Streaming and partial rendering in NextJS - Step-by-Step Execution

Concept Flow - Streaming and partial rendering

Start Server Rendering

↓

Render Initial HTML Chunk

↓

Send Chunk to Browser

↓

Browser Displays Partial Content

↓

Render Next Chunk on Server

↓

Send Next Chunk to Browser

↓

Browser Updates with More Content

↓

Repeat Until Full Page Rendered

↓

Complete Rendering

The server renders and sends parts of the page step-by-step, letting the browser show content early and update progressively.

Execution Sample

NextJS

import React, { Suspense } from 'react';

export default function Page() {
  return (
    <>
      <Suspense fallback={<p>Loading header...</p>}>
        <Header />
      </Suspense>
      <MainContent />
    </>
  );
}

This Next.js component shows a loading fallback for the Header first, then streams MainContent, showing partial content early while Header loads.

Execution Table

Step	Server Action	Content Sent	Browser Action	Visible Content
1	Start rendering Page component	No content yet	Waiting for data	Blank page
2	Render <Suspense> fallback for Header	<p>Loading header...</p>	Display fallback	Shows 'Loading header...' text
3	Render MainContent component	<main>Main content here</main>	Append main content	Shows 'Loading header...' + Main content
4	Header component resolves	<header>Header content</header>	Replace fallback with Header	Shows Header + Main content
5	Finish rendering all components	Full page sent	Page fully loaded	Complete page visible

💡 All components rendered and streamed; browser shows full page progressively.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
ContentSent	''	'<p>Loading header...</p>'	'<p>Loading header...</p><main>Main content here</main>'	'<header>Header content</header><main>Main content here</main>'	Full HTML page
BrowserVisible	''	'Loading header...'	'Loading header... + Main content'	'Header + Main content'	Full page content

Key Moments - 2 Insights

Why does the browser show 'Loading header...' before the actual header content?

Because the <Suspense> fallback is sent first (see execution_table step 2), letting the browser show something while waiting for the Header component to load.

How does streaming improve user experience compared to waiting for the full page?

Streaming sends parts of the page early (steps 2 and 3), so the user sees content sooner instead of a blank page until everything loads (see execution_table).

Visual Quiz - 3 Questions

Look at the execution table, what content is visible in the browser after Step 3?

A 'Main content here'

B 'Loading header...'

C 'Loading header... + Main content'

D Blank page

💡 Hint

Check the 'Visible Content' column after Step 3 in the execution_table.

At which step does the browser start showing the main content?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Content Sent' and 'Visible Content' columns for when

Main content here

is sent.

If the <Suspense> fallback was removed, what would the browser show after Step 2?

A Blank page

B Header content immediately

C Loading header...

D Main content

💡 Hint

Refer to the role of fallback in execution_table step 2 and variable_tracker.

Concept Snapshot

Next.js streaming sends HTML chunks progressively.
Use <Suspense> with fallback to show placeholders.
Browser renders partial content early.
Improves perceived load speed.
Page updates as more chunks arrive.
Full page visible when done.

Full Transcript

Streaming and partial rendering in Next.js means the server sends parts of the page step-by-step. First, it sends a fallback UI like 'Loading header...' so the browser shows something quickly. Then, the main content is sent and appended below the fallback. Next, the server sends the actual header content, replacing the fallback. This way, users see parts of the page early instead of waiting for everything to load. The process repeats until the full page is rendered and visible. This improves user experience by reducing blank screen time and showing content progressively.

Practice

1. What is the main benefit of streaming in Next.js when rendering pages?

easy

A. It caches the whole page on the client side before showing anything.

B. It delays sending the entire page until all data is loaded.

C. It disables server-side rendering completely.

D. It sends parts of the page to the browser as soon as they are ready.

Solution

1. Step 1: Understand streaming concept in Next.js

   Streaming means sending parts of the page to the browser immediately when they are ready, not waiting for the whole page.

2. Step 2: Compare options with streaming behavior

   It sends parts of the page to the browser as soon as they are ready. matches this behavior exactly, while others describe delaying or caching, which are not streaming.

