(data => {
        setUsers(data);
        setLoading(false);
      });
  }, []);

  return (
    // Show loading message if loading is true, else show user list
    // Your code here
  );
}

Hint

Use a ternary operator inside JSX to show <p>Loading...</p> when loading is true, else map over users to show names.

4

Add accessibility and export the component

Ensure the ul element has an aria-label attribute with the value "User list" for accessibility. Confirm the component is exported as default.

NextJS

import React, { useState, useEffect } from 'react';

export default function UserList() {
  const [users, setUsers] = useState([]);
  const [loading, setLoading] = useState(true);

  useEffect(() => {
    fetch('https://jsonplaceholder.typicode.com/users')
      .then(response => response.json())
      .then(data => {
        setUsers(data);
        setLoading(false);
      });
  }, []);

  return (
    
      {loading ? (
        Loading...
      ) : (
        // Add aria-label to ul
        // Your code here
      )}
    
  );
}

Hint

Add aria-label="User list" to the ul element for screen readers.

Practice

1. What is the main purpose of a loading state in a Next.js component?

easy

A. To speed up the data fetching process automatically

B. To show users that data is being fetched and the app is working

C. To permanently hide the data from users

D. To prevent users from clicking buttons

Solution

1. Step 1: Understand loading state purpose

   Loading states inform users that data is being fetched and the app is busy.

2. Step 2: Compare options

   Only To show users that data is being fetched and the app is working correctly describes this purpose; others are incorrect or unrelated.

3. Final Answer:

   To show users that data is being fetched and the app is working -> Option B

4. Quick Check:

   Loading state = user feedback [OK]

Hint: Loading states show progress to users [OK]

Common Mistakes:

• Thinking loading states speed up data fetching
• Confusing loading state with error state
• Ignoring user feedback during data fetch

2. Which of the following is the correct way to declare a loading state using React hooks in a Next.js component?

easy

A. const loading = useState(false);

B. let loading = true;

C. var loading = useState(true);

D. const [loading, setLoading] = useState(false);

Solution

1. Step 1: Identify correct useState syntax

   useState returns an array with state and setter, so destructuring is needed.

2. Step 2: Check options

   const [loading, setLoading] = useState(false); correctly uses array destructuring; others misuse useState or declare variables incorrectly.

3. Final Answer:

   const [loading, setLoading] = useState(false); -> Option D

4. Quick Check:

   useState syntax = destructuring [OK]

Hint: useState returns [state, setter], use array destructuring [OK]

Common Mistakes:

• Not destructuring useState result
• Using var or let instead of const
• Assigning useState directly to a variable

3. Given this Next.js component snippet, what will be rendered initially?

import { useState, useEffect } from 'react';

export default function DataLoader() {
  const [loading, setLoading] = useState(true);
  const [data, setData] = useState(null);

  useEffect(() => {
    setTimeout(() => {
      setData('Hello World');
      setLoading(false);
    }, 1000);
  }, []);

  if (loading) return <div>Loading...</div>;
  return <div>{data}</div>;
}

medium

A. Nothing renders

B. <div>Hello World</div>

C. <div>Loading...</div>

D. Error: data is null

Solution

1. Step 1: Check initial state values

   loading is true initially, so the component returns the loading message.

2. Step 2: Understand useEffect timing

   Data and loading update after 1 second, so initially only loading message shows.

3. Final Answer:

   <div>Loading...</div> -> Option C

4. Quick Check:

   Initial loading = true means show loading [OK]

Hint: Initial loading true means show loading message first [OK]

Common Mistakes:

• Assuming data shows immediately
• Ignoring initial loading state
• Expecting error when data is null

4. Identify the bug in this Next.js loading state code:

import { useState, useEffect } from 'react';

export default function Fetcher() {
  const [loading, setLoading] = useState(false);
  const [data, setData] = useState(null);

  useEffect(() => {
    setLoading(true);
    fetch('/api/data')
      .then(res => res.json())
      .then(json => {
        setData(json);
        setLoading(false);
      });
  }, []);

  if (loading) return <div>Loading...</div>;
  return <div>{JSON.stringify(data)}</div>;
}

medium

A. Initial loading state should be true, not false

B. Missing dependency array in useEffect

C. setLoading(true) should be after fetch

D. fetch call is missing await keyword

Solution

1. Step 1: Check initial loading state

   Loading starts false, but fetch begins immediately, so UI may skip loading message.

2. Step 2: Understand effect of initial loading false

   Because loading is false initially, component renders data area before fetch completes, showing null or empty.

3. Final Answer:

   Initial loading state should be true, not false -> Option A

4. Quick Check:

   Loading true initially shows loading UI correctly [OK]

Hint: Start loading as true to show loading UI immediately [OK]

Common Mistakes:

• Setting loading false initially hides loading UI
• Ignoring initial state impact on render
• Misplacing setLoading calls

5. You want to show a loading spinner while fetching data and then display the data or an error message if fetching fails. Which approach correctly handles loading, success, and error states in a Next.js component?

hard

A. Use three state variables: loading (boolean), data (object|null), error (string|null); update them accordingly during fetch lifecycle

B. Use only one state variable for data and show loading until data is not null

C. Use loading state only and ignore errors to simplify code

D. Fetch data outside component and pass as props to avoid loading states

Solution

1. Step 1: Identify states needed for full fetch lifecycle

   Loading, data, and error states cover all cases: waiting, success, and failure.

2. Step 2: Evaluate options

   Use three state variables: loading (boolean), data (object|null), error (string|null); update them accordingly during fetch lifecycle uses all three states properly; others miss error handling or loading feedback.

3. Final Answer:

   Use three state variables: loading (boolean), data (object|null), error (string|null); update them accordingly during fetch lifecycle -> Option A

4. Quick Check:

   Loading + data + error states = robust UI [OK]

Hint: Track loading, data, and error separately for clear UI states [OK]

Common Mistakes:

• Ignoring error state leads to silent failures
• Using only data state misses loading feedback
• Fetching data outside component loses dynamic loading UI