Loading states for data in NextJS - Step-by-Step Execution

Concept Flow - Loading states for data

Component mounts

↓

Start fetching data

↓

Show loading state

↓

Data fetch resolves

↓

If success

→Show data

↓

If error

→Show error message

↓

User sees updated UI

When a component loads, it starts fetching data and shows a loading message. Once data arrives, it updates the UI or shows an error if fetching fails.

Execution Sample

NextJS

import { useState, useEffect } from 'react';

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

  useEffect(() => {
    fetch('/api/data')
      .then(res => {
        if (!res.ok) {
          throw new Error('Network response was not ok');
        }
        return res.json();
      })
      .then(json => {
        setData(json);
        setLoading(false);
      })
      .catch(err => {
        setError(err.message);
        setLoading(false);
      });
  }, []);

  if (loading) return <p>Loading...</p>;
  if (error) return <p>Error: {error}</p>;
  return <pre>{JSON.stringify(data, null, 2)}</pre>;
}

This React component fetches data on mount, shows "Loading..." while waiting, then displays the data once loaded or an error message if fetching fails.

Execution Table

Step	Action	State Before	State After	UI Rendered
1	Component mounts	loading: true, data: null, error: null	loading: true, data: null, error: null	"Loading..." shown
2	Fetch starts	loading: true, data: null, error: null	loading: true, data: null, error: null	"Loading..." shown
3	Fetch resolves with data	loading: true, data: null, error: null	loading: false, data: {items: [...]}, error: null	Data JSON shown
4	Component re-renders	loading: false, data: {items: [...]}, error: null	loading: false, data: {items: [...]}, error: null	Data JSON shown
5	No more updates	loading: false, data: {items: [...]}, error: null	loading: false, data: {items: [...]}, error: null	Data JSON shown

💡 Data loaded and loading set to false, so loading state ends and data is displayed.

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
loading	true	true	false	false
data	null	null	{items: [...]}	{items: [...]}
error	null	null	null	null

Key Moments - 2 Insights

Why does the UI show "Loading..." first before showing data?

Because the loading state is true initially (see execution_table step 1), so the component renders the loading message until data arrives.

What triggers the component to re-render and show the data?

When setLoading(false) and setData(...) are called after fetch resolves (step 3), React updates state causing a re-render (step 4) to show data.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'loading' after step 3?

A null

B true

C false

D undefined

💡 Hint

Check the 'State After' column for step 3 in the execution_table.

At which step does the UI switch from showing "Loading..." to showing the data?

A Step 1

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'UI Rendered' column in the execution_table to see when data is shown.

If the fetch never resolves, what will the UI show according to the execution flow?

A "Loading..." forever

B Data JSON

C Error message

D Blank screen

💡 Hint

Refer to the concept_flow where loading state shows until data arrives or error occurs.

Concept Snapshot

Loading states in Next.js:
- Start with loading=true and data=null
- Fetch data inside useEffect on mount
- Show loading UI while loading is true
- On fetch success, set data and loading=false
- Component re-renders to show data
- Always handle loading and error states for good UX

Full Transcript

This example shows how a Next.js React component manages loading states when fetching data. Initially, the component sets loading to true and data to null. When the component mounts, it starts fetching data asynchronously. While waiting, it renders a loading message. Once the fetch resolves successfully, it updates the data state and sets loading to false. This triggers a re-render, and the component then displays the fetched data. The execution table traces each step, showing state changes and UI updates. Key moments clarify why loading shows first and what triggers the data display. The visual quiz tests understanding of state values and UI changes during the process.