3. Final Answer:

   It sends parts of the page to the browser as soon as they are ready. -> Option D

4. Quick Check:

   Streaming = send parts early [OK]

Hint: Streaming means show parts early, not wait for all [OK]

Common Mistakes:

• Thinking streaming waits for full page
• Confusing streaming with client caching
• Assuming streaming disables server rendering

2. Which of the following is the correct way to use Suspense for partial rendering in Next.js?

easy

A. } />

B. } />

C. }>

D. }>

Solution

1. Step 1: Recall Suspense usage pattern

   Suspense wraps the async component and uses the fallback prop to show a loading state while waiting.

2. Step 2: Match correct syntax

   <Suspense fallback=<Loading />><AsyncComponent /></Suspense> correctly wraps AsyncComponent inside Suspense with fallback. Others misuse fallback or component placement.

3. Final Answer:

   <Suspense fallback=<Loading />><AsyncComponent /></Suspense> -> Option C

4. Quick Check:

   Suspense wraps async with fallback [OK]

Hint: Suspense wraps async component with fallback prop [OK]

Common Mistakes:

• Placing fallback inside async component
• Not wrapping async component with Suspense
• Using fallback incorrectly as a child

3. Given this Next.js server component code using streaming:

export default async function Page() {
  const user = await fetchUser();
  return (
    <>
      
Welcome {user.name}
      Loading posts...
}>
        
      
    </>
  );
}

What will the user see first in the browser?

medium

A. Only 'Loading posts...' until all data loads.

B. The heading with user name, then 'Loading posts...' while posts load.

C. The full page with heading and posts at once.

D. An empty page until fetchUser finishes.

Solution

1. Step 1: Analyze async data fetching and rendering

   The user data is awaited before rendering, so the heading with user name shows immediately.

2. Step 2: Understand Suspense fallback behavior

   The Posts component is wrapped in Suspense with a fallback, so 'Loading posts...' shows while posts load.

3. Final Answer:

   The heading with user name, then 'Loading posts...' while posts load. -> Option B

4. Quick Check:

   Await user shows heading first, Suspense fallback next [OK]

Hint: Awaited data shows first; Suspense fallback shows during async child load [OK]

Common Mistakes:

• Thinking Suspense fallback shows before awaited user data
• Assuming full page waits for all data
• Ignoring Suspense fallback usage

4. Identify the error in this Next.js streaming code snippet:

export default async function Page() {
  return (
    Loading...
}>
      
    
  );
}

function AsyncData() {
  const data = fetchData();
  return <p>{data.message}</p>;
}

medium

A. AsyncData is not marked async and fetchData is not awaited.

B. Suspense cannot wrap components in Next.js.

C. fetchData should be called inside Page, not AsyncData.

D. The fallback prop must be a string, not JSX.

Solution

1. Step 1: Check AsyncData function for async usage

   AsyncData calls fetchData() but does not await it or mark function async, causing a promise to be rendered.

2. Step 2: Confirm Suspense usage and fallback validity

   Suspense usage is correct, and fallback can be JSX, so no error there.

3. Final Answer:

   AsyncData is not marked async and fetchData is not awaited. -> Option A

4. Quick Check:

   Async function must await async calls [OK]

Hint: Async components must await async calls inside [OK]

Common Mistakes:

• Not marking component async when using await
• Rendering unresolved promises
• Misunderstanding Suspense fallback types

5. You want to stream a page showing user info and their posts. User info loads fast, posts load slowly. How do you combine streaming and partial rendering to show user info immediately and posts with a loading fallback?

hard

A. Make Page async, await user data, render user info immediately, wrap Posts in Suspense with fallback.

B. Render user info and posts inside one Suspense with fallback for both.

C. Fetch posts first, then user info, and render all at once without Suspense.

D. Use client-side fetching for posts and server-side for user info without Suspense.

Solution

1. Step 1: Await fast user data in Page component

   Mark Page async and await user data so user info renders immediately.

2. Step 2: Wrap slow posts component in Suspense with fallback

   Wrap Posts in Suspense with a loading fallback to show partial rendering while posts load.

3. Final Answer:

   Make Page async, await user data, render user info immediately, wrap Posts in Suspense with fallback. -> Option A

4. Quick Check:

   Await fast data, Suspense slow data [OK]

Hint: Await fast data, Suspense slow parts with fallback [OK]

Common Mistakes:

• Wrapping all content in one Suspense losing immediate render
• Not awaiting fast data before rendering
• Using client fetching without Suspense for server